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National SemiconductorApplication Note 868Santanu RoyJanuary 1993

Interfacing the HPC andLM629 for Motion Control

INTRODUCTION

Control of servo motors has a wide range of applications

including industrial control factory automation position andvelocity servomechanism and robotics The basic tasks in-

volved in such motion controls include position velocityand acceleration measurement implementation of PID algo-

rithm detection of overrun and stress conditions and com-munication back to a central controller The HPC high per-formance microcontroller in conjunction with LM629 motion

controller chip provides a solution for handling these re-quired control tasks with extremely high degree of precision

and very little software overhead

The HPC is a highly integrated and high performance mem-ber of the HPCTM   family of National’s microcontroller The

availability of a wide variety of on-chip peripherals such ashigh speed timers high speed IOs input capture ADPWM UART and MICROWIRETM   provides a flexible archi-

tecture for a wide variety of high performance applicationsat a reasonable cost The LM629 is a National’s dedicated

motion-control processor designed for use with a variety of

DC and brushless DC servomotors which provide a quad-rature incremental position feedback signal for close-loop

operation The LM629 stand alone can perform all the inten-sive real-time computational tasks required for high per-

formance digital motion control As a result in a multi-tasksystem using the HPC where one of the tasks requires pre-cision servo control can be achieved with this chip-set with

minimal software overhead and very little CPU time

THE INTERFACE DESIGN

Figure 1   shows the interface between the HPC and the

LM629 via an 8-bit parallel bus As shown in the figure theHPC (host controller) interfaces with the LM629 data bus

(D0–D7) on the lower half of PORTA (MUXed addressdatabus) The HPC in this application is used in 8-bit Expanded-

Normal mode (strapping the HBE   pin to VCC EXM pin toground and EA bit in the PSW set to a one) The advantageof such a configuration is the upper half of the addressdata

bus (PORTA bits 815) will latch the address which is usedto generate the required chip select for the LM629 This

eliminates the requirement of an address latch for the de-coder logic

The delay and the logic associated with HPC write strobe

(WR) is added to increase the write-data hold time (asviewed from the LM629 end) effectively causing the WRpulse to rise early The LM629 clock is provided by the CK2

output of the HPC Since the LM629 has a maximum run-ning speed of 8 MHz the fastest HPC could run in such

applications will be 16 MHz The 74HC245 is used to de-

crease the read-data hold time) which is necessary wheninterfacing HPC with slower peripherals

TLDD11770–1

FIGURE 1 Interface between the HPC and LM629

HPCTM and MICROWIRETM are trademarks of National Semiconductor Corporation

C1995 National Semiconductor Corporation RRD-B30M75Printed in U S A

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The LM629 Host Interrupt (HI) output could be hardwired to

an input capture port (I2 in this case) or the NMI pin of theHPC for handshaking purposes

The Port Select (PS) input is hardwired to the pin B1 of theHPC for selecting command or data when communicating

with the LM629

The LM629 gets its RESET from the HPC which is low for aminimum of 8 CK2 periods to satisfy its reset requirement

USER-SYSTEM INTERFACE

Figure 2  shows a typical system implemented for handshak-ing between the user and the motion controller block In this

specific application this is developed using the HPC’s on-chip UART communicating at 4800 baud using a standard10 MHz HPC input clock (CKI) Any terminal (eg  a VT100)hooked to the HPC UART via the MAX232 should serve thepurpose The user can then input data required to control

the motor trajectory and tuning the PlD filter for precisionmotion control via this interface

TLDD11770–2

FIGURE 2 User-System Interface

PWM MOTOR DRIVE INTERFACE

Figure 3   shows an LMD 18200a 3A H-bridge driver is in-terfaced to the LM629 PWM outputs to provide power am-

plification for driving brushcommutator and brushless DCmotors The motor used in this application is a brush DC

motor with a HP HEDS 5300 incremental optical shaft en-coder with 2-channel (without the index pulse) and 500 cpsproviding TTL compatible digital output

TLDD11770–3

FIGURE 3 DC Motor Output Driver Stage

MOTION CONTROL SOFTWARE

Appended at the end of this application note is a flowchart(Figure 4)  for initiating a simple motor movement and a sam-

ple software (in ‘‘C’’) provided for the benefit of the readerNote that the requirements of individual routines are verymuch application specific and left for the user to develop as

required

TUNING THE PID

When connecting up a drive system for the first time there

could be a possibility that the loop phasing is incorrect Asthis may cause severe oscillation it is recommended to usea low value for the proportional gain say Kp   e 1 (with KdKi and il all set to zero) which will provide a weak level ofdrive to the motor If the system does oscillate with this Kp

value then the motor connections should be reversed

Having determined that the loop phasing is correct Kp cannow be increased to about 20 to see that the control system

basically works This value of Kp should hold the motorshaft reasonably stiffly returning the motor to the set posi-tion which will be zero until trajectory values have been

input and a position move performed If oscillation and un-acceptable ringing still occurs Kp should be reduced until it

stops Low values of acceleration and velocity can now beinput of around 100 and a position move commanded tosay 1000 counts All values suggested here are decimal

It is useful at this stage to try different values of acceleration

and velocity to get a feel for the system limitation Thesecan be determined by reporting desired and actual velocity

and acceleration to see that the error is not increasing with-out bound

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TLDD11770–4

FIGURE 4 Flowchart of a Simple Motor Move

CONCLUSION

The combination chip-set HPC and LM629 (also the LM628)form the core of a powerful solution to position servo prob-lems Commanded by the HPC microcontroller the most

powerful single-chip microcontroller available this uniquecombination is the key to a flexible and easy-to-implement

coordinated multi-axis motion system

REFERENCES

1 Special Purpose Linear Devices Data BookNSC

2 HP HEDS-5000 series Optical Encoder Data Book

3 Linear Data BookNSC

4 HPC User’s ManualNSC

5 Motion Control HandbookNSC

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