nippon pulse basic mechanics presentation

Basic Mechanics – Physics and Motors

ELECTROMATEToll Free Phone (877) SERVO98

Toll Free Fax (877) SERV099www.electromate.com

[email protected]

Sold & Serviced By:

Motion Control RequirementsMotion Control Applications must be:1. Predictable2. Verifiable 3. Controllable

Design Considerations:• Accuracy

– Stiffness– Environment



[email protected]

Sold & Serviced By:

Presenter

Presentation Notes

No matter what your motion control application it must be: Predictable Verifiable Controllable In order for this to happen with your application, you must keep the fallowing three things in mind: Accuracy Stiffness Vibration

Design Considerations

•Accuracy



[email protected]

Sold & Serviced By:

4

Accuracy Accuracy vs. Precision

• Accuracy doesn’t mean Precision

• Accuracy represents the ability to get closest to a defined point.

• Precision is the ability to execute a move and return to the same point (regardless of accuracy).



[email protected]

Sold & Serviced By:

Presenter

Presentation Notes

Accuracy vs. Precision Precision also called Repeatability Accuracy is not the same thing as precision. "Accuracy" represents the ability to get closest to the true point you are expecting. "Precision" measures the ability to repeat a move and get back to the same point (regardless of accuracy). “Precision” can be achieved regardless of Accuracy. I like to the following illustration. A watch dial is graduated in l/5th of a second intervals between each minute mark. Thus the watch is precise, but unbeknownst to the person using the watch to observe the time, the watch is five minutes slow. Reading a time to the nearest 1/5th second with this watch, while being precise, is ridiculous, because the time reading is five whole minutes away from the true time - the watch is inaccurate.

5

Accuracy Accuracy vs. Precision



[email protected]

Sold & Serviced By:

Presenter

Presentation Notes

Accuracy & Precision (Repeatability) Accuracy – Difference between expected position and achieved position. Precision (Repeatability) Bi-directional – The error from nominal when repeatedly approaching a position from opposite directions. Precision (Repeatability) Uni-directional – The error from nominal when repeatedly approaching a position from the same direction. Here you will see a graph that helps illustrate this.

Accuracy ResolutionWhile Resolution is not Accuracy…

It is part of what makes up Accuracy

Two parts of Resolution1. Electrical Resolution2. Mechanical Resolution



[email protected]

Sold & Serviced By:

Presenter

Presentation Notes

And then there’s resolution It can be divided into the fallowing points: Resolution is the fineness of position precision that is attainable by a motion system. The precision of repetitive positioning is dependent on the resolution of the linear encoder. In addition, it is also necessary to have sufficient machine rigidity. In the same way, the absolute positioning precision is also fundamentally dependent on the linear encoder. Resolution, Electrical – The smallest increment that can be “commanded” by a servo system (minimum programmable move increment). Precision of the feedback system (scale, reader head, & controller logic) sets this figure. Resolution, Mechanical – The smallest increment that can be “controlled” by a motion system (minimum actual mechanical move increment). Mechanical precision is often coarser than electronic resolution due to: friction; station; deflections; etc.


•Accuracy– Stiffness



[email protected]

Sold & Serviced By:

What Exactly is Stiffness?SE

RVO

STIFFNESS

MOTOR

MECHANICALELECTROMATE

Toll Free Phone (877) SERVO98Toll Free Fax (877) SERV099

[email protected]

Sold & Serviced By:

Presenter

Presentation Notes

Stiffness Mechanical Servo Motor

StiffnessSE

RVO

STIFFNESS

MOTOR

MECHANICALELECTROMATE


[email protected]

Sold & Serviced By:

Presenter

Presentation Notes

Motor

Stiffness Motor

• Motor Stiffness– Mechanical stiffness

– Ability to respond to commanded motion

– Flux pattern (Magnetic Servo)



[email protected]

Sold & Serviced By:

Presenter

Presentation Notes

Mechanical stiffness Motor built-in stiffness - The epoxy structure of an ironless motor has a low inherent stiffness. The motor rigidity is given by the copper coils that are inserted and is dependent on how the coils are physically put together: separated or overlapped. An overlapping configuration would lead to a higher bending stiffness compared to a construction with independent coils whereas the lateral stiffness will be almost identical in both overlapping and separated configuration. The steel structure of an iron core motor make it obviously much stiffer than an ironless solution. The Cylindrical design of the Linear Shaft Motor is makes it the much stiffer than an ironless or iron core solution.. Ability to respond to commanded motion Flux pattern (Magnetic Servo)

