Measuring movement of golfers with an accelerometer

18th October, 2012 Changsu Jung

Contents

Introduction Background Related work Analysis of golf swings Design Implementation Evaluation and result Conclusion and Future work

2

Introduction Many studies have been carried out for measuring golfers’ movement – But required complicated and expensive devices – Not convenient and just for elite players

The goals of this thesis – To provide simple and convenient way of

measurement system – Design and implementation of Android application

• Using an accelerometer • Extracting core information from acceleration data • Identifying critical points of the swing movement

– Implementation of feedback • different kinds of feedback related to the golfer’s performance

3

Background Accelerometer – Measures acceleration generated by the movement of an object and the

change of velocity – Commonly used for measuring physical activities

One of the main interests of golfers is how to increase distance – Obtain maximum club-head speed at impact timing – Force = Mass x Acceleration – An accelerometer was chosen to measure the movement of a golfer

The coordinate system of Android – Sensor coordinate system, screen coordinate system – Remapping between the two coordinate system

4 Y = 9.81 ㎨ Back

Y-

X- X+

Y+

Z-

Sensor Y

Y-

X+ X-

Y+

Z+

Sensor X

Sensor Y Sensor X

X = 9.81 ㎨

Y-

X+ X-

Y+

Z+

Sensor Y

Sensor X

Related Work Motion sensor at the grip [1] – three-dimensional accelerometer

and a gyroscope – show the acceleration graph

5

• Two accelerometers in the shaft and the club head [2] – identified and calculated

swing durations

club head

grip

Acceleration graphs

6

-20

-15

-10

-5

0

5

10

15

200

5152

1085

116

382

2194

027

582

3351

139

341

4497

150

572

5640

262

063

6752

173

142

7884

184

222

8960

595

150

1005

8210

6302

1118

2511

7454

1230

7112

8748

1344

3414

0121

1458

1115

1342

1568

9116

2420

1681

2217

3825

1794

32

Acc

eler

atio

n (㎨

)

X-axis

Y-axis

Z-axis

-20-15-10-505

101520

051

5210

851

1638

221

940

2758

233

511

3934

144

971

5057

256

402

6206

367

521

7314

278

841

8422

289

605

9515

010

0582

1063

0211

1825

1174

5412

3071

1287

4813

4434

1401

2114

5811

1513

4215

6891

1624

2016

8122

1738

2517

9432

Acc

eler

atio

n (㎨

)

X-axis

Y-axis - 9.8

Z-axis

-20

-15

-10

-5

0

5

10

15

20

030

7561

7693

5512

475

1562

818

705

2182

524

954

2828

231

653

3494

638

055

4114

544

276

4729

950

504

5364

556

865

6005

563

198

6636

569

426

7265

675

774

7901

682

126

8527

588

505

9177

595

195

9866

610

1982

1051

1310

8543

1118

5311

5065

1182

9412

1626

1248

8412

8276

Acc

eler

atio

n (㎨

)

X-axis

Y-axis - 9.8

The original acceleration graph of golf swings

Y – gravity(9.8)

The acceleration data of the x and y – gravity(9.8)

Analysis

The definition of critical points

7

-13

-8

-3

2

7

12

19 71 143

226

282

361

431

511

591

672

763

843

914

1015

1093

1172

1243

1315

1382

1452

1523

1592

1662

1733

1803

1862

1941

2024

2082

2161

2237

2333

2413

2503

2583

2653

2712

2782

2853

2923

Acc

eler

atio

n(㎨

)

X

Y

Backswing Point

Downswing Point

Minimum peak point of the x-axis

Maximum peak point of the x-axis

Minimum peak point of the y-axis

Maximum peak point of the y-axis

End point

Phase1 Phase2 Phase3

Phase1: backswing, Phase2: downswing, Phase3: follow-through http://blog.snowlove.co.kr/30056382913

Analysis

The movement during the backswing

8

X+

Y+

Z+

Attached to the front side (chest)

Attached to the backside (waist)

