Part 3 – Position control
Transcript of 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
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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.
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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
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Position measurement Magnetic technology � Principle
� The counting of the sinusoidal periods yields the relative displacement
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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.
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Position measurement Synchro-resolver
� Principle
� Rotating transformer principle. � The rotor is excited by a sinusoidal time-
varying magnetic field at a constant high frequency.
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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
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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.
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Position measurement Optical technologies � Advantages
� Robustness � Cost � Immunity to electromagnetic disturbances � High resolution � Absolute or relative measurement
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Position measurement Optical technologies : relative sensors
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t
A
t
B
t
A
B
+90°
!90°
t
Positive direction
Negative direction
Position measurement Optical technologies : relative sensors
� Quad precision
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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.
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Position measurement Optical technologies : relative sensors
� Interpolation
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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
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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 ?
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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
Position measurement Optical technologies : relative sensors
� Frequency vs periods per revolution :
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Position measurement Optical technologies : absolute sensors
� Position is encoded on the wheel mask. � Use of multiple tracks.
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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.
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Position measurement Optical technologies : examples
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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.
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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.
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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.
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Position measurement Optical technologies : hybrid encoder PC interface
� Hybrid solution : example, Heindehain.
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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.
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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.
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Position control General purpose position controllers � Pluggable board. � I/Os and basic position control functions. � Embedded OS. � Example : PMAC board.
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Position control Distributed systems
� Access to the hardware through fieldbus.
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Position control Supervised systems
� The controllers are interconnected with a network.
� Supervision can monitor the whole system.
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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
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