Control systems technology

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

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.

V^s

V^cc x

x = A V_s V_cc

Position measurement

Resistive technology

—  Advantages

–  Cost

–  Size

–  Simplicity

–  Absolute measurement

—  Drawbacks

–  Lack of robustness

–  Accuracy (nonlinearities)

–  Resolution

–  Noise

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°

Positive direction

Negative direction

Position measurement

Optical technologies : relative sensors

—  Quad precision

Position measurement

Optical technologies : relative sensors

—  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.

Position measurement

Optical technologies : relative sensors

—  Interpolation

Position measurement

Optical technologies : relative sensors

—  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.

Position measurement

Optical technologies : relative sensors

—  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

n =100000x60/512=11719 rpm

Position measurement

Optical technologies : relative sensors

—  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

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 blox^TM 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