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Football Wristband for Measuring Throwing Speed. Group 18 Kevin He & Darryl Ma ECE Senior Design November 30, 2006. Motivation. Provides a cost effective solution for measuring speed Does not require a dedicated operator Can be applied to other sports: baseball, boxing, cricket, etc.

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Football Wristband for Measuring Throwing Speed

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Football Wristband for Measuring Throwing Speed

Group 18

Kevin He & Darryl Ma

ECE Senior Design

November 30, 2006


  • Provides a cost effective solution for measuring speed

  • Does not require a dedicated operator

  • Can be applied to other sports: baseball, boxing, cricket, etc.


  • Accuracy within 10% of measurements obtained from radar gun

  • Weighs less than 500 grams

  • Lasts up to 5 hours per battery/charge

  • Temperature is within 3C of ambient

  • Able to measure a velocity range of 0mph to 70mph in increments of 1mph

Original Design Overview

  • Hardware

    • Power Supply Module, Pressure Sensor, Accelerometer, LCD, PIC Microcontroller

  • Software

    • PIC programming for user interface and speed calculation algorithm

Block Diagram

Block Diagram of the Device

System Schematic

Original Hardware Design

  • Power Supply Module

    • Converts 3V from coin cell battery to stable 5VDC

  • Pressure Sensor

    • Informs microcontroller when the ball is in the user’s hand and when the ball has been released

  • Accelerometer

    • Measures acceleration of the wrist

  • LCD

    • Displays velocity and user prompts

  • PIC Microcontroller

    • Performs integration function to calculate speed

Power Supply Module

  • Battery Rating: 160mAh

  • Converts 3VDC to a stable 5VDC

  • Total Current Drain: 20-22mA

Schematic of Power Supply Module

Power Supply Output

  • Vmax = 5.445V

  • Vaverage = 5.4305V

  • Vmin = 5.416V

  • Vripple = 14.69mV

Voltage output waveform from power supply module with 120kΩ load resistance

Pressure Sensor

  • Polyester variable resistor (32kΩ to 420kΩ)

  • Used voltage divider circuit to convert varying resistances into varying voltages


Schematic of Pressure Sensor Circuit

Pressure Sensor

Pressure Sensor Resistance Vs Voltage

  • Vhigh => hand unoccupied

  • Vlow => ball in hand

Resistance of Pressure Sensor (Ω)

Output Voltage Vs Pressure Sensor Resistance


  • ±5g ADXL320 Dual Axis Accelerometer

  • 5V supply with 2mA current drain

  • Capacitors attach across output terminals for signal conditioning


Capacitors for signal conditioning

Accelerometer Sensitivity

  • Verify datasheet sensitivity value of 312mV/g

Voltage output from accelerometer when dropped

Accelerometer Orientation

  • Voltage outputs change depending on orientation


  • Displays user prompts and final ball velocity

  • 16X2 RT1602C character LCD

  • 5V supply with 1mA current drain

PIC Microcontroller

  • PIC16F877A

  • Performs integration of acceleration input to calculate ball velocity

  • 5V supply with 8mA current drain

  • Operating frequency: 8.00MHz

Original Software Design

  • Written in PIC-C

  • Purpose

    • Obtain Inputs from Pressure Sensor and Accelerometer

    • Calculate Velocity

    • Output results to LCD

Original Flow Chart

Positive and negative accelerations cancel to zero at apex of wind-up

Riemann Sum

Acceleration to speed conversion

a(t) = acceleration function

v(ti) = velocity at time, ti

Baseline Detection System

  • When the PIC senses a relatively stable signal, it resets the net velocity to zero

Voltage output from accelerometer when simulating a throw

Integration sum is reset to zero here.


  • Major Problems we encountered:

    1) Poor battery life

    2) PCB/Protoboard Issues

    3) Inconsistent Results

Poor Battery Life

  • Battery rating of 160mAh and current drain of 21mA suggests battery life of ~8 hours.

  • Actual battery life only about ~4 hours, with frequent fadeouts

  • Traced problem to battery brand

  • Energizer is the way to go!

PCB/Protoboard Issues

  • Main issues were related to durability

    • Protoboard wires and components often came loose

      • Solution: Make PCB

    • PCB wire pads were often stripped of copper lining

      • Solution: Use 30 gauge wires to reinforce connections

Inconsistent Results

  • Causes Explored

    1) Accelerometer output noise

    2) Insufficient sample size

    3) Accelerometer Orientation

    4) Pressure Sensor Sensitivity

Accelerometer Output Noise

Measurement of the accelerometer’s axis show a large uncertainty in the output voltage. The uncertainty is on the order of ~1V, as seen on the graph.