Stiffness Mounting / Drive location



[email protected]

Sold & Serviced By:

Presenter

Presentation Notes

Mounting stiffness - By design, the mounting surface of an ironless motor (for example) corresponds generally from 10% to 25% of the active surface of the motor. The attachment surface is located on one side of the motor. Besides allowing almost no thermal conduction, this kind of mounting can be problematic when high position stability or very tight settling times are required. The main reason is that the current that is flowing in the motor phases can excite the transversal natural frequency of the motor, leading to oscillations once in position.

Stiffness Mounting / Drive location



[email protected]

Sold & Serviced By:

Presenter

Presentation Notes

In an iron core and Shaft motor, the mounting surface is generally 100% of the active surface of the motor and centered above the motor. The stiffness of this mounting is therefore much higher. As long as the carriage is stiff enough to withstand the attraction forces, no vibration problem should be foreseen. The Linear Shaft Motor does not have this problem.


•Accuracy–Stiffness–Environment



[email protected]

Sold & Serviced By:

Presenter

Presentation Notes

External forces

Environment Position Feedback Control Schemes



[email protected]

Sold & Serviced By:

Presenter

Presentation Notes

The fallowing sketches are presented to illustrate general concepts of common position feedback control schemes and should be viewed as a “food for thought” exercise. In most applications, work is being performed on or near the slide’s mounting surface, either by moving a target on the slide with respect to a fixed tool or by moving the tool on a slide with respect to a fixed target (something moving & something stationary). In either case, motion control should be near the slide. Cost vs. precision generally dictates the feedback method used for positioning systems. This figure illustrates an open-loop stepper-motor driven linear stage. With this approach, a control system sends a string of pulses to the motor which causes it to rotate. For example, 10,000 pulses to a 2,000 pulses/rev motor will cause it to rotate 5 times. Directly coupled to a 5-pitch lead screw, 10,000 pulses will cause the slide to move precisely 1.0 inch (maybe). “Precisely” 1.0 inch will only happen if the stepper motor rotates exactly 1,800° and the lead of the screw is “exactly 0.200” and there is no compliance (zero backlash) in the nut and the coupling is stiff and there is no lost motion caused by stiction in the bearing systems. All of these “If, ands, or buts” can be minimized by using very high priced precision components. Fortunately, there are less costly, alternative approaches to consider.




[email protected]

Sold & Serviced By:

Presenter

Presentation Notes

This figure replaces the stepper motor with a servo motor/rotary encoder combination. Now the rotation precision can be sensed and corrections made if necessary. All of the other “and” parameters listed above are still present though.




[email protected]

Sold & Serviced By:

Presenter

Presentation Notes

This figure replaces the motor-mounted rotary encoder with a linear encoder scale and a slide-mounted reader head. Though this bypasses all the potential error causing parameters mentioned above, position control is only good to the point of reader head and scale interfacing. Work is typically not performed at the encoder but somewhere above the slide’s surface. As shown in an earlier section, angular errors could adversely affect precision at the point of work.




[email protected]

Sold & Serviced By:

Presenter

Presentation Notes

This figure illustrates a method for bypassing error potentials all the way to the point of work. In this illustration, a laser provides position feedback “where” the work is being performed (at point A).




[email protected]

Sold & Serviced By:

Environment External forces

• Side Loading

• Motor resonance

• System harmonicsELECTROMATE


[email protected]

Sold & Serviced By:

Vibration – Nearby Conditions

• Other machinery

• Road traffic

• Building sway



[email protected]

Sold & Serviced By:

Review of physics• Newton’s law for translation:

F = m a F in Newtons, m in kg, a in m/s2.

• Acceleration a = dv / dt

• Kinetic energy E = ½ m v2

E in Joules, m in kg, v in m/s.ELECTROMATE


[email protected]

Sold & Serviced By:

Physics of translation• Momentum p = m v and so F = dp /

dt

• In the absence of force, momentum is conserved.

