Football Wristband for Measuring Throwing Speed Group 18 Kevin He & Darryl Ma ECE Senior Design November 30, 2006.

Football Wristband for Measuring Throwing

Speed

Group 18Kevin He & Darryl Ma

ECE Senior DesignNovember 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.

Objectives

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 3C 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

5VDC

Schematic of Pressure Sensor Circuit

Pressure Sensor

Pressure Sensor Resistance Vs Voltage

Vhigh => hand unoccupied Vlow => ball in hand

Output Voltage Vs Pressure Sensor Resistance

Resistance of Pressure Sensor (Ω)

Accelerometer

±5g ADXL320 Dual Axis Accelerometer

5V supply with 2mA current drain Capacitors attach across output

terminals for signal conditioning

Accelerometer

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

LCD

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

Riemann Sum

Acceleration to speed conversion

a(t) = acceleration function v(ti) = velocity at time, ti

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

Baseline Detection System

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

Integration sum is reset to zero here.

Voltage output from accelerometer when simulating a throw

Debugging

Major Problems we encountered:1) Poor battery life2) PCB/Protoboard Issues3) 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 Explored1) Accelerometer output noise2) Insufficient sample size3) Accelerometer Orientation4) 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

Stability Testing: Output from PIC stable within 9.8mV

Testing Accuracy of PIC Reading

from Accelerometer

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

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

Radar Gun Speed (mph)

Device Speed (mph)

Trial #1 26 31.8Trial #2 28 26.2Trial #3 25 14

1) 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.

2) Inhibits throw

3) Reduces production cost significantly

Threshold Algorithm

Threshold

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:

25 1 9.8 / 1( ) 1

1024 0.312 1 0.44704 /

V g m s mphVelocity AccelerationVoltage ms

V g m s

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).

1( 0.0005 5) [27 ( 0.0005 5)]

3ActualVelocity Velocity Velocity

Our final equation including offset is:

Velocity Correction Factor

Measurements from Radar Gun (mph)

Measurements from our device (mph) – without

correction factor(Velocity * 0.0005)

Measurements from our device

(mph) – with correction factor

Trial #1

27 31.5 26.7

Trial #2

25 26 23

Trial #3

28 35.2 29.1

Trial #4

32 41.4 33.3

Trial #5

27 32 27

Trial #6

35 46 36.3

Trial #7

38 49.5 38.7

Trial #8

28 34.8 28.9

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

Verification

Radar Gun Vs Wristband Correlation Check Tolerance Analysis Temperature Measurements

Radar Gun Vs Wristband

Radar Gun Measurement

Measurement from Device

Percentage Difference

27 30 10%

29 26.6 9.02%

34 33.9 0.29%

38 36.5 4.11%

36 35.8 0.56%

25 24 4.17%

27 27.5 1.82%

28 26 7.69%

33 34.2 3.51%

32 33.3 3.90%

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

Relative Speed

Trial #1

Trial #2

Trial #3

Trial #4 Trial #5

Slow 6mph 3mph 4.6mph 4.2mph 7.6mph

Medium 11.1mph

13.1mph

10.3mph

12.5mph 14.4mph

Fast 30.2mph

28mph 23.7mph

30.6mph 27.1mphResults 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 = 1.891VSensitivity = 0.312V/gg = ∆Voltage/Sensitivityg = 5.66g

Tolerance Analysis on Y-Axis

Tolerance Analysis

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

∆Voltage = 2.031VSensitivity = 0.312V/gg = ∆Voltage/Sensitivityg = 6.5096g

Tolerance Analysis on X-Axis

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

SWOT Analysis

Strengths: Easily removable and comfortable Powered by one 3V coin cell

battery Cost effective Large range of measurement

Weaknesses: Inconsistent results due to

human variation Slightly inhibits throw Measures acceleration in one

plane

Opportunities: Useful for other sports

applications

Threats: Low consumer demand

Comparison with Radar Gun

Cost Accuracy

Precision Ease of Use

Measurement Range

Battery Life

RadarGun

$85 - $400

Within +/-0.5mph

+/- 1mph Requires dedicated operator

Relative angle to ball should be less than 15 degrees for best accuracy

~20 hours using specialized batteries, or 6 AA batteries

Our Device

$80 for prototype, $45 if mass- produced

Within +/- 10% of Radar Gun

+/- 0.2mph

Does not require dedicated operator

Accuracy independent of ball path

~4 hours using a single 3V coin cell battery

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

Credits

Ms. Hye Sun Park Mr. Mark Smart Professor Jonathan Makela Coach Dan Hartleb & Coach Eric

Snider

Questions?