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LM628 Programming Guide

INTRODUCTION

The LM628/LM629 are dedicated motion control processors.Both devices control DC and brushless DC servo motors, aswell as, other servomechanisms that provide a quadratureincremental feedback signal. Block diagrams of typicalLM628/LM629-based motor control systems are shown inFigures 1, 2.

As indicated in the figures, the LM628/LM629 are bus pe-ripherals; both devices must be programmed by a host pro-cessor. This application note is intended to present a con-crete starting point for programmers of these precisionmotion controllers. It focuses on the development of shortprograms that test overall system functionality and lay thegroundwork for more complex programs. It also presents amethod for tuning the loop-compensation PID filter. (Note 1)

REFERENCE SYSTEM

Figure 15 is a detailed schematic of a closed-loop motor con-trol system. All programs presented in this paper were devel-oped using this system. For application of the programs inother LM628-based systems, changes in basic programmingstructure are not required, but modification of filter coeffi-cients and trajectory parameters may be required.

I. PROGRAM MODULES

Breaking programs for the LM628 into sets of functionalblocks simplifies the programming process; each block ex-ecutes a specific task. This section contains examples of theprincipal building blocks (modules) of programs for theLM628.

Note 1: For the remainder of this paper, all statements about the LM628 also apply to the LM629 unless otherwise noted.

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FIGURE 1. LM628-Based Motor Control System

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FIGURE 2. LM629-Based Motor Control System

National SemiconductorApplication Note 693Steven HuntJanuary 1999

LM628

Program

ming

Guide

AN

-693

© 1999 National Semiconductor Corporation AN010860 www.national.com

BUSY-BIT CHECK MODULE

The first module required for successful programming of theLM628 is a busy-bit check module.

The busy-bit, bit zero of the status byte, is set immediatelyafter the host writes a command byte, or reads or writes thesecond byte of a data word. See Figure 5. While the busy-bitis set, the LM628 will ignore any commands or attempts totransfer data.

A busy-bit check module that polls the Status Byte and waitsuntil the busy-bit is reset will ensure successful host/LM628communications. It must be inserted after a commandwrite, or a read or write of the second byte of a dataword. Figure 3 represents such a busy-bit check module.This module will be used throughout subsequent modulesand programs.

Reading the Status Byte is accomplished by executing a RD-STAT command. RDSTAT is directly supported by LM628hardware and is executed by pulling CS, PS, and RD logiclow.

INITIALIZATION MODULE

In general, an initialization module contains a reset com-mand and other initialization, interrupt control, and data re-porting commands.

The example initialization module, detailed in Table 1, con-tains a hardware reset block and a PORT 12 command.

Hardware Reset Block

Immediately following power-up, a hardware reset must beexecuted. Hardware reset is initiated by strobing RST (pin27) logic low for a minimum of eight LM628 clock periods.The reset routine begins after RST is returned to logic high.During the reset execution time, 1.5 ms maximum, theLM628 will ignore any commands or attempts to transferdata.

A hardware reset forces the LM628 into the state describedin what follows.

1. The derivative sampling coefficient, dS, is set to one, andall other filter coefficients and filter coefficient input buff-ers are set to zero. With dS set to one, the derivativesampling interval is set to 2048/fCLK.

2. All trajectory parameters and trajectory parameters inputbuffers are set to zero.

3. The current absolute position of the shaft is set to zero(“home”).

4. The breakpoint interrupt is masked (disabled), and theremaining five interrupts are unmasked (enabled).

5. The position error threshold is set to its maximum value,7FFF hex.

6. The DAC output port is set for an 8-bit DAC interface.

Figure 4 illustrates a hardware reset block that includes anLM628 functionality test. This test should be completed im-mediately following all hardware resets.

Reset Interrupts

An RSTI command sequence allows the user to reset the in-terrupt flag bits, bits one through six of the status byte. SeeFigure 5. It contains an RSTI command and one data word.

The RSTI command initiates resetting the interrupt flag bits.Command RSTI also resets the host interrupt output pin (pin17).

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FIGURE 3. Busy-bit Check Module

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FIGURE 4. Hardware Reset Block

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TABLE 1. Initialization Module (with Hardware Reset)

Port Bytes Command Comments

(Note 5) hardwarereset

Strobe RST, pin 27,logic low for eightclock periodsminimum.

wait The maximum timeto completehardware resettasks is 1.5 ms.During this resetexecution time, theLM628 will ignoreany commands orattempts to transferdata.

c(Note 2)

xx(Note 3)

RDSTAT This commandreads the statusbyte. It is directlysupported byLM628 hardwareand can beexecuted at anytime by pulling CS,PS, and RD logiclow. Statusinformation remainsvalid as long as RDis logic low.

decision If the status byte isC4 hex or 84 hex,continue. Otherwiseloop back tohardware reset.

c 1D RSTI This commandresets only theinterrupts indicatedby zeros in bits onethrough six of thenext data word. Italso resets bitfifteen of theSignals Registerand the hostinterrupt output pin(pin 17).

Busy-bit Check Module

d xx HB(Note 4)

don’t care

d 00 LB Zeros in bits onethrough six indicateall interrupts will bereset.



c xx RDSTAT This commandreads the statusbyte.

decision If the status byte isC0 hex or 80 hex,continue. Otherwiseloop back tohardware reset.

c 06 PORT12 The reset defaultsize of the DACport is eight bits.This commandinitializes the DACport for a 12-bitDAC. It should notbe issued insystems with an8-bit DAC.


Note 2: The 8-bit host I/O port is a dual-mode port; it operates in commandor data mode. The logic level at PS (pin 16) selects the mode. Port c repre-sents the LM628 command port-commands are written to the command portand the Status Byte is read from the command port. A logic level of “0” at PSselects the command port. Port d represents the LM628 data port — data isboth written to and read from the data port. A logic level of “1” at PS selectsthe data port.

Note 3: x - don’t care

Note 4: HB - high byte, LB - low byte

Note 5: All values represented in hex.