• Momentum conservation implies energy conservation.



[email protected]

Sold & Serviced By:

Physics of rotation• Rotation is more complex; Euler’s equation:

T = I α + ω x I ω T (torque) in N-m, ω in radians/sec, α in

radians/sec2, I in kg-m2, α = dω / dt

• I is a 3x3 matrix, not necessarily diagonal.

• If T = 0, then I α = - ω x I ω

which is usually non-zero. So α is non-zero, ω changes with time, and the object wobbles.



[email protected]

Sold & Serviced By:

Physics of rotation• Angular momentum is q = I ω

• The rotation equation simplifies to T = dq / dt because

dq/dt = I dω/dt + dI/dt ω = I α + ω x I ω

• So even though an object wobbles when there is no external force, the angular momentum is conserved: q = I ω



[email protected]

Sold & Serviced By:

Physics of rotation• Kinetic energy of rotation is ½ ωT I ω

• In the absence of external torque, kinetic energy of rotation is conserved.

• But angular momentum conservation does not imply energy conservation.



[email protected]

Sold & Serviced By:

Work

• Work done by a force = F x (Joules) where x is the distance (m) through which the force acts.

• Work done by a torque = T θ (Joules) ELECTROMATE


[email protected]

Sold & Serviced By:

Power• Power is rate of doing work.

• Power of a force = F v (Watts).

• Power of a torque = T ω (Watts).

• Power often expressed in horsepower = 746 Watts



[email protected]

Sold & Serviced By:

Motors• Motors come in several flavors:

– DC motors– Stepper motors– (AC) induction motors– (AC) Single-phase motors– (AC) Synchronous motors

• The first two are highly controllable, and usually what you would use in an application. But we quickly review the others.



[email protected]

Sold & Serviced By:

3-phase AC• Three or four wires that carry the same voltage at 3

equally-spaced phases:

• Single phase AC requires two wires (only 1/3 the current or power of 3-phase).



[email protected]

Sold & Serviced By:

AC induction Motors• Induction motors – simple, cheap, high-power, high

torque, simplest are 3-phase.

• Speed up to 7200 rpm: speed ~ 7200 / # “poles” of the motor.

• Induction motors are brushless (no contacts between moving and fixed parts). Hi reliability.

• Efficiency high: 50-95 %



[email protected]

Sold & Serviced By:

Single-phase AC Motors• Single-phase (induction) motors – operate from

normal AC current (one phase). Household appliances.

• Single-phase motors use a variety of tricks to start, then transition to induction motor behavior.

• Efficiency lower: 25-60%

• Often very low starting torque.



[email protected]

Sold & Serviced By:

Synchronous AC Motors• Designed to turn in synchronization

with the AC frequency. E.g. turntable motors.

• Low to very high power.

• Efficiency ??ELECTROMATE


[email protected]

Sold & Serviced By:

DC Motors• DC motor types:

– DC Brush motor– “DC” Brushless motor– Stepper motor



[email protected]

Sold & Serviced By:

DC Brush Motors• A “commutator” brings current to the

moving element (the rotor).

• As the rotor moves, the polarity changes, which keeps the magnets pulling the right way. DEMO

• Highly controllable, most common DC motor.



[email protected]

Sold & Serviced By:

http://www.microchip.com/1010/suppdoc/design/mtrcntrl/menufaq/mtrtypes/

DC Brush Motors• At fixed load, speed of rotation is

proportional to applied voltage. – Changing polarity reverses rotation.

• To first order, torque is proportional to current.

• Load curve:• Motors which

approximate this ideal well are called DC servo motors.



[email protected]

Sold & Serviced By:

DC Brushless Motors• Really an AC motor with electronic commutation.

• Permanent magnet rotor, stator coils are controlled by electronic switching. DEMO

• Speed can be controlled accurately by the electronics.

• Torque is often constant over the speed range.



[email protected]

Sold & Serviced By:

http://www.microchip.com/1010/suppdoc/design/mtrcntrl/menufaq/mtrtypes/

Stepper Motors• Sequence of (2 or

more) poles is activated in turn, moving the stator in small “steps”.

• Very low speed / high angular precision is possible without reduction gearing by using many rotor teeth.

