Part 3 – Position control

Control systems technology University of Strasbourg Telecom Physique Strasbourg, ISAV option Master IRIV, AR track Part 3 – Position control

Outline � Position measurement

�  Technologies �  Absolute/relative measurement �  Measurement transmission �  Interface

� Position control �  Dedicated system �  Embedded system �  Distributed system �  Supervised system

� Example 06/12/12 [email protected] 2

Position measurement Resistive technology � Principle

�  Variable resistor connected to the system. �  The output voltage is an image of the position. �  Analog measurement.

�  A is the full displacement. Can be a distance or an angle.

06/12/12 [email protected] 3

Vs

Vcc

x

x = A

Vs

Vcc

Position measurement Resistive technology � Advantages

�  Cost �  Size �  Simplicity �  Absolute measurement

� Drawbacks �  Lack of robustness �  Accuracy (nonlinearities) �  Resolution �  Noise �  Maximal velocity


Position measurement Magnetic technology � Principle

� The counting of the sinusoidal periods yields the relative displacement


Position measurement Magnetic technology � Advantages

�  No friction yields better robustness. �  Suited to high velocities.

� Drawbacks �  Limited number of pole pairs : limited resolution. �  Voltage tends toward zero with velocity : difficult to

measure at low speed.


Position measurement Synchro-resolver

� Principle

� Rotating transformer principle. � The rotor is excited by a sinusoidal time-

varying magnetic field at a constant high frequency.


Position measurement Synchro-resolver � Advantages

�  Robustness. �  The output magnitude does not depend on the

velocity. �  The use of 2 secondary windings allows for

direction sensing. �  Interpolation can enhance resolution.

� Drawbacks �  Only one period per turn �  Maximal velocity is limited by the transformer

frequency


Position measurement Optical technologies � Principle

� A mask is inserted between a light source and a light sensor.

� Displacements yield time-varying illumination at the sensor side.


Position measurement Optical technologies � Advantages

�  Robustness �  Cost �  Immunity to electromagnetic disturbances �  High resolution �  Absolute or relative measurement


Position measurement Optical technologies : relative sensors


t

A

t

B

t

A

B

+90°

!90°

t

Positive direction

Negative direction


� Quad precision



� Notes �  500 points per turn yield 2000 impulses per turn in

quad-precision mode. �  A third channel called “Z” or “I” or “C” gives one

pulse per turn. It can be used for calibration purpose.

�  Channels “A”, “B” and “C” can be completed be complemented channels “A/”, “B/” or “C/” for transmission robustness purpose.

�  For technological reasons, the number of points per turn is limited to approximately 5000.



�  Interpolation



� Example �  The sampling period of a position loop is 500us. We

use a 512 ppt encoder. Find the minimal velocity when counting with quad-precision. Same question with an interpolation of 512.

� Without interpolation


At least one increment within one period. Thus, at least 2000 increments per second. One turn yields 512x4 pulses. Thus the minimal velocity is 2000/(512x4) turns per second which is approx. 60 rpm.


� With interpolation

�  What is the maximal velocity considering that the maximal frequency for channels “A” and “B” is 100 kHz ?


At least one increment within one period. Thus, at least 2000 increments per second. One turn yields 5122 pulses. Thus the minimal velocity is 2000/5122 turns per second which is approx. 0.46 rpm.

In both cases, the channels frequency is 512xn/60 with n the rpm of the shaft. Thus nmax=100000x60/512=11719 rpm


�  Frequency vs periods per revolution :


Position measurement Optical technologies : absolute sensors

� Position is encoded on the wheel mask. � Use of multiple tracks.


Position measurement Optical technologies : relative vs absolute sensors

� Relative sensors �  High resolution through interpolation �  Need an initialization. �  Missed impulses yields on offset.

� Absolute sensors �  Position is encoded usually using a binary code. �  Position is known at startup. �  Resolution is lower than with relative sensors. �  Usually used in combination with relative sensors.

Absolute measurement at the load side and relative measurement at the motor side.


Position measurement Optical technologies : examples


Heidenhain

Heidenhain

Heidenhain

US digital Agilent : HEDL 5540

Position measurement Optical technologies : connection

�  Incremental encoder �  Square or sine signals directly sent to the interface. �  Max distance : 100 m �  Max frequency : 1 MHz �  Redundancy of information to enhance robustness.

� Absolute encoder �  Serial link (SSI standard). �  4 wires. �  Up to 10 Mbps. �  Up to 1200 m.


Position measurement Optical technologies : incremental encoder PC interface

� Without interpolation �  Counter with complementary channels pre-filtering. �  Initialization procedure.

� With interpolation �  Insertion of an interpolation electronic board.


Position measurement Optical technologies : absolute encoder PC interface

�  SSI interface : �  Synchronous Serial

Interface. �  Frequency is imposed

by the board. �  Compatible with a

wide range of encoders.


Position measurement Optical technologies : hybrid encoder PC interface

� Hybrid solution : example, Heindehain.


Position control Rapid prototyping � Reduce try and error cycle length. � Hardware in the loop. �  Synthesis of the control loop using bloc.

diagram representation. � Automatic code generation. � Live parameters tuning. � Example : dSPACE.


Position control Dedicated system � Dedicated system with custom IOs and

specific software. � Real-time dedicated OS. �  Software dedicated to application. � Examples :

�  Adept Motion bloxTM robot controller. �  Maxon EPOS motor drive.


Position control General purpose position controllers � Pluggable board. �  I/Os and basic position control functions. � Embedded OS. � Example : PMAC board.


Position control Distributed systems

� Access to the hardware through fieldbus.


Position control Distributed systems

�  Fieldbus comparison :


Position control Supervised systems

� The controllers are interconnected with a network.

�  Supervision can monitor the whole system.


Links �  http://www.profibus.com/

http://www.automation.siemens.com/ �  http://www.heidenhain.com/ �  http://www.renco.com/ : encoder manufacturer �  http://www.deltatau.com/ : embedded general

purpose position controller manufacturer �  http://www.maxon.ch/ : high quality medium power

motor and drive manufacturer �  http://www.micromotorisation.com/ : French

reseller of motors and drives


Part 3 – Position control

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