Immediately following the RSTI command, a single dataword is written. The first byte is not used. Logical zeros inbits one through six of the second byte reset the correspond-ing interrupts. See Figure 6. Any combination of the interruptflag bits can be reset within a single RSTI command se-quence. This feature allows interrupts to be serviced accord-ing to a user-programmed priority.

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FIGURE 5. Status Byte Bit Allocation

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In the case of the example module, the second byte of theRSTI data word, 00 hex, resets all interrupt flag bits. SeeTable 1.

DAC Port Size

During both hardware and software resets, the DAC outputport defaults to 8-bit mode. If an LM628 control loop utilizesa 12-bit DAC, command PORT12 should be issued immedi-ately following the hardware reset block and all subsequentresets. Failure to issue command PORT12 will result in er-ratic, unpredictable motor behavior.

If the control loop utilizes an 8-bit DAC, command PORT12must not be executed; this too will result in erratic, unpredict-able motor behavior.

An LM629 will ignore command PORT8 (as it provides an8-bit sign/magnitude PWM output). Command PORT12should not be issued in LM629-based systems.

Software Reset Considerations

After the initial hardware reset, resets can be accomplishedwith either a hardware reset or command RESET (softwarereset). Software and hardware resets execute the sametasks (Note 6) and require the same execution time, 1.5 msmaximum. During software reset execution, the LM628 willignore any commands or attempts to transfer data.

The hardware reset module includes an LM628 functionalitytest. This test is not required after a software reset.

Table 2 details an initialization module that uses a softwarereset.Note 6: In the case of a software reset, the position error threshold remainsat its pre-reset value.

TABLE 2. Initialization Module (with Software Reset)


c 00 RESET See InitializationModule text.

wait The maximum timeto complete RESETtasks is 1.5 ms.

c 06 PORT12 The RESET defaultsize of the DAC portis eight bits. Thiscommand initializesthe DAC port for a12-bit DAC. It shouldnot be issued in asystem with an 8-bitDAC.


c 1D RSTI This command resetsonly the interruptsindicated by zeros inbits one through sixof the next dataword. It also resetsbit fifteen of theSignals Register and(pin 17) the hostinterrupt output pin.


d xx HB Don’t care

d 00 LB Zeros in bits onethrough six indicateall interrupts will bereset.


Comments

Figure 7 illustrates, in simplified block diagram form, theLM628. The profile generator provides the control loop input,desired shaft position. The quadrature decoder provides thecontrol loop feedback signal, actual shaft position. At the firstsumming junction, actual position is subtracted from desiredposition to generate the control loop error signal, position er-ror. This error signal is filtered by the PID filter to provide themotor drive signal.

After executing the example initialization module, the follow-ing observations are made. With the integration limit term (iL)and the filter gain coefficients (kp, ki, and kd) initialized tozero, the filter gain is zero. Moreover, after a reset, desiredshaft position tracks actual shaft position. Under these con-ditions, the motor drive signal is zero. The control systemcan not affect shaft position. The shaft should be stationary

high byte

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low byte

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FIGURE 6. Interrupt Mask/Reset Bit Allocations

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and “free wheeling”. If there is significant drive amplifier off-set, the shaft may rotate slowly, but with minimal torque ca-pability.Note: Regardless of the free wheeling state of the shaft, the LM628 continu-

ously tracks shaft absolute position.

FILTER PROGRAMMING MODULE

The example filter programming module is shown in Table 4.

Load Filter Parameters (Coefficients)

An LFIL (Load FILter) command sequence includes com-mand LFIL, a filter control word, and a variable number ofdata words.

The LFIL command initiates loading filter coefficients into in-put buffers.

The two data bytes, written immediately after LFIL, comprisethe filter control word. The first byte programs the derivativesampling coefficient, ds (i.e. selects the derivative samplinginterval). The second byte indicates, with logical ones in re-spective bit positions, which of the remaining four filter coef-ficients will be loaded. See Figure 8,Table 3. Any combina-tion of the four coefficients can be loaded within a single LFILcommand sequence.

Immediately following the filter control word, the filter coeffi-cients are written. Each coefficient is written as a pair of databytes, a data word. Because any combination of the four co-efficients can be loaded within a single LFIL command se-quence, the number of data words following the filter controlword can vary in the range from zero to four.

In the case of the example module, the first byte of the filtercontrol word, 00 hex, programs a derivative sampling coeffi-cient of one. The second byte, x8 hex, indicates only the pro-portional gain coefficient will be loaded.

Immediately following the filter control word, the proportionalgain coefficent is written. In this example, kp is set to ten withthe data word 000A hex. The other three filter coefficients re-main at zero, their reset value.

Update Filter

The update filter command, UDF, transfers new filter coeffi-cients from input buffers to working registers. Until UDF isexecuted, the new filter coefficients do not affect the transfercharacteristic of the filter.

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FIGURE 7. LM628 — Simplified Block Diagram Form

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TABLE 3. Derivative — Term Sampling Interval Selection Codes

Filter Control Word Bit Position d s SelectedDerivative-Term

Sampling Interval — T d15 14 13 12 11 10 9 8

0 0 0 0 0 0 0 0 1 Ts

0 0 0 0 0 0 0 1 2 2Ts

0 0 0 0 0 0 1 0 3 3Ts

0 0 0 0 0 0 1 1 4 4Ts

• • •

• • •

• • •

1 1 1 1 1 1 1 1 256 256Ts

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FIGURE 8. Filter Control Word Bit Allocation

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TABLE 4. Filter Programming Module


c 1E LFIL This command initiatesloading the filtercoefficients inputbuffers.


dd

00x8

HBLB

These two bytes are thefilter control word. A 00hex HB sets thederivative samplinginterval to 2048/fCLK bysetting ds to one. A x8hex LB indicates only kp

will be loaded. Theother filter parameterswill remain at zero, theirreset default value.


dd

000A

HBLB

These two bytes set kp

to ten.


c 04 UDF This command transfersnew filter coefficientsfrom input buffers toworking registers. UntilUDF is executed,coefficients loaded viathe LFIL command donot affect the filtertransfer characteristic.