• Can also “micro- step” by activating both coils at once.



[email protected]

Sold & Serviced By:

Driving Stepper Motors• Note: signals to the stepper motor

are binary, on-off values (not PWM).

• In principle easy: activate poles as A B C D A… or A D C B A…Steps are fixed size, so no need to sense the angle! (open loop control). ELECTROMATE


[email protected]

Sold & Serviced By:

Driving Stepper Motors• But in practice, acceleration and

possibly jerk must be bounded, otherwise motor will not keep up and will start missing steps (causing position errors).

• i.e. driver electronics must simulate inertia of the motor. ELECTROMATE


[email protected]

Sold & Serviced By:

Stepper Motor example• Step angle: 1.8°• Voltage: 3.2 V• Holding torque: 0.97 N-m• Rotor inertia: 250 g-cm2

• Weight: 1.32 lb (0.6 Kg.)• Length: 2.13" (54 mm)• Power output = 3W

• Precision stepper motor: 0.02°

/step, 1 rpm, 3W



[email protected]

Sold & Serviced By:

DC Motor example• V = 12 volts• Max Current = 4 A• Max Power Out = 25 W• Max efficiency = 74%• Max speed = 3500 rpm• Max torque = 1.4 N-m • Weight = 1.4 lbs• Forward or reverse (brushed)



[email protected]

Sold & Serviced By:

DC Motors – micro sizes• Conventional (brush)

DC motor: 6mm x 15mm• 13,000 rpm• 0.11 m Nm• Power 0.15 W• V from 1.5 to 4.5 V



[email protected]

Sold & Serviced By:

Brushless DC Motors • Brushless DC motor:

16mm x 28mm• 65,000 rpm• 50 m Nm• Power 11 W• V = 12 V



[email protected]

Sold & Serviced By:

DC Motors – gearing• Gearing allows you to trade off speed

vs. torque.

• An n:1 reduction gearing decreases speed by n, but increases torque by n.

• Ratios from 3:1 to many 1000s :1 are available in compact “gearheads” that attach to motors.



[email protected]

Sold & Serviced By:

DC Motors – gearing• But gears cost efficiency (20% - 50%)

• Gears decrease precision (due to backlash).

• Reduction gear train is normally not backdriveable (can’t use for “force control”).



[email protected]

Sold & Serviced By:

DC torque motors• Some high-end motors are available for direct

drive servo or force applications (no gears). • They have low speed (a few rpm), high

precision (with servo-ing), and moderate torque.

• Typically have large diameter vs. length, and use rare-earth magnetic material.

• Cost $100’s



[email protected]

Sold & Serviced By:

Feedback• Shaft encoders can be fitted to almost any DC

motor. They provide position sensing.

• Many motor families offer integrated encoders.

• Strain gauges can be used to sense force directly. Or DC brush motor current can be used to estimate force.



[email protected]

Sold & Serviced By:

Linear movement• There are several ways to produce linear

movement from rotation:• Rotary to linear gearing:



[email protected]

Sold & Serviced By:

Linear movement• Ball screws: low linear speed, good

precision• Motor drives shaft, stages move (must be

attached to linear bearing to stop from rotating).



[email protected]

Sold & Serviced By:

Linear movement• Belt drive: attach moving stage to a

toothed belt:

• Used in inkjet printers and some large XY robots.



[email protected]

Sold & Serviced By:

True Linear movement• There are some true linear magnetic drives. • BEI-Kimco voice coils:• Up to 1” travel• 100 lbf• > 10 g acceleration• 6 lbs weight• 500 Hz corner

frequency.

• Used for precision vibration control.ELECTROMATE


[email protected]

Sold & Serviced By:

Summary• AC motors are good for inexpensive high-

power applications where fine control isn’t needed.

• DC motors provide a range of performance:– DC brush: versatile, “servo” motor, high speed, torque– DC brushless: speed/toque depend on electronics– Stepper: simple control signals, variable speed/accuracy

without gearing, lower power– Direct-drive (torque) motors, expensive, lower torque

• Linear actuation via drives, or voice coils.ELECTROMATE


[email protected]

Sold & Serviced By:

nippon pulse basic mechanics presentation

Technology