Y+

X-

Z+

The coordinate systems according to the placement

Analysis

Default placement

of a smart phone

Analysis of backswing movement Backswing: Phase1 – A golfer moves the golf club backwards, the acceleration values

are changing according to the Android coordinate system – Non-zero area: moving until the top swing – Zero area: top swing

9

-20

-15

-10

-5

0

5

10

15

20

6443

764

496

6456

764

637

6472

664

807

6496

565

035

6513

665

215

6528

465

355

6542

665

516

6559

765

666

6574

665

816

6594

666

005

6607

466

144

6622

566

285

6635

066

427

6649

666

566

6663

566

707

6679

666

866

6694

667

026

6711

7

Acc

eler

atio

n (㎨

)

second(ms)

Rounddown(X,0)

zero area non-zero area

Timestamp X Round-down(X) 65586 1.762734 1 65597 1.762734 1 65606 1.916016 1 65616 1.762734 1 65637 1.801055 1 65646 1.801055 1 65657 1.839375 1 65666 1.532813 1 65676 1.532813 1 65692 1.111289 1 65696 0.651445 0 65717 -0.07664 0 65737 -0.53648 0 65746 -0.80473 0 65766 -0.72809 0 65780 -0.88137 0 65786 -0.88137 0 65796 -0.84305 0 65807 -0.84305 0 65816 -1.49449 -1

Analysis

Analysis of other swing motions

Downswing: Phase 2 – A golfer turns his trunk from the right to the left – The acceleration values increase in the negative direction – From the downswing to the minimum peak point(impact)

Follow-through: Phase 3 – From the minimum peak point to the end point – decreasing to the negative direction and a golfer moves to

the positive direction

10

Analysis

Analysis of the end point Finish: end point – A golfer’s movement is stopped and the acceleration values can be

almost zero. – Checked the red-colored areas of the all swing data. – Common patterns exist: zero values more than 5 times consecutively

11

No. Timestamp X Y Times

tamp X Y Timestamp X Y

1 3638 0 0 18291 0 0 34299 0 0 2 3659 0 0 18411 0 0 34319 0 0 3 3669 0 0 18420 0 0 34329 0 0 4 3679 0 0 18431 0 0 34349 0 0 5 3689 0 0 18440 0 0 34359 0 0 6 3699 0 0 18451 0 0 34368 0 0

-20

-15

-10

-5

0

5

10

15

20

042

3283

1112

541

1682

021

005

2562

029

900

3416

838

460

4298

047

360

5153

056

026

6014

064

484

6897

173

210

7740

081

741

8617

990

430

9474

098

931

1032

6010

7440

1118

7911

5979

1202

8012

4716

1288

3913

3120

Acc

eler

atio

n (㎨

)

second(ms)

Round down(X) Round down(Y-9.81)

Analysis

Software architecture

12 Database

Design

Displaying acceleration value Each timeslot has a representative acceleration value – Due to the limited screen size – The maximum value is chosen as the representative

• because it means the maximum force and the fastest movement

13

Xi = Max(|Maxi|, |Mini|)

abs(Maxi) > abs(Mini) abs(Maxi) < abs(Mini)

Design

Beep sound feedback

Type Sound Icons

Normal point Normal sound

Maximum peak point

High pitch

Minimum peak point

Low pitch

Two peak point in a timeslot

Combined pitch

14

Design

Data collection

15

start collecting acceleration data for 20 seconds

feedback screen

Sampling option: SENSOR_DELAY_FASTEST – collect acceleration data almost every 10 msec

Collection time – 10, 20, 30, 40, 50 seconds – A golfer can choose in the settings menu

The number of swings – 1, 5, 10 times

For example – If the collection time is 20 seconds and 5 times

5 times

display

10 seconds later

Implementation

The sequence of detecting points

16

-20

-15

-10

-5

0

5

10

15

2064

437

6449

664

567

6463

764

726

6480

764

965

6503

565

136

6521

565

284

6535

565

426

6551

665

597

6566

665

746

6581

665

946

6600

566

074

6614

466

225

6628

566

350

6642

766

496

6656

666

635

6670

766

796

6686

666

946

6702

667

117

Acc

eler

atio

n (㎨

)

second(ms)

