september 2004 slide 1 of 49 dr. ir. w. jongkind ae4-s02 spacecraft mechatronics displacement...

September 2004slide 1 of 49

Dr. ir. W. JongkindAE4-S02 Spacecraft Mechatronics

Displacement Sensors

AE4-S02 Spacecraft MechatronicsDisplacement Sensors

Dr. ir. W. Jongkind

TU-Delft2004 – 2005




Introduction (1)

Potentiometers. Linear as well as rotary potentiometer are applied.

– Potentiometers are used in situations where accuracy is not of major importance,

– Accuracies may vary from 0.3 % to 5 %. – Device is normally cheap.

Incremental Encoders.Linear and rotary incremental encoders are applied.

– Rather inexpensive devices.– The performance depends on the resolution of the encoder slit pattern. – They can can be very accurate indeed.




Introduction (2)

Absolute Encoders. Again linear and rotary absolute encoders exist

– The code pattern is in the majority of cases a Gray code pattern, binary code patterns are much less common

Electrical Transformers– For very accurate linear displacement measurements often Linear

Variable Differential Transformers (LVDT)are applied– The rotary displacement can be accurately measured with rotary electrical

transformer devices such as the Resolver or Synchro




Making the Right Choice

The following requirements and constraint should be addressed:

– Required resolution– Required accuracy– Environmental constraints– Integration aspects– Availability

The selected displacement sensor is the most important factor and deciding for overall performance of a system especially when the device forms part of a feedback loop




Potentiometer (1)

Operating range from 1 mm to 1m

xKxL

VV pout




Potentiometer (2)

pout KV

V max2

2




Encoders

Typically used as shaft angle encoders

The device output is in digital form– Digital output needs in general to be transformed with the aid of a

computing device to obtain magnitude and direction of movement as well as position or angle information

Consist of a pattern impressed upon a part of the system that characterizes the motion

Two main classes of optical encoders:– Absolute encoders and– Incremental encoders– For velocity measurement nearly always incremental encoders are

applied




Incremental Encoder




Readout System




Sense of Rotation




Linear Encoder Codes




Circular Encoder Codes




Gray Encoder Output




Encoder Construction




Incremental Encoder Construction




Gray EncoderConstruction




Transformer Based Displacement Sensors

Linear Variable Differential Transformer

Synchro’s and Resolvers




Linear Variable Differential Transformer




Coil Voltages




Coils in Series Opposition




LVDT Animation




Coil Voltages




Amplitude versus Displacement




Output of an LVDT

Starting at the primary or excitation side of the LVDT:

kernel. theofnt displacemeith linearly w changes quantity The

)(

:are coilssecondary at the voltagesinduced The

21

2121

22

11

-MMdt

diMMvv

dt

diMv

dt

diMv

dt

diLRi v

pss

ps

ps

ppppp




Signal conditioning Scheme




Signal Conditioning of a LVDT (1)

Location of the displacement transducer coil– consider the phase of the output– as well as the magnitude

The output phase of the position sensor is compared with the excitation phase and it can be:

– In or out of phase with the excitation, depending upon which half of the coil the center of the armature is in




Signal Conditioning of a LVDT (2)

This type of signal conditioning systems is available in IC form.

The Analog Devices type AD 598 IC uses this technique.

21

210

ss

ss

vv

vvAv




Principle of a Resolver




Resolver Operation Principle (1)

The rotor of the resolver is exited by an AC reference voltage of 400 Hz typically

As the rotor turns and the stator remains static an angular difference in orientation between rotor and stator develops:

– va=Kvexitesinωt sinθ– vb=Kvexitesinωt cosθ

The shaft angle θ is obtained by first multiplying the original output signals by cosφ and sinφ respectively




Resolver Detector System





Kvsinωt sinθ cosφ and Kvsinωt cosθ cosφ

Subtracting gives: Kvsinωt sin(θ-φ)

(θ-φ) is the angular error

Demodulated gives: A sin(θ-φ)

Signal to a phase sensitive detector followed by an integrator and a Voltage Controlled Oscillator (VCO)

Detection of angle θ is based on nulling the error angle (θ-φ)





The VCO controls an up/down counter containing the digital equivalent of angle θ

The whole manipulation is performed in closed loop fashion

Since the difference between θ and φ is nulled, the up/down counter supplies θ

In practice all calculations are performed on--chip such as the AD2S90 from Analog Devices




Excitation and Read-Out Chipset




Principle of a Synchro




Application of Displacement Sensors (1)

Application of LVDT's– On SOHO the LASCO experiment is flown– The device requires very accurate positioning measurement. This

accurate position measurement is obtained by applying LVDT's– The LVDT's were able to measure with a resolution of 0.01 μm over a

range of 30 m

An other experiment on board of SOHO, the SUMER EUV Spectrometer also applies a LVDT for linear position measurement. This sensor has a resolution of 12 bits




LASCO




SOHO-LASCO Experiment




SOHO LASCO LVDT Characteristics

Stroke in micrometers





Application of Resistive Encoders– The CAPS instrument on the Cassini spacecraft is equipped with a resistive

encoder to measure the angular position of the instrument





Application of Optical Encoders:– In the pointing and scanning mechanism for ATLID and SPOT5 an optical

encoder is applied to obtained angular position. The resolution is 21 bits, its static accuracy is 15 μ rad and its bandwidth is a few hundred Hz

– For angular position measurement of a tether reel use of an incremental encoder generates 4000 pulses per revolution on two channels in quadrature.

– In the scan mechanism for the Master Limb Sounding Instrument it is proposed to measure the angle of the elevation axis with an incremental encoder





– Scanning mechanism of the MIPAS Interferometer on board ENVISAT an incremental encoder is used. The outer diameter of the encoder is 182 mm containing 18000 equally spaced lines. The accuracy is 1 arcsec

– An 21 bits optical encoder is applied in a pointing mechanism for a Earth Observation Satellite. The resolution is 3 μrad, precision over a range of 3600 is better than 15 μrad

– The scan mechanism of the Atmospheric Lidar Instrument makes use of an encoder

– In the scanning mechanism for SPOT5 a high resolution pointing and scanning mechanism was required. The encoder applied had 21 bit resolution




Encoder





Resolver Applications– The Infrared Atmospheric Sounder Interferometer contains the IASI

instrument. Part of the instrument is a scan sub-system for a mirror. The shaft angle is measured by a Resolver

– The resolver assembly accuracy is 10-4 rad– The Global Ozone Scan Monitoring Experiment also contains a resolver

– Accuracy is 10 arcsec– European Robotic Arm (ERA) is equipped with a 6040 Rotasyn




Resolver




Rotasyn Resolver

september 2004 slide 1 of 49 dr. ir. w. jongkind ae4-s02 spacecraft mechatronics displacement...

Documents

synchro slide

rotary displacement

w incremental encoders

w absolute encoders

feedback loop slide

rotary incremental encoders

w potentiometers

motion w