1. September 2004 slide 1 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors AE4-S02 Spacecraft Mechatronics Displacement Sensors Dr. ir. W. Jongkind TU-Delft 2004 – 2005

2. September 2004 slide 2 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Introduction (1) w 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. w 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.

3. September 2004 slide 3 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Introduction (2) w 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 w 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

4. September 2004 slide 4 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Making the Right Choice w The following requirements and constraint should be addressed: – Required resolution – Required accuracy – Environmental constraints – Integration aspects – Availability w 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

5. September 2004 slide 5 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Potentiometer (1) w Operating range from 1 mm to 1m

6. September 2004 slide 6 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Potentiometer (2)

7. September 2004 slide 7 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Encoders w Typically used as shaft angle encoders w 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 w Consist of a pattern impressed upon a part of the system that characterizes the motion w Two main classes of optical encoders: – Absolute encoders and – Incremental encoders – For velocity measurement nearly always incremental encoders are applied

8. September 2004 slide 8 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Incremental Encoder

9. September 2004 slide 9 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Readout System

10. September 2004 slide 10 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Sense of Rotation

11. September 2004 slide 11 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Linear Encoder Codes

12. September 2004 slide 12 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Circular Encoder Codes

13. September 2004 slide 13 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Gray Encoder Output

14. September 2004 slide 14 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Encoder Construction

15. September 2004 slide 15 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Incremental Encoder Construction

16. September 2004 slide 16 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Gray EncoderConstruction

17. September 2004 slide 17 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Incremental Encoder

18. September 2004 slide 18 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Transformer Based Displacement Sensors w Linear Variable Differential Transformer w Synchro’s and Resolvers

19. September 2004 slide 19 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Linear Variable Differential Transformer

20. September 2004 slide 20 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Coil Voltages

21. September 2004 slide 21 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Coils in Series Opposition

22. September 2004 slide 22 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors LVDT Animation

23. September 2004 slide 23 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Coil Voltages

24. September 2004 slide 24 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Amplitude versus Displacement

25. September 2004 slide 25 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Output of an LVDT w Starting at the primary or excitation side of the LVDT:

26. September 2004 slide 26 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Signal conditioning Scheme

27. September 2004 slide 27 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Signal Conditioning of a LVDT (1) w Location of the displacement transducer coil – consider the phase of the output – as well as the magnitude w 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

28. September 2004 slide 28 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Signal Conditioning of a LVDT (2) w This type of signal conditioning systems is available in IC form. w The Analog Devices type AD 598 IC uses this technique.

29. September 2004 slide 29 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Principle of a Resolver

30. September 2004 slide 30 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Resolver Operation Principle (1) w The rotor of the resolver is exited by an AC reference voltage of 400 Hz typically w As the rotor turns and the stator remains static an angular difference in orientation between rotor and stator develops: – v a =Kv exite sinωt sinθ – v b =Kv exite sinωt cosθ w The shaft angle θ is obtained by first multiplying the original output signals by cosφ and sinφ respectively

31. September 2004 slide 31 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Resolver Detector System

32. September 2004 slide 32 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Resolver Operation Principle (2) w K v sinωt sinθ cosφ and K v sinωt cosθ cosφ w Subtracting gives: K v sinωt sin(θ-φ) w (θ-φ) is the angular error w Demodulated gives: A sin(θ-φ) w Signal to a phase sensitive detector followed by an integrator and a Voltage Controlled Oscillator (VCO) w Detection of angle θ is based on nulling the error angle (θ-φ)

33. September 2004 slide 33 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Resolver Detector System

34. September 2004 slide 34 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Resolver Operation Principle (3) w The VCO controls an up/down counter containing the digital equivalent of angle θ w The whole manipulation is performed in closed loop fashion w Since the difference between θ and φ is nulled, the up/down counter supplies θ w In practice all calculations are performed on--chip such as the AD2S90 from Analog Devices

35. September 2004 slide 35 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Resolver Detector System

36. September 2004 slide 36 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Excitation and Read-Out Chipset

37. September 2004 slide 37 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Principle of a Synchro

38. September 2004 slide 38 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Application of Displacement Sensors (1) w 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 w 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

39. September 2004 slide 39 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors LASCO

40. September 2004 slide 40 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors SOHO-LASCO Experiment

41. September 2004 slide 41 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors SOHO-LASCO Experiment

42. September 2004 slide 42 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors SOHO LASCO LVDT Characteristics Stroke in micrometers

43. September 2004 slide 43 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Application of Displacement Sensors (2) w 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

44. September 2004 slide 44 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Application of Displacement Sensors (3) w 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

45. September 2004 slide 45 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Application of Displacement Sensors (4) – 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 360 0 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

46. September 2004 slide 46 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Encoder

47. September 2004 slide 47 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Application of Displacement Sensors (5) w 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

48. September 2004 slide 48 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Resolver

49. September 2004 slide 49 of 49 Dr. ir. W. Jongkind AE4-S02 Spacecraft Mechatronics Displacement Sensors Rotasyn Resolver