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EMG With Weight Adjustment Group 2 Brandon Ceaser, Andy Eddleman, and Robert Lewton ECE445 Senior Design November 28, 2

EMG With Weight Adjustment Group 2 Brandon Ceaser, Andy Eddleman, and Robert Lewton ECE445 Senior Design November 28, 2007. Introduction.

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EMG With Weight Adjustment Group 2 Brandon Ceaser, Andy Eddleman, and Robert Lewton ECE445 Senior Design November 28, 2

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  1. EMG With Weight AdjustmentGroup 2 Brandon Ceaser, Andy Eddleman, and Robert LewtonECE445 Senior DesignNovember 28, 2007

  2. Introduction • The scope of this project was to take a signal from the bicep obtained by the EMG circuit that would then adjust a motor that acts as a variable weight load. • This project includes application in Biomedical, Controls and Signal Processing, and Power disciplines of electrical engineering.

  3. System Overview • Hardware • EMG • Motor and gearbox • Motor controller • PC user interface • Software • Labview

  4. Features • Simple PC user interface • Minimal weight training equipment • User calibration • Multiple difficulty levels • Dual switches for safe operation

  5. How EMG Works • Action potential from motor nerve depolarizes muscle fiber membrane • EMG measures resulting electrochemical reaction • Signal acquired through skin electrodes that envelope muscle of interest • Reference electrode placed in area with minimal electric association

  6. Skin Electrodes • Unilect 4560M wet gel electrodes (38x60mm) • Ag/AgCl sensor material • Inter-electrode distance of approximately 8mm (avoiding tendons)

  7. EMG Circuit • Original EMG Circuit • Skin electrodes • Protective resistors • High-pass filters • AD622 instrumentation amplifier • Improved EMG Circuit • Two (2) LM324 quad op-amps • LM741 op-amp • Four (4) 1N4150 diodes • Capacitor/Resistor circuitry

  8. EMG Circuit Schematic

  9. Amplification • AD622 Instrumentation Amplifier • Gain calculated as G = 50.5k/Rg + 1 • Used approximate gain of 1000 Vmax: 3.1mV Vmin: -6.9mV Vmax: 2.25V Vmin: -2.25V

  10. EMG Signal and Fatigue • Typical frequency range of 20-500Hz • Amplitude of signal increases during a flex • Signal amplitude directly correlated to amount of strain and level of fatigue (Vmax reaches more than 2v)

  11. EMG Rectification and Enveloping • Output of AD622 rectified by first LM324 • LM741 (active LPF) creates inverted envelope of the output of first LM324 • Second LM324 rectifies the inverted envelope

  12. EMG Testing • Gain resistance threshold • Useful signal from 51.1Ω (G = 1000) to 1.3kΩ (G = 40) • When G = 1000, input voltage greater than 19mV causes AD622 to saturate • Inter-electrode distance • Varied from 8mm to 3mm • Smaller distance leads to larger signal amplitude

  13. EMG Testing (contd.) 8mm Vmax: 1.19V Vmin: 125mV 5mm Vmax: >2V 3mm Vmin: 62.5mV Vmax: 4.06V Vmin: 2V

  14. EMG Testing (contd.) • Effects of sweat (water) on signal acquisition. • Typical signal ranges for different levels of stress and body types. Brandon Robert

  15. Lifting Apparatus • Original Design • Motor and Gearbox • Preacher curl bench • Motor control circuit w/ current feedback • Improved Design • Interlock safety switches • Standing curl platform

  16. Lifting Platform • The lifting Platform was a wooden platform with the motor and gearbox mounted at the front left. • For safety a set of interlock switches was installed. • A normally closed switch was placed on the bar. • A normally open switch was placed by the users foot.

  17. Motor Choice: Permanent Magnet DC Motor • 12/24Vdc Permanent Magnet Dc motor given by Professor Krein. • A Permanent Magnet Dc motor was selected due to its linear relationship between toque and current. • The nameplate characteristics of the motor are: • Speed: 1800/4200 RPM • Power: 1/6-1/3 Horsepower • Full Load Current Rating: 16Amps http://www.grainger.com/Grainger/items/4Z529

  18. Motor Characteristics • Km constant: • Armature Resistance (Ra) ≈ 0.9 ohms • (this will change with the armature position) • No load current ≈ 1.1 amps • Km=0.05841 volts*sec/rad • Torque for a Permanent Magent Dc motor is: • Max Tmech = 0.934558 Nm ≈ 8.271534 in-lbs

  19. Gear Box Choice • Our final design included gear ratio to 5:1. • This gear ratio and our cable drum (approx. 1.25 inch in diameter) should give us a maximum load of approximately 70.806 lbs. • A test was done with the hanging scale to determine the pull force versus the duty cycle.

