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Digital Tachometer

Digital Tachometer. Participant: Naveen K Boggarpu Place : EPE-PEMC 2006, Portoroz, Slovenia. Supervisor: Richard C. Kavanagh Date : 31/08/06. Agenda. Introduction Design Techniques Implementation Conclusion. 1.Introduction. 1.1 Aim of the project:

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Digital Tachometer

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  1. Digital Tachometer Participant: Naveen K Boggarpu Place : EPE-PEMC 2006, Portoroz, Slovenia. Supervisor: Richard C. Kavanagh Date : 31/08/06 Marie Curie Fellowship for ESRT

  2. Agenda • Introduction • Design Techniques • Implementation • Conclusion Marie Curie Fellowship for ESRT

  3. 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 Marie Curie Fellowship for ESRT

  4. 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 Marie Curie Fellowship for ESRT

  5. 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 . Marie Curie Fellowship for ESRT

  6. 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 Marie Curie Fellowship for ESRT

  7. 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 T1is eliminated. Marie Curie Fellowship for ESRT

  8. 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 Marie Curie Fellowship for ESRT

  9. 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 Marie Curie Fellowship for ESRT

  10. 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 Marie Curie Fellowship for ESRT

  11. 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 Marie Curie Fellowship for ESRT

  12. 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 Marie Curie Fellowship for ESRT

  13. 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 Marie Curie Fellowship for ESRT

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