Comments

After executing both the example initialization and examplefilter programming modules, the following observations aremade. Filter gain is nonzero, but desired shaft position con-tinues to track actual shaft position. Under these conditions,the motor drive signal remains at zero. The shaft should bestationary and “free wheeling”. If there is significant driveamplifier offset, the shaft may rotate slowly, but with minimaltorque capability.

Initially, kp should be set below twenty, ds should be set toone, and ki, kd, and il should remain at zero. These valueswill not provide optimum system performance, but they willbe sufficient to test system functionality. See Tuning the PIDFilter.

TRAJECTORY PROGRAMMING MODULE

Table 5 details the example trajectory programming module.

Load Trajectory Parameters.

An LTRJ (Load TRaJectory) command sequence includescommand LTRJ, a trajectory control word, and a variablenumber of data words.

The LTRJ command initiates loading trajectory parametersinto input buffers.

The two data bytes, written immediately after LTRJ, com-prise the trajectory control word. The first byte programs,with logical ones in respective bit positions, the trajectorymode (velocity or position), velocity mode direction, and

stopping mode. See Stop Module. The second byte indi-cates, with logical ones in respective bit positions, which ofthe three trajectory parameters will be loaded. It also indi-cates whether the parameters are absolute or relative. SeeFigure 9. Any combination of the three parameters can beloaded within a single LTRJ command sequence.

Immediately following the trajectory control word, the trajec-tory parameters are written. Each parameter is written as apair of data words (four data bytes). Because any combina-tion of the three parameters can be loaded within a singleLTRJ command sequence, the number of data words follow-ing the trajectory control word can vary in the range fromzero to six.

In the case of the example module, the first byte of the tra-jectory control word, 00 hex, programs the LM628 to operatein position mode. The second byte, 0A hex, indicates velocityand position will be loaded and both parameters are abso-lute. Four data words, two for each parameter loaded, followthe trajectory control word.

Start Motion Control

The start motion control command, STT (STarT), transfersnew trajectory parameters from input buffers to working reg-isters and begins execution of the new trajectory. Until STT isexecuted, the new trajectory parameters do not affect shaftmotion.Note: At this point no actual trajectory parameters are loaded. Calculation of

trajectory parameters and execution of example moves is left for alater section.

high byte

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low byte

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FIGURE 9. Trajectory Control Word Bit Allocation

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TABLE 5. Trajectory Programming Module


c 1F LTRJ This command initiatesloading the trajectoryparameters inputbuffers.


dd

000A

HBLB

These two bytes are thetrajectory control word.A 0A hex LB indicatesvelocity and position willbe loaded and bothparameters areabsolute.


dd

xxxx

HBLB

Velocity is loaded in twodata words. These twobytes are the high dataword.


dd

xxxx

HBLB

velocity data word (low)


dd

xxxx

HBLB

Position is loaded in twodata words. These twobytes are the high dataword.


dd

xxxx

HBLB

position data word (low)


c 01 STT STT must be issued toexecute the desiredtrajectory.


STOP MODULE

This module demonstrates the programming flow required tostop shaft motion.

While the LM628 operates in position mode, normal stoppingis always smooth and occurs automatically at the end of aspecified trajectory (i.e. no stop module is required). Underexceptional conditions, however, a stop module can be usedto affect a premature stop.

While the LM628 operates in velocity mode, stopping is al-ways accomplished via a stop module.

The example stop module, shown in Table 5, utilizes anLTRJ command sequence and an STT command.

Load Trajectory Parameters

Bits eight through ten of the trajectory control word select thestopping mode. See Figure 9.

In the case of the example module, the first byte of the tra-jectory control word, x1 hex, selects motor-off as the desiredstopping mode. This mode stops shaft motion by setting themotor drive signal to zero (the appropriate offset-binary codeto apply zero drive to the motor).

Setting bit nine of the trajectory control word selects stopabruptly as the desired stopping mode. This mode stopsshaft motion (at maximum deceleration) by setting the targetposition equal to the current position.

Setting bit ten of the trajectory control word selects stopsmoothly as the desired stopping mode. This mode stopsshaft motion by decelerating at the current user-programmedacceleration rate.Note: Bits eight through ten of the trajectory control word must be used ex-

clusively; only one of them should be logic one at any time.


The start motion control command, STT, must be executedto stop shaft motion.

Comments

After shaft motion is stopped with either an “abrupt” or a“smooth” stop module, the control system will attempt to holdthe shaft at its current position. If forced away from this de-sired resting position and released, the shaft will move backto the desired position. Unless new trajectory parametersare loaded, execution of another STT command will restartthe specified move.

After shaft motion is stopped with a “motor-off” stop module,desired shaft position tracks actual shaft position. Conse-quently, the motor drive signal remains at zero and the con-trol system can not affect shaft position; the shaft should bestationary and free wheeling. If there is significant drive am-plifier offset, the shaft may rotate slowly, but with minimaltorque capability. Unless new trajectory parameters areloaded, execution of another STT command will restart thespecified move.

TABLE 6. Stop Module (Motor-Off)




dd

x100

HBLB

These two bytes are thetrajectory control word.A x1 hex HB selectsmotor-off as the desiredstopping mode. A 00hex LB indicates notrajectory parameterswill be loaded.


c 01 STT The start motion controlcommand, STT, mustbe executed to stopshaft motion.


II. PROGRAMS

This section focuses on the development of four brief LM628programs.

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LOOP PHASING PROGRAM

Following initial power-up, the correct polarity of the motordrive signal must be determined. If the polarity is incorrect(loop inversion), the drive signal will push the shaft awayfrom its desired position rather than towards it. This results in“motor runaway”, a condition characterized by the motor run-ning continuously at high speed.

The loop phasing program, detailed in Table 7, contains boththe example initialization and filter programming modules. Italso contains an LTRJ command sequence and an STTcommand.Note: Execution of this simple program is only required the first time a new

system is used.


An LTRJ (Load TRaJectory) command sequence includescommand LTRJ, a trajectory control word, and a variablenumber of data words.

