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Electronic Bench Press Spotter. ECE 345 – Senior Design Jon Donenberg, Barry Horwitz. Summary. 4 Major Areas of Inquiry Concept Original Design Scale-model Implementation Testing and Analysis Successes, Challenges, and Recommendations. Concept. Description of Problem.

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electronic bench press spotter

Electronic Bench Press Spotter

ECE 345 – Senior Design

Jon Donenberg, Barry Horwitz

  • 4 Major Areas of Inquiry
    • Concept
    • Original Design
    • Scale-model Implementation
    • Testing and Analysis
    • Successes, Challenges, and Recommendations
description of problem
Description of Problem
  • Weightlifters work near maximum lifting ability to promote muscle development
  • Can be dangerous when the weightlifter reaches muscle ‘failure’ if a system to relieve him or her of the bulk of the weight is not present
  • Task usually fulfilled by another weightlifter – a ‘spotter’
  • Can design an electronic system to fulfill this role
performance benchmarks
Performance Benchmarks
  • Speed
    • typical weightlifters lift in the speed range of 1cm/s to 4 cm/s
    • Falling weight may approach 1 m/s
  • Detection Accuracy
    • eliminate input signal errors
  • Safety
    • electrical and mechanical failsafes must ensure a robust device that will not cause injury (and/or lawsuits)
original design7
Original Design
  • Full-scale Bench Press System
    • 200+lbs capacity
    • Requires kW of power capacity
    • DC motors rated for such work can run in the thousands of dollars
    • Financially infeasible (would break the 345 bank)
    • Decided instead on 1/5 scale model with reduced weight
    • Operational elements of circuitry are the same; only motor size and controller specs change
original design input circuit
Original Design – Input Circuit
  • Originally based on old sensor array using reflection instead of direct IR signal
    • Each sensor output is logic LO unless the bar is in front reflecting the beam
  • In this manner, the sensors were fed to 8-3 Priority Encoders
  • Four 3-bit strings fed to two 4-bit full adders (most significant bit a function of encoder outputs)
  • Sensor info added to produce a 5-bit output string
original design input circuit9
Original Design – Input Circuit
  • Problems
    • Sensor reflection based on ideas of ECE110 project lab; not optimal for our project
    • Original Encoders (MC10165) output logic LO @ 3.5V; attempted using comparators to lower the level to zero proved useless (even Mark couldn't figure it out)
  • Solution
    • changing the sensor location to direct beam would necessitate inverters on each input; or a new circuit design
    • new circuit design more simple, using OR gates
original design output circuit
Original Design – Output Circuit
  • Originally planned to use a smaller motor controller limited to 3A, LMD18200 chip
  • Problem
    • After testing the motor with our system, we needed a controller rated for higher currents
  • Solution
    • Devantech MD03 – Up to 20A continuous current
input circuitry ir sensor array
Input Circuitry – IR Sensor Array
  • Maximum Detector Characteristics:
    • Emitter-Collector Voltage: 5V
    • Ic Collector Current: 50mA
    • Power Dissipation: 150mW
    • Peak Sensitivity Wavelength: 850nm
    • Bandwidth Range: 620-890nm
    • Angle of half-sensitivity: +/-20o
  • Maximum Emitter Characteristics:
    • Continuous Forward Current: 150mA
    • Radiant Power Output: 15mW
    • Peak Emission Wavelength: 950nm
input circuitry ir sensor array13
Input Circuitry – IR Sensor Array
  • Emitter-Detector Circuit
    • Emitter and Detector on ‘line of sight’
    • Detector output reads logic HI unless the emitter IR signal is blocked
input circuitry ir sensor array14
Input Circuitry – IR Sensor Array
  • Emitter & Detector Arrays
  • Measured Total Detector Current: ~200mA
  • Total Emitter Current: ~100mA
input circuitry binary encoder
Input Circuitry – Binary Encoder
  • 32 Bit Priority Encoder
    • 5-bit string output is sent to Basic-X to designate location of bar within the sensor array
input circuitry binary encoder16
Input Circuitry – Binary Encoder
  • 3 Levels
  • Typical Propagation Delays
    • NAND gates – 125 ns
    • 83 Encoders – 10ns
    • Inverters – 20ns
  • Maximum Propagation Delays
    • NAND gates – 250ns
    • 83 Encoders – 15ns
    • Inverters – 33ns
  • Total System Maximum Delay ~0.3us
processing circuitry basicx 24
Processing Circuitry – BasicX-24
  • BasicX Characteristics:
    • Program Execution Speed: 16 us/16-bit ADD
    • Operating Voltage: 5.0V
    • Current Requirements: 20 mA
    • Maximum Current (w/ loads): 80mA
    • Onboard Timer Speed: 512 Hz
processing circuitry code
Processing Circuitry – Code
  • Major Executable Functions
    • Main() – primary executable function; runs at startup
    • Initialize() – set all variables to initial values
    • InputValue() – check and identify sensor input
    • InitializePWM() – turn on internal PWM at specified operating frequency
    • PutPinPWM() – toggle PWM high/low according to an input duty cycle
    • CheckInput() – process sensor input and determine the current output state
processing circuitry code19
Processing Circuitry – Code
  • Output State Sub-Functions
    • Embedded in CheckInput(), UpCode(), and DownCode()
    • Reversal Compensation – Pulls bar to top of system and holds it in place until the user is ready to begin lifting again
    • UpCoil – Coils lift cord as bar is raised
    • Velocity Compensation – Applies counterforce to get user back to the set minimum velocity
    • Stuck  Bump Compensation – Applies a slight upward force to the bar when the user is frozen in place for more than 4 seconds
processing circuitry code20
Processing Circuitry – Code
  • Basic PWM Implementation:
    • Turn on PWM at 14.456 kHz
    • Calculate Duty Cycle
    • Apply Duty Cycle to PWM output (applied within a loop architecture)
    • Set Duty Cycle to non-operational value
  • Code Skeleton:

