Digital Tachometer

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Digital Tachometer

• Participant Naveen K Boggarpu Place
  EPE-PEMC 2006,
  
  Portoroz, Slovenia.
• Supervisor Richard C. Kavanagh Date
  31/08/06

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Agenda

• Introduction
• Design Techniques
• Implementation
• Conclusion

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1.Introduction

• 1.1 Aim of the project
• To improve the performance of high-bandwidth
  servo systems using the Constant Sample-time
  Digital Tachometer (CSDT) method and particularly
  to design a new improved technique to compensate
  for high frequency sensor (encoder) errors.
• 1.2 Objectives
• On-line learning of sensor characteristics
  to enable use of low-cost encoders
• a. New techniques to obtain
  improved tachometer output
• b. Matlab model for selected
  technique
• c. Experimental implementation

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• 1.3 Background of the project
• Why not sensorless?
• a) Accurate trajectory following
  for very high bandwidth robotics
• b) NC machines
• c) Low speed machines
• Compensate errors due to misalignment of edges
• High-resolution encoders

Fig.1.1 Basic block diagram of a digital
tachometer
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2.Design Techniques
2.1 Speed measuring Techniques a)
Pulse counting technique b)
Elapsed time technique

• 2.2 Pulse counting method
• Time interval is fixed
• Effect of quantization at low shaft speed is main
  drawback
• Poor transient response and very poor resolution
  except at very high speeds
• Velocity resolution can be improved by
  quadrature decoding

Fig.2.1 Pulse counting technique
Where is fixed sample time, N is the pulse
count .
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• 2.3 Elapsed time method
• In this method the output from hardware is
  inversely proportional to speed
• Suitable for low shaft speeds
• Better transient response than pulse count
  method
• This method has poor resolution at high speeds
  and poor dynamic response at low speeds.

Fig2.2 Elapsed time technique
Where is the input frequency to the
counter, pulse counts of high freq between
two successive pulses
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• 2.5 CSDT Method
• CSDT stands for Constant Sample Time Digital
  Tachometer
• This method is an optimised version of pulse
  counting method with better accuracy and
  transient response CSDT method
• Greater speed range

Fig2.4 Original CSDT model
Where is N is pulse count for on sample period Ts
. In simplified CSDT method T1 is eliminated.
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• 2.5.1 Simplified CSDT
• T1(i) is eliminated in velocity calculation
• 2.5.2 CSDT at low speed
• At low speeds the lag increases and an additional
  observer is required for some closed loop
  applications
• Modified equation to calculate very low speeds
  is given below

Fig2.5 Simplified CSDT method
Fig2.6 Low speed CSDT method
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3.Implementation
Fig.3.1 Experimental setup

• Drive DS1104 is used to control the speed of the
  motor
• Encoder used is a three channel incremental
  encoder
• FPGA is used to calculate the auxiliary time and
  digital position from encoder input
• D-space is used to perform the high speed
  calculations

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• 4.1 FPGA
• FPGA is designed using Xilinx Project Manager
• Project is implemented using schematics design
  approach
• Auxiliary time and digital position are
  transferred through 8-bit databus
• 4.2 DSP
• DS1104 DSP chip is used
• Calculation of velocity and control is
  implemented using Controldesk software

Fig.3.2 FPGA design
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• 4.3 Simulation
• MATLAB is used as simulation tool
• Code is written to simulate the encoder output
  and calculate the velocity using CSDT method
• Compensation is implemented in simulation by
  creating a lookup table using digital position
  and auxiliary timing data

Fig 3.3 CSDT velocity before compensation
Fig 3.4 CSDT velocity after compensation
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Fig 3.6.1 Digital position of encoder (Top), Fig
3.6.2 Auxiliary time, Bottom (Middle), Fig 3.6.3
Velocity at every edge (Bottom)
Fig 3.5 Experimental setup

• 4.4 Online learning
• FPGA is calculating digital position and
  auxiliary time using the encoder output
• Digital position and the auxiliary time are
  shown in Fig 3.6.1 and Fig 3.6.2 respectively
• In Fig 3.6.3 the velocities at different edges
  are shown

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5.Conclusion

• CSDT method is selected as it has good transient
  response and better accuracy
• FPGA is used to calculation of auxiliary time and
  digital position
• Simulation model is implemented
• Working on offline programming
• Future works
• Online implementation
• Working on bidirectional motion