In the case of the Loop Phasing Program, the first byte of thetrajectory control word, 00 hex, programs the LM628 to oper-ate in position mode. The second byte, 00 hex, indicates notrajectory parameters will be loaded (i.e. in this program,zero data words follow the trajectory control word). The threetrajectory parameters will remain at zero, their reset value.


The start motion control command, STT (STarT), transfersnew trajectory parameters from input buffers to working reg-isters and begins execution of the new trajectory. Until STT isexecuted, the new trajectory parameters do not affect shaftmotion.

TABLE 7. Loop Phasing Program


Initialization Module

Filter Programming Module



dd

0000

HBLB

These two bytes are thetrajectory control word.A 00 hex LB indicatesno trajectory parameterswill be loaded.



Comments

Execution of command STT results in execution of the de-sired trajectory. With the acceleration set at zero, the profilegenerator generates a desired shaft position that is both con-stant and equal to the current absolute position. See Figure7. Under these conditions, the control system will attempt to

hold the shaft at its current absolute postion. The shaft willfeel lightly “spring loaded”. If forced (CAREFULLY) awayfrom its desired position and released, the shaft will springback to the desired position.

If the polarity of the motor drive signal is incorrect (loop inver-sion), motor runaway will occur immediately after executionof command STT, or after the shaft is forced (CAREFULLY)from its resting position.

Loop inversion can be corrected with one of three methods:interchanging the shaft position encoder signals (channel Aand channel B), interchanging the motor power leads, or in-verting the motor command signal before application to themotor drive amplifier. For LM629 based systems, loop inver-sion can be corrected by interchanging the motor powerleads, interchanging the shaft position encoder signals, orlogically inverting the PWM sign signal.

SIMPLE ABSOLUTE POSITION MOVE

The Simple Absolute Position Move Program, detailed inTable 8, utilizes both the initialization and filter programmingmodules, as well as, an LTRJ command sequence and anSTT command.

Factors that influenced the development of this program in-cluded the following: the program must demonstrate simpletrajectory parameters calculations, the program must dem-onstrate the programming flow required to load and executean absolute position move, and correct completion of themove must be verifiable through simple observation.

Move: The shaft will accelerate at 0.1 rev/sec2 until itreaches a maximum velocity of 0.2 rev/sec, and then decel-erate to a stop exactly two revolutions from the starting posi-tion. See Figure 10.Note: Absolute position is position measured relative to zero (home). An ab-

solute position move is a move that ends at a specified absolute posi-tion. For example, independent of the current absolute position of theshaft, if an absolute position of 30,000 counts is specified, uponcompletion of the move the absolute position of the shaft will be 30,000counts (i.e. 30,000 counts relative to zero). The example program callsfor a position move of two revolutions. Because the starting absoluteposition is 0 counts, the move is accomplished by specifying an abso-lute position of 8000 counts. See Table 8.

The Quadrature Incremental Encoder

As a supplement to the trajectory parameters calculations, abrief discussion is provided here to differentiate between en-coder lines and encoder counts .

A quadrature incremental shaft encoder encodes shaft rota-tion as electrical pulses. Figure 11 details the signals gener-ated by a 3-channel quadrature incremental encoder. TheLM628 decodes (or “counts”) a quadrature incremental sig-nal to determine the absolute position of the shaft.

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The resolution of a quadrature incremental encoder is usu-ally specified as a number of lines . This number indicatesthe number of cycles of the output signals for each completeshaft revolution. For example, an N-line encoder generatesN cycles of its output signals during each complete shaftrevolution.

By definition, two signals that are in quadrature are 90˚ out ofphase. When considered together, channels A and B (Figure11) traverse four distinct digital states during each full cycle

of either channel. Each state transition represents onecount of shaft motion. The leading channel indicates the di-rection of shaft rotation.

Each line, therefore, represents one cycle of the output sig-nals, and each cycle represents four encoder counts.

The reference system uses a one thousand line encoder.

Sample Period

Sampling of actual shaft position occurs at a fixed frequency,the reciprocal of which is the system sample period. The sys-tem sample period is the unit of time upon which shaft accel-eration and velocity are based.

The reference system uses an 8 MHz clock. The sample pe-riod of the reference system follows directly from the defini-tion.

Trajectory Parameters Calculations

The shaft will accelerate at 0.1 rev/sec2 until it reaches amaximum velocity of 0.2 rev/sec, and then decelerate to astop exactly two revolutions from the starting position.

Trajectory parameters calculations for this move are detailedin Figure 12.

Comments

After completing the move, the control system will attempt tohold the shaft at its current absolute position. The shaft willfeel lightly “spring loaded”. If forced away from its desiredresting position and released, the shaft will move back to thedesired position.

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FIGURE 10. Velocity Profile for SimpleAbsolute Position Move Program

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FIGURE 11. 3-Channel Quadrature Encoder Signals

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FIGURE 12. Calculations of Trajectory Parameters for Simple Absolute Position Move

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TABLE 8. Simple Absolute Position Move Program




c 1F LTRJ This command initiatesloading the trajectoryparameters input buffers


dd

002A

HBLB

These two bytes are thetrajectory control word.A 2A hex LB indicatesacceleration, velocity,and position will beloaded and all threeparameters areabsolute.

dd

0000

HBLB

Acceleration is loaded intwo data words. Thesetwo bytes are the highdata word. In this case,the acceleration is 0.1rev/sec2.


dd

0002

HBLB

acceleration data word(low)

dd

0000

HBLB

velocity is loaded in twodata words. These twobytes are the high dataword. In this case, thevelocity is 0.2 rev/sec.


dd

346E

HBLB



dd

0000

HBLB

Position is loaded in twodata words. These twobytes are the high dataword. In this case, theposition loaded is eightthousand counts. Thisresults in a move of tworevolutions in theforward direction.


dd

1F40

HBLB




SIMPLE RELATIVE POSITION MOVE

This program demonstrates the programming flow requiredto load and execute a relative position move. See Table 9.