Output voltage waveform from accelerometer when simulating a throw

Accelerometer Noise Solution

Our solution was to add capacitors across the power terminals of the PIC, oscillator, and accelerometer. This further stabilized the input voltage waveform, which helped the accelerometer output consistent results.

Output voltage waveform after adding capacitors to PIC, oscillator, and accelerometer

Stability Testing: Output from PIC stable within 9.8mV

Testing Accuracy of PIC Reading from Accelerometer

Insufficient Sample Size

  • What is the processing time per sample?

  • Are there enough samples to perform an Riemann Sum integration?

Accelerometer Orientation

  • Changing the accelerometer orientation changes plane of acceleration

Pressure Sensor Sensitivity

  • Is the pressure sensor sensitive enough to detect the football?

    • Lab tests show that even a gentle grip causes a sizeable voltage drop

    • However, while actually throwing a ball, the user may loosen his grip even though the ball is still in his/her hand

Removal of Pressure Sensor

  • The trials below represent three out of ten trials that returned results. The other seven trials produced no measurement due to insufficient number of samples. The number of samples is directly controlled by the pressure sensor.

  • Inhibits throw

  • Reduces production cost significantly

Threshold Algorithm


  • Starts integrating as soon as the 1.4g threshold is reached and continues to integrate until acceleration falls below this threshold

  • Eliminates need for pressure sensor and baseline detection

  • Will give relative velocity rather than exact velocity, so offset factor needed

Velocity Correction Factor

  • Why is it needed?

    • Threshold algorithm only measures acceleration beyond a certain limit, so not all acceleration is captured

    • The acceleration per sample is sqrt(x2 + y2), but taking the square root every sample reduces the number of total samples we can take, so we only used x2 + y2

Velocity Correction Factor

  • We know there is a correlation between the device speed and radar gun speed, so we need to apply an offset to make them equal

  • The velocity is:

  • Our final equation including offset is:

  • The offset factor was determined solely based on experimental data. The speeds that we validated were the ones we could obtain with the radar gun (25mph – 40mph).

Velocity Correction Factor

Average percentage difference

Without Correction Factor: 17.9%

With Correction Factor: 6.68%

Std. Dev. of percentage difference

Without Correction Factor: 3.25%

With Correction Factor: 2.61%

New Flowchart

Summary of Final Design

  • Original Power Supply Circuit

  • Original LCD

  • Added capacitors to clean up accelerometer output

  • Removed Pressure Sensor

  • Modified Velocity Algorithm


  • Radar Gun Vs Wristband

  • Correlation Check

  • Tolerance Analysis

  • Temperature Measurements

Radar Gun Vs Wristband

Percentage Difference

Average: 4.507%

Std Dev.: 3.381%

Correlation Check

  • Since the radar gun only measured speeds greater than 25mph and we were not able to throw the football faster than 38mph, we performed a correlation check to make sure there was some correlation between the relative speed of the arm and the measured throw speed.

Correlation Check Results

Results show a definite correlation between the relative speed of the arm and the measured throw speeds (throws were performed empty-handed)

Tolerance Analysis

  • Concern: Accelerating over 5g could damage the accelerometer or other components

∆Voltage = 1.891V

Sensitivity = 0.312V/g

g = ∆Voltage/Sensitivity

g = 5.66g

Tolerance Analysis on Y-Axis

Tolerance Analysis

∆Voltage = 2.031V

Sensitivity = 0.312V/g

g = ∆Voltage/Sensitivity

g = 6.5096g

Tolerance Analysis on X-Axis

Waveform shows no saturation at accelerations greater than 5g and when integrated back into device, there were no adverse effects.

Temperature Measurements

  • Room Temperature: 24.8°C

  • Device-Wristband Surface: 26.8°C

  • Wristband-Skin Surface: 26.4°C

  • Normal Skin Temperature: 32.9°C

  • Fulfills our performance requirement of ±3°C of ambient temperature

The Wristband


Easily removable and comfortable

Powered by one 3V coin cell battery

Cost effective

Large range of measurement

SWOT Analysis


  • Inconsistent results due to human variation

  • Slightly inhibits throw

  • Measures acceleration in one plane


  • Useful for other sports applications


  • Low consumer demand

Comparison with Radar Gun

Ethical Considerations

  • Safety

    • Temperature

    • Electrostatic Discharge (ESD)

  • Being honest/realistic about what our device is capable of

Future Steps

  • More accelerometers to achieve absolute acceleration, and enable more accurate measurements

  • Convert all parts to surface mount components to reduce device size

  • Improve durability

  • Improve battery life

  • Apply for a Patent


  • Ms. Hye Sun Park

  • Mr. Mark Smart

  • Professor Jonathan Makela

  • Coach Dan Hartleb & Coach Eric Snider


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