Do While (Execution Condition == True)

DutyCycle = Operational Value

Call PutPinPWM(PinOC1A, DutyCycle)

Update Execution Condition


DutyCycle = non-operational Value

Call PutPinPWM(PinOC1A, DutyCycle)

processing circuitry code21
Processing Circuitry – Code
  • Velocity Compensation
    • System subject to a minimum lifting velocity (default = 3 cm/s = 2 sensor lengths per second)
    • If user falls below minimum velocity, system applies a counterforce to the bar equal and opposite to the amount of force required to raise the velocity back up to 3 cm/s
processing circuitry code22
Processing Circuitry – Code
  • Velocity Compensation
    • Acceleration = 9.81 m/s2 + (Vminimum2 – Vactual2)/(2*min. sensor resolution)
    • Trequired = (mass)*(acceleration)*(pulley radius)
    • Trequired/TFL = Vrequired/VFL
    • PWM Duty Cycle = Trequired/TFL

TFL, VFL are characteristics of the motor

output circuitry motor controller
Output Circuitry – Motor Controller
  • Devantech MD03 – 50-Volt 20-Amp H-Bridge Motor Driver
  • Voltage - 5v logic supply and 5V – 50V for the motor
  • Current - 50mA Max for logic and up to 20a for the motor
  • Mode 2 - 0v-5v (or PWM equivalent) with separate direction control
  • Current Limiter – Preset to 20 A
output circuitry motor controller24
Output Circuitry – Motor Controller

www.owlnet.rice.edu/~elec201/ Book/hardware.html

system mechanical characteristics
System Mechanical Characteristics
  • Vertical Sliders
    • Track Length – 18 in.
  • Operating Mass
    • Bar Weight: 632.0 g
    • Slider Weight: X2 = 457.4 g
    • Supplemental Mass Weight: X2 = 1281.6 g
  • Sensor Array
    • Array Lengths – 0.4 m
    • Sensor Spacing – 1.5 cm
  • Pulley Radius – 1 cm
output circuitry motor
Output Circuitry -- Motor
  • Motor Sizing: Power requirements
    • Pmax = (mass)*(gravity acceleration)*(min. sensor resolution)/(min. time resolution) = m*g*(max system velocity) = 94.3463W
  • Motor Sizing: Current requirements
    • Imax = Pmax/(Operative Voltage) = 7.86A
output circuitry motor27
Output Circuitry -- Motor
  • Motor Sizing: Torque requirements
    • Max acceleration = 9.81 m/s2 + (Vminimum2)/(2*min. sensor resolution)
    • Tmax = (mass)*(max acceleration)*(pulley radius) = 0.2317 N-m
  • Motor Sizing: Speed requirements
    • Pull-up Speed Vup = 1 m/s = 954.9 rpm
output circuitry motor28
Output Circuitry -- Motor
  • Motor Characteristics:
    • Model Type: 4z144
    • Operating Voltage: 12V
    • Full Load Torque: 0.2892N-m
    • Maximum Speed: 1750 rpm
    • Rated Current: 6.9A-8.1A
    • Peak Current: 17.5A
    • Rated Power: 53.29 W
    • Peak Power: 124.3 W