Move: Independent of the current resting position of theshaft, the shaft will complete thirty revolutions in the reversedirection. Total time to complete the move is fifteen seconds.Total time for acceleration and deceleration is five seconds.

Note: Target position is the final requested position. If the shaft is stationary,and motion has not been stopped with a “motor-off” stop module, thecurrent absolute position of the shaft is the target position. If motionhas been stopped with a “motor-off” stop module, or a position movehas begun, the absolute position that corresponds to the endpoint ofthe current trajectory is the target position. Relative position is positionmeasured relative to the current target position of the shaft. A relativeposition move is a move that ends the specified “relative” number ofcounts away from the current target position of the shaft. For example,if the current target position of the shaft is 10 counts, and a relative po-sition of 30,000 counts is specified, upon completion of the move theabsolute position of the shaft will be 30,010 counts (i.e. 30,000 countsrelative to 10 counts).


The first byte of the trajectory control word, 00 hex, programsposition mode operation. The second byte, 2B hex, indicatesall three trajectory parameters will be loaded. It also indi-cates both acceleration and velocity will be absolute valueswhile position will be a relative value.

Trajectory Parameters Calculations

Independent of the current resting position of the shaft, theshaft will complete thirty revolutions in the reverse direction.Total time to complete the move is fifteen seconds. Total timefor acceleration and deceleration is five seconds.

The reference system utilizes a one thousand line encoder.The number of counts for each complete shaft revolution andthe total counts for this position move are determined.

With respect to time, two-thirds of the move is made at maxi-mum velocity and one-third is made at a velocity equal toone-half the maximum velocity (Note 7). Therefore, totalcounts traveled during acceleration and deceleration periodsis one-fifth the total counts traveled. See Figure 13.

The reference system uses an 8 MHz clock. The sample pe-riod of the reference system is determined.

The number of samples during acceleration (and decelera-tion) is determined.

Using the number of counts traveled during acceleration andthe number of samples during acceleration, acceleration isdetermined.

Total counts traveled while at maximum velocity is four-fifthsthe total counts traveled.

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Note 7: Average velocity during acceleration and deceleration periods isone-half the maximum velocity.

TABLE 9. Simple Relative Position Move Program






dd

002B

HBLB

These two bytes are thetrajectory control word.A 2B hex LB indicatesall three parameters willbe loaded and bothacceleration andvelocity will be absolutevalues while position willbe a relative value.


dd

0000

HBLB

Acceleration is loaded intwo data words. Thesetwo bytes are the highdata word. In this case,the acceleration is 17counts/sample2.


dd

0011

HBLB

acceleration data word(low)


dd

0002

HBLB

Velocity is loaded in twodata words. These twobytes are the high dataword. In this case,velocity is 161,087counts/sample.


dd

753F

HBLB



dd

FFFE

HBLB

Position is loaded in twodata words. These twobytes are the high dataword. In this case, theposition loaded is−120,000 counts. Thisresults in a move ofthirty revolutions in thereverse direction.


dd

2B40

HBLB





The number of samples while at maximum velocity is deter-mined.

Using the total counts traveled while at maximum velocityand the number of samples while at maximum velocity, ve-locity is determined.

Both acceleration and velocity values are scaled.

Acceleration and velocity are rounded to the nearest integerand all three trajectory parameters are converted to hexa-decimal.

BASIC VELOCITY MODE MOVE WITH BREAKPOINTS

This program demonstrates basic velocity mode program-ming and the (typical) programming flow required to set bothabsolute and relative breakpoints. See Table 10.

Move: The shaft will accelerate at 1.0 rev/sec2 until itreaches a maximum velocity of 2.0 rev/sec. After completingtwenty forward direction revolutions (including revolutionsduring acceleration), the shaft will accelerate at 1.0 rev/sec2

until it reaches a maximum velocity of 4.0 rev/sec. After com-pleting twenty forward direction revolutions (including revolu-tions during acceleration), the shaft will decelerate (at 1.0rev/sec2) to a stop. See Figure 14.

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FIGURE 13. Velocity Profile for SimpleRelative Position Move Program

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Mask Interrupts

An MSKI command sequence allows the user to determinewhich interrupt conditions result in host interrupts; interrupt-ing the host via the host interrupt output (pin 17). It containsan MSKI command and one data word.

The MSKI command initiates interrupt masking.

Immediately following the MSKI command, a single dataword is written. The first byte is not used. Bits one throughsix of the second byte determine the masked/unmasked sta-tus of each interrupt. See Figure 6. Any zeros in this 6-bitfield mask (disable) the corresponding interrupts while anyones unmask (enable) the corresponding interrupts.

In the case of the examlple program, the second byte of theMSKI data word, 40 hex, enables the breakpoint interrupt. Allother interrupts are disabled (masked).

When interrupted, the host processor can read the StatusByte to determine which interrupt condition(s) occurred. SeeFigure 5.Note: Command MSKI controls only the host interrupt process. Bits one

through six of the Status Byte reflect actual conditions independent ofthe masked/unmasked status of individual interrupts. This feature al-lows interrupts to be serviced with a polling scheme.

Set Breakpoints (Absolute and Relative)

An SBPA command sequence enables the user to set break-points in terms of absolute shaft position. An SBPR com-mand sequence enables setting breakpoints relative to thecurrent target position. When a breakpoint position isreached, bit six of the status byte, the breakpoint interruptflag, is set to logic high. If this interrupt is enabled (un-masked), the host will be interrupted via the host interruptoutput (pin 17).

An SBPA (or SBPR) command initiates loading/setting abreakpoint. The two data words, written immediately follow-ing the SBPA (or SBPR) command, represent the breakpointposition.

The example program contains a relative breakpoint set at80,000 counts relative to position zero (the current target po-sition). This represents a move of twenty forward directionrevolutions. When this position is reached, the LM628 inter-rupts the host processor, and the host executes a sequenceof commands that increases the maximum velocity, resetsthe breakpoint interrupt flag, and loads an absolute break-point.