  20. Pulling Force Vs. Duty Cycle Measurements were made using a 50lbs. Max digital hanging scale. The Bar Weight was 3.375lbs.

  21. Torque control circuit • The torque control of the motor was implemented by a buck converter. • The buck converter is a dc transformer that turns a voltage source into a current source through the duty ratio. • Duty cycle decreases • current increases, voltage decreases. • Duty cycle increases • current decreases, voltage increases. Overall buck converter circuit

  22. Torque Control Circuit (cont.) • For our application: • FQP50N06 Mosfet • NTE6081 Diode • MIC4424CN Gate Driver. • The MIC4424CN Gate Driver is needed to provide enough voltage from gate to source and the current necessary for the gate capacitance of the power mosfet. Freq: 100kHz Duty cycle: 50% Square wave 5Vpp

  23. Torque Control Circuit Test • To verify a stall test was done were current was monitored while duty cycle was changed. Measurements were made using a Fluke Digital Multimeter in series with the motor terminal.

  24. Back EMF • Due to our need to pull against the motor it becomes a generator causing an increase in: • Voltage • Current • Torque

  25. Current Mode Control • To compensate for this back EMF issue a Current mode controller was attempted to be implemented. • The current mode control uses • Differential Amplifier • Current Mode Control Chip • DC Voltage Control Signal

  26. Differential Amplifier • The Differential Amplifier, amplifies the current sensed by the 0.01 ohm sense resistor by a factor of 6. • This gives us 0.962 V at 16 amps, which close to the desirable value of 1 V. Differential Amplifier Circuit Freq: 100 kHz Dc offset: 0.1 Vdc Vpp: 0.16V Tri. Wave

  27. Current Mode Controller (UC3843) • To determine the switching frequency an external resistor(R5) and capacitor(C1) are added and is approximately 80.63Hz. • The Differential amplifier output is input into an internal comparator with the Rsense pin and compared at a 1V level control voltage.

  28. Current Mode Controller (UC3843)(cont.) • We were able to get the current controller to respond with a function generator, and power supply, but once integrated into the motor circuit we were unable to get a response from the controller. Freq: 50 kHz Dc offset: 0.2V Vpp: 0.82 V Control voltage: 0v Sawtooth

  29. Improvements • To improve this circuit a current mode controller should be successfully implemented. Whether the UC3843 or a different one. • Future inspection of the use of a PID controller for current and speed control. • For safety purposes a mechanical brake should be fixed to the drum that is engaged when power to the motor is off, and disengaged when the motor is running.

  30. Control Design • Original Control Design • PSoC MicroController • Take in EMG voltage Signal (trouble taking in Values) • Output PWM signal (could not easily change Duty Ratio) • Improved Control Design • Labview • Connect Computer running labview to Agilent 54642A Oscilloscope and Phillips PM5139 Function Generator using GPIB IEEE 488 cables • Take in EMG voltage Signal from Agilent Scope • Output PWM signal from Function Generator

  31. Workout Program Control Block Diagram

  32. Workout Block Diagram

  33. Calibration Diagram

  34. Agilent 54642A Oscilloscope -Input Phillips PM5139 Function Generator - Output Nation Instruments LabVIEW - Control

  35. Range of Values and Duty Ratio • Test Duty Ratio = ((Calibration Value – Test Value) + Test Constant )*10 • If (Test Duty Ratio > Test High Constant) or (Test Duty Ratio < Test Low Constant) we will use Test Constant * 10 as our Duty Ratio or else we will use Test Duty Ratio • Test Duty Ratio is independent of any control. To acquire, set the duty ratio at Test Constant *10 and then and read voltage from EMG.

  36. Front Panel

  37. Output PWM at Calibration Duty Ratio PM 5139 Function Generator

  38. Output Test Reading Capture Test Calibration Agilent 54642A Osciliscope

  39. Choose Measurement Record and Compare Agilent 54642A Osciliscope

  40. Take Difference, Check Range, Use if in Range, Use 30, 50, or 70 if not in Range PM 5139 Function Generator Output PWM And Loop

  41. Revised Control Input-Output • Use NI’s CompactDAQ System • NI 9411 Signal Generator • NI 9201 Voltmeter • Connect CompactDAQ chassis via USB to computer and use National Instruments Labview to control as in previous system

  42. Strengths EMG with envelope detection made it possible to sample bicep signal. Motor control allowed us to control the initial pull of the motor. Use of Labview allows calibration and different difficulty levels for multiple users. Weaknesses Motor noise detected by the EMG. Current ramping due to back EMF. Improvements All components placed on the lifting platform. Safety brake for commercial use. Conclusions: Strengths, Weaknesses, and Improvements

  43. Credits • Professor Philip Krein • Professor Patrick Chapman • Professor Kenneth Gentry • Professor Raymond Fish • Professor Scott Carney • TA Tony Mangognia

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