performance benchmarks30
Performance Benchmarks
  • Speed Analysis
    • Minimum speed resolution for microprocessor:
    • 4.1 m/s >> 1m/s
    • Four orders of

magnitude better

than necessary

performance benchmarks31
Performance Benchmarks
  • Speed Analysis
    • Optimal operating speed ~ 3cm/s
    • Sensor reads continue through our entire testing range
performance benchmarks32
Performance Benchmarks
  • Detection Accuracy & Safety
    • Glitch frequency increases as velocity decreases due to misreads when no sensor is being tripped
    • How to fix the


performance benchmarks33
Performance Benchmarks
  • Detection Accuracy & Safety
    • When no sensor is being tripped, input signal will read 0 or a much lower sensor due to propagation delays in input changes
    • While moving upward, the next sensor should be above the current sensor
    • While moving downward, the next sensor should be below the current sensor
    • Experimental results show that skipping more than three sensors is extremely unlikely
    • Solution – software error buffers
performance benchmarks34
Performance Benchmarks

If (moving upward) then ' error check

If (current sensor greater than last sensor) then

If (difference between current sensor and read sensor < 2) then

If (previously moving downward) then

upErrorBuffer = 1 'turn on error buffer

End If

If (previously moving upward) then

If (error buffer is active) then

increment error buffer

End If

If (upErrorBuffer = 2) then

Debug.Print "UP "; cstr(Alast); "->"; cstr(A)

upErrorBuffer = 0

Call upCode()

ElseIf (upErrorBuffer = 0) then

Debug.Print "UP "; cstr(Alast); "->"; cstr(A)

Call upCode()

End If

End If

prevState = 1

downErrorBuffer = 0

End If

End If

End If

performance benchmarks35
Performance Benchmarks
  • Detection Accuracy & Safety
  • Error checking virtually eliminates glitches except at excessive speeds
  • Safety is guaranteed at normal operational speeds
performance benchmarks36
Performance Benchmarks
  • Electromechanical Safety
  • Operational Currents for reversal trials well within safe range for motor
  • Mechanical stop


  • With error checking, system glitches were virtually eliminated
  • Operational currents fall well within suggested parameters
  • Prototype model performs to specifications
  • Input Circuit Design and Implementation
  • Early misunderstanding of power requirements caused scrapping of original output circuit configuration and the death of a few unfortunate capacitors
  • Occasionally sluggish response time at startup (from bottom) due to spikes in current requirements – in the worst case could cause operational failure
  • Potential Solution: “Stiffening Capacitor”
    • Used in cars with large sound amplification systems
    • Capacitor is placed in parallel with motor; charged to the operating voltage (12V)
    • Capacitor can be used to stabilize voltage level at the motor and dump excess current into the motor when peak exceeds power source ratings
    • Similar to a low-pass filter; voltage drop is an ‘AC wave’ that is filtered out
    • (Thanks to Prof. Swenson for insight on this topic!)
  • How does model scale to full-size, home use?
    • Need to run a much larger motor – 90V DC off wall AC current is typical
    • Power requirements to operate on 300lb load ~ 4 kW; at 90V current setup would require ~50A!
    • Must reduce current requirements
    • Gear box can be used to step down required torque and reduce current requirements
    • Tradeoff with maximum operative speed is acceptable due to the relatively low-speed nature of the application
    • 48V battery with 400 Amp-hrs would likely last for months since motor runs for very short periods of time
    • Wall source can run 90V motor
    • Most home fuses are ~20A
    • Could use large stiffening capacitor to handle brief, peak current requirements
    • Can also use a gear box in this implementation to reduce current requirements
  • Capacitor size requirements can be massive
  • Rule of thumb – 1F/kW of required power
  • Scale model application requires ~ 0.1 F Capacitor
  • ~4F for full-scale operation
  • These things are huge!
  • If this guy explodes, you may lose your house



Special Thanks to:

  • Mark Weigert
  • Scott McDonald (and the rest of the machine shop)
  • Profs. Swenson & Carney
  • Greg Sorenson
  • Coffee