The example program contains an absolute breakpoint set at160,000 counts. When this absolute position is reached, theLM628 interrupts the host processor, and the host executesa Smooth Stop Module.

Breakpoint positions for this example program are deter-mined.


This example program contains two LTRJ command se-quences. The trajectory control word of the first LTRJ com-mand sequence, 1828 hex, programs forward direction ve-locity mode, and indicates an absolute acceleration and anabsolute velocity will be loaded. The trajectory control wordof the second LTRJ command sequence, 180C hex, pro-grams forward direction velocity mode, and indicates a rela-tive velocity will be loaded. See Figure 9.

Trajectory parameters calculations follow the same formatas those detailed for the simple absolute position move. SeeFigure 12.

AN010860-16

FIGURE 14. Velocity Profile for Basic Velocity Mode with Breakpoints Program

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TABLE 10. Basic Velocity Mode Move withBreakpoints Program




c 1C MSKI Mask interrupts.


d xx HB don’t care

d 40 LB A 40 hex LB enables(unmasks) thebreakpoint interrupt. Allother interrupts aredisabled (masked).


c 21 SPBR This command initiatesloading a relativebreakpoint.


dd

0001

HBLB

A breakpoint is loadedin two data words.These two bytes are thehigh data word. In thiscase, the breakpoint is80,000 counts relativeto the currentcommanded targetposition (zero).


d 38 HB breakpoint data word(low)

d 80 LB


c 1F LTRJ Load trajectory.


dd

1828

HBLB

These two bytes are thetrajectory control word.A 18 hex HB programsforward directionvelocity mode operation.A 28 hex LB indicatesacceleration andvelocity will be loadedand both values areabsolute.


dd

0000

HBLB

Acceleration is loaded intwo data words. Thesetwo bytes are the highdata word. In this case,the acceleration is 1.0rev/sec2.


d 00 HB acceleration data word(low)

d 11 LB



dd

0002

HBLB

Velocity is loaded in twodata words. These twobytes are the high dataword. In this case,velocity is 2.0 rev/s.


d 0C HB velocity data word (low)

d 4A LB


c 01 STT Start motion control.




dd

180C

HBLB

These two bytes are thetrajectory control word.A 18 hex HB programsforward directionvelocity mode operation.A 0C hex LB indicatesonly velocity will beloaded and it will be arelative value.


dd

0002

HBLB

Velocity is loaded in twodata words. These twobytes are the high dataword. In this case,velocity is 2.0 rev/s.Because this is arelative value, thecurrent velocity will beincreased by 2.0 rev/s.The resultant velocitywill be 4.0 rev/s.


d 0C HB velocity data word (low)

d 4A LB

wait This wait represents thehost processor waitingfor an LM628 breakpointinterrupt.

c 01 STT Start motion control.


c 1D RSTI Reset interrupts.



d 00 LB Zeros in bits onethrough six reset allinterrupts.

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TABLE 10. Basic Velocity Mode Move withBreakpoints Program (Continued)



c 20 SPBA This command initiatesloading an absolutebreakpoint.


dd

0002

HBLB

A breakpoint is loadedin two data words.These two bytes are thehigh data word. In thiscase, the breakpoint is160,000 countsabsolute.



d 71 HB breakpoint data word(low)

d 00 LB

wait This wait represents thehost processor waitingfor an LM628 breakpointinterrupt.

“Smooth” Stop Module

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III. TUNING THE PID FILTER

BACKGROUND

The transient response of a control system reveals importantinformation about the “quality” of control, and because a stepinput is easy to generate and sufficiently drastic, the tran-sient response of a control system is often characterized bythe response to a step input, the system step response.

In turn, the step response of a control system can be charac-terized by three attributes: maximum overshoot, rise time,and settling time. These step response attributes are definedin what follows and detailed graphically in Figure 16.

1. The maximum overshoot, Mp, is the maximum peakvalue of the response curve measured from unity. Theamount of maximum overshoot directly indicates therelative stability of the system.

2. The rise time, tr, is the time required for the response torise from ten to ninety percent of the final value.

3. The settling time, ts, is the time required for the responseto reach and stay within two percent of the final value.

A critically damped control system provides optimum perfor-mance. The step response of a critically damped control sys-tem exhibits the minimum possible rise time that maintainszero overshoot and zero ringing (damped oscillations). Fig-ure 17 illustrates the step response of a critically dampedcontrol system.

INTRODUCTION

The LM628 is a digital PID controller. Theloop-compensation filter of a PID controller is usually tunedexperimentally, especially if the system dynamics are notwell known or defined.

The ultimate goal of tuning the PID filter is to critically dampthe motor control system — provide optimum tracking andsettling time.

As shown in Figure 7, the response of the PID filter is thesum of three terms, a proportional term, an integral term, anda derivative term. Five variables shape this response. Thesefive variables include the three gain coefficients (kp, ki, andkd), the integration limit coefficient (il), and the derivativesampling coefficient (ds). Tuning the filter equates to deter-mining values for these variable coefficients, values that criti-cally damp the control system .

Filter coefficients are best determined with a two-step experi-mental approach. In the first step, the values of kp, ki, and kd

(along with il and ds) are systematically varied until reason-ably good response characteristics are obtained. Manualand visual methods are used to evaluate the effect of eachcoefficient on system behavior. In the second step, an oscil-loscope trace of the system step response provides detailedinformation on system damping, and the filter coefficients,determined in step one, are modified to critically damp thesystem.Note: In step one, adjustments to filter coefficient values are inherently

coarse, while in step two, adjustments are inherently fine. Due to thiscoarse/fine nature, steps one and two complement each other, and thetwo-step approach is presented as the “best” tuning method. The PIDfilter can be tuned with either step one or step two alone.

STEP ONE — MANUAL VISUAL METHOD

Introduction

In the first step, the values of kp, ki, and kd (along with il andds) are systematically varied until reasonably good responsecharacteristics are obtained. Manual and visual methods areused to evaluate the effect of each coefficient on system be-havior.Note: The next four numbered sections are ordered steps to tuning the PID

filter.

1. Prepare the System

The initialization section of the filter tuning program is ex-ecuted to prepare the system for filter tuning. See Table 11.This section initializes the system, presets the filter param-eters (kp, ki, il = 0, kd = 2, ds = 1), and commands the controlloop to hold the shaft at the current position.

After executing the initialization section of the filter tuningprogram, both desired and actual shaft positions equal zero;the shaft should be stationary. Any displacement of the shaftconstitutes a position error, but with both kp and ki set tozero, the control loop can not correct this error.

2. Determine the Derivative Gain Coefficient

The filter derivative term provides damping to eliminate oscil-lation and minimize overshoot and ringing, stabilize the sys-tem. Damping is provided as a force proportional to the rateof change of position error, and the constant of proportional-ity is kd x ds. See Figure 18.

Coefficients kd and ds are determined with an iterative pro-cess. Coefficient kd is systematically increased until the shaftbegins high frequency oscillations. Coefficient ds is then in-creased by one. The entire process is repeated until ds

reaches a value appropriate for the system.

The system sample period sets the time interval between up-dates of position error. The derivative sampling interval is an

AN010860-17

FIGURE 16. Unit Step Response Curve ShowingTransient Response Attributes

AN010860-18

FIGURE 17. Unit Step Response of aCritically Damped System

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integer multiple of the system sample period. See Table 3. Itsets the time interval between successive position errorsamples used in the derivative term, and, therefore, directlyaffects system damping. The derivative sampling intervalshould be five to ten times smaller than the system mechani-cal time constant — this means many systems will requirelow ds. In general, however, kd and ds should be set to givethe largest kd x ds product that maintains acceptably low mo-tor vibrations.Note: Starting kd at two and doubling it is a good method of increasing kd.

Manually turning the shaft reveals that with each increase of kd, the re-sistance of the shaft to turning increases. The shaft feels increasinglysluggish and, because kd provides a force proportional to the rate ofchange of position error, the faster the shaft is turned the more slug-gish it feels. For the reference system, the final values of kd and ds are4000 and 4 respectively.

TABLE 11. Initialization Section — Filter TuningProgram


c 00 RESET See Initialization ModuleText


wait The maximum time tocomplete RESET tasksis 1.5 ms.

c 06 PORT12 The RESET default sizeof the DAC port is eightbits. This commandinitializes the DAC portfor a 12-bit DAC. Itshould not be issued insystems with an 8-bitDAC.


c 1D RSTI This command resetsonly the interruptsindicated by zeros inbits one through six ofthe next data word. Italso resets bit fifteen ofthe Signals Registerand the host interruptpin (pin 17).



d 00 LB Zeros in bits onethrough six indicate allinterrupts will be reset.


c 1C MSKI This command masksthe interrupts indicatedby zeros in bits onethrough six of the nextdata word.



d 04 LB A 04 hex LB enables(unmasks) the trajectorycomplete interrupt. Allother interrupts aredisabled (masked). SeeFigure 6.


c 1E LFIL This command initiatesloading the filtercoefficients inputbuffers.

Proportional Term

AN010860-19

Integral Term

AN010860-20

Derivative Term

AN010860-21

FIGURE 18. Proportional, Integral, and Derivative (PID)Force Components

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TABLE 11. Initialization Section — Filter TuningProgram (Continued)



dd

00x2

HBLB

These two bytes are thefilter control word. A 00hex HB sets thederivative samplinginterval to 2048/fCLK bysetting ds to one. A x2hex LB indicates only kd

will be loaded. Theother filter parameterswill remain at zero, theirreset default value.


dd

0002

HBLB

These two bytes set kd

to two.


c 04 UDF This command transfersnew filter coefficientsfrom input buffers toworking registers. UntilUDF is executed,coefficients loaded viathe LFIL command donot affect the filtertransfer characteristic.




dd

0000

HBLB

These two bytes are thetrajectory control word.A 00 hex LB indicatesno trajectory parameterswill be loaded.



3. Determine the Proportional Gain Coefficient

Inertial loading causes following (or tracking) error, positionerror associated with a moving shaft. External disturbancesand torque loading cause displacement error, position errorassociated with a stationary shaft. The filter proportionalterm provides a restoring force to minimize these position er-rors. The restoring force is proportional to the position errorand increases linearly as the position error increases. SeeFigure 18. The proportional gain coefficient, kp, is the con-stant of proportionality.

Coefficient kp is determined with an iterative process — thevalue of kp is increased, and the system damping is evalu-ated. This is repeated until the system is critically damped.

System damping is evaluated manually. Manually turning theshaft reveals each increase of kp increases the shaft “stiff-ness”. The shaft feels spring loaded, and if forced away fromits desired holding position and released, the shaft “springs”back. If kp is too low, the system is over damped, and theshaft recovers too slowly. If kp is too large, the system is un-der damped, and the shaft recovers too quickly. This causesovershoot, ringing, and possibly oscillation. The proportionalgain coefficient, kp, is increased to the largest value thatdoes not cause excessive overshoot or ringing. At this pointthe system is critically damped, and therefore provides opti-mum tracking and settling time.Note: Starting kp at two and doubling it at each iteration is a good method of

increasing kp. The final value of kp for the reference system is 40.

4. Determine the Integral Gain Coefficient

The filter proportional term minimizes the errors due to iner-tial and torque loading. The integral term, however, providesa corrective force that can eliminate following error while theshaft is spinning and the deflection effects of a static torqueload while the shaft is stationary. This corrective force is pro-portional to the position error and increases linearly withtime. See Figure 18. The integral gain coefficient, ki, is theconstant of proportionality.

High values of ki provide quick torque compensation, but in-crease overshoot and ringing. In general, ki should be set tothe smallest value that provides the appropriate compromisebetween three system characteristics: overshoot, settlingtime, and time to cancel the effects of a static torque load. Insystems without significant static torque loading, a ki of zeromay be appropriate.

The corrective force provided by the integral term increaseslinearly with time. The integration limit coefficient, il, acts as aclamping value on this force to prevent integral wind-up, abacklash effect. As noted in Figure 18, il limits the summationof error (over time), not the product of ki and this summation.In many systems il can be set to its maximum value, 7FFFhex, without any adverse effects. The integral term has noeffect if il is set to zero.

For the test system, the final values of ki and il are 5 and1000 respectively.

STEP TWO — STEP RESPONSE METHOD

Introduction

The step response of a control system reveals important in-formation about the “quality” of control — specifically, de-tailed information on system damping.

In the second step to tuning the PID filter, an oscilloscopetrace of the control system step response is used to accu-rately evaluate system damping, and the filter coefficients,determined in step one, are fine tuned to critically damp thesystem.

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Software Considerations

The step generation section of the filter tuning program pro-vides the control loop with a repetitive small-signal step in-put. This is accomplished by repeatedly executing a smallposition move with high maximum velocity and high accel-eration. See Figure 19 and Table 12.

AN010860-22

FIGURE 19. Step Generation Section of Filter TuningProgram

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TABLE 12. Step Generation Section — Filter TuningProgram




dd

002B

HBLB

These two bytes are thetrajectory control word.A 2B hex LB indicatesacceleration, velocity,and position will beloaded and bothacceleration andvelocity are absolutewhile position is relative.


dd

0004

HBLB

Acceleration is loaded intwo data words. Thesetwo bytes are the highdata word.


d 93 HB acceleration data word(low)

d E0 LB


dd

0007

HBLB

Velocity is loaded in twodata words. These twobytes are the high dataword.


d A1 HB velocity data word (low)

d 20 LB


dd

0000

HBLB

Position is loaded in twodata words. These twobytes are the high dataword.


d 00 HB position data word (low)

d C8 LB





c xx RDSTAT This command readsthe Status Byte. It isdirectly supported byLM628 hardware andcan be executed at anytime by pulling CS, PS,and RD logic low.Status informationremains valid as long asRD is logic low.

decision If the TrajectoryComplete interrupt bit isset, continue. Otherwiseloop back to RDSTAT.

c 1D RSTI This command resetsonly the interruptsindicated by zeros inbits one through six ofthe next data word. Italso resets bit fifteen ofthe Signals Registerand the host interruptpin (pin 17).


d 00 LB Zeros in bits onethrough six indicate allinterrupts will be reset.

wait This wait block inserts adelay betweenrepetitions of the stepinput. The delay isapplication specific, buta good range of valuesfor the delay is 5 ms to5000 ms.

loop Loop back to STT.

Hardware Considerations

For a motor control system, an oscilloscope trace of the sys-tem step response is a graph of the real position of the shaftversus time after a small and instantaneous change in de-sired position.

For an LM628-based system, no extra hardware is neededto view the system step response. During a step, the voltageacross the motor represents the system step response, andan oscilloscope is used to generate a graph of this response(voltage).

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For an LM629-based system, extra hardware is needed toview the system step response. Figure 20 illustrates a circuitfor this purpose. During a step, the voltage output of this cir-cuit represents the system step response, and an oscillo-scope is used to generate a graph of this response.

The oscilloscope trigger signal, a rectangular pulse train, istaken from the host interrupt output pin (pin 17) of theLM628/LM629. This signal is generated by the combinationof a trajectory complete interrupt and a reset interrupts(RSTI) command. See Figure 19.Note: The circuit of Figure 20 can be used to view the step response of an

LM628-based system.

Observations

What follows are example oscilloscope traces of the step re-sponse of the reference system.Note 8: All traces were generated using the circuit of Figure 20.

Note 9: All traces were generated using the following “step” trajectory pa-rameters: relative position, 200 counts; absolute velocity, 500,000 counts/sample; acceleration, 300,000 counts/sample/sample. These values gener-ated a good small-signal step input for the reference system; other systemswill require different trajectory parameters. In general, step trajectory param-eters consist of a small relative position, a high velocity, and a high accelera-tion.

The position parameter must be relative. Otherwise, a define home command(DFH) must be added to the main loop of the step generation section — filtertuning program. See Figure 19.

The circuit for viewing the system step response uses an 8-bitanalog-to-digital converter. See Figure 20. To prevent converter overflow, thestep position parameter must not be set higher than 200 counts.

Note 10: The circuit of Figure 20 produces an “inverted” step responsegraph. The oscilloscope input was inverted to produce a positive-going (morefamiliar) step response graph.

Figure 21 represents the step response of an under dampedcontrol system; this response exhibits excessive overshootand long settling time. The filter parameters used to gener-ate this response were as follows: kp, 35; ki, 5; kd, 600; ds, 4;il, 1000. Figure 21 indicates the need to increase kd, the de-rivative gain coefficient.

Figure 22 represents the step response of an over dampedcontrol system; this response exhibits excessive rise timewhich indicates a sluggish system. The filter parametersused to generate this response were as follows: kp, 35; ki, 5;kd, 10,000; ds, 7; il, 1000. Figure 22 indicates the need to de-crease kd and ds.

Figure 23 represents the step response of a criticallydamped control system; this response exhibits virtually zeroovershoot and short rise time. The filter parameters used togenerate this response were as follows: kp, 40; ki, 5; kd,4000; ds, 4; il, 1000.

AN010860-23

FIGURE 20. Circuit for Viewing the System Step Response with an Oscilloscope

AN010860-24

FIGURE 21. The Step Response of an Under Damped Control System

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NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DE-VICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMI-CONDUCTOR CORPORATION. As used herein:1. Life support devices or systems are devices or sys-

tems which, (a) are intended for surgical implant intothe body, or (b) support or sustain life, and whose fail-ure to perform when properly used in accordancewith instructions for use provided in the labeling, canbe reasonably expected to result in a significant injuryto the user.

2. A critical component is any component of a life supportdevice or system whose failure to perform can be rea-sonably expected to cause the failure of the life supportdevice or system, or to affect its safety or effectiveness.

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AN010860-25

FIGURE 22. The Step Response of an Over Damped Control System

AN010860-26

FIGURE 23. The Step Response of a Critically Damped Control System

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