X Y

2

3 4 1 6

1. The minimum peak point of the x-axis 2. The maximum peak point of the x-axis 3. The downswing point from 1 4. The backswing point from 3 5. The end point from 1 6. The minimum peak point of the y-axis 7. The maximum peak point of the y-axis

7

5

Implementation

Round-down operation To make the acceleration data simple – can ignore small differences from the acceleration data – useful to find similar patterns

17

X Y Round-down(X)

Round-down(Y)

0.7664063 8.430469 0 8

5.7863674 7.6640639 5 7

5.5564456 0.804727 5 0

7.4724612 -4.4451562 7 -4

8.5837507 -4.9049998 8 -4

7.3575001 -9.0052734 7 -9

7.3575001 -9.0052734 7 -9

2.3758595 -11.1128907 2 -11

-3.1039455 -9.9632813 -3 -9

-15

-10

-5

0

5

10

15

19 143

282

431

591

763

914

1093

1243

1382

1523

1662

1803

1941

2082

2237

2413

2583

2712

2853

Acc

eler

atio

n(㎨

)

second(ms)

X

-15

-10

-5

0

5

10

15

19 131

262

402

532

691

843

985

1151

1272

1402

1534

1662

1781

1915

2051

2183

2352

2503

2643

2763

2893

Acc

eler

atio

n(㎨

)

second(ms)

rounddown(X,0)

Implementation

Detection of the two peak points Using two threshold values – To filter weak and small movements such as warming-up or

noises

Find the minimum value and the maximum value Keep comparing acceleration values until finding a transition point – to avoid misjudging the jagged patterns as the peak points

18

Implementation

-13

-8

-3

2

7

12

19 71 143

226

282

361

431

511

591

672

763

843

914

1015

1093

1172

1243

1315

1382

1452

1523

1592

1662

1733

1803

1862

1941

2024

2082

2161

2237

2333

2413

2503

2583

2653

2712

2782

2853

2923

Acc

eler

atio

n(㎨

)

second(ms)

X

Y

Detection of the downswing

Finding the downswing point: – The first zero point from the minimum

peak of the x-axis – Analyze acceleration data in the reverse

direction – If not detect, use the first point of the

swing data

19

The minimum peak

Searching the first zero point from the peak point

The collected data round-down value -0.84305 0 (start point) -1.34121 -1 -2.4525 -2 -3.02731 -3 -3.75539 -3 -4.10027 -4 -5.97797 -4 -7.12758 -5 -7.1659 -7 -8.04727 -7 -9.15856 -8

-9.54176 -9

(the minimum peak point)

Implementation

Detection of the back swing

20

Backswing

-20

-10

0

10

2018 102

195

282

355

452

532

602

686

765

832

932

1014

1092

1175

1256

1336

1453

1542

1642

1732

1825

1902

2002

2112

2192

2263

2352

2442

2543

2644

2713

2803

2892

2993

Acc

eler

atio

n(㎨

)

second(ms)

Rounddown(X,0)

non-zero area

zero area

Searching for the zero area before the downswing – r = rounddown(x); – if(r==0) ++zero_count; – if(zero_count >= TOP_SWING_CRITERION) topSwingFound = true;

Find a non-zero area in front of the zero area – if((topSwingFound == true) && (r > 0 || r < 0)), then non_zero_count++; – if(non_zero_count >= BACK_SWING_CRITERION) backSwingFound = true;

If not detected – Return the downswing timestamp

TOP_SWING_CRITERION=4 BACK_SWING_CRITERION=4

Implementation

-15

-10

-5

0

5

10

1519 81 162

255

333

421

511

603

717

802

892

985

1093

1181

1262

1342

1425

1505

1592

1672

1751

1831

1915

2012

2082

2171

2265

2382

2485

2563

2653

2723

2803

2893

2982

Acc

eler

atio

n(㎨

)

second(ms)

rounddown(X,0)

Detection of the end point

End point - X, Y values are zero at least 5 times in series - If(rounddown(x[i]) == 0),

- then zero_count++; - If(zero_count >= END_CRITERION),

- then x[i] = the end point

If not detect, – use the end index(the last timestamp) of the swing data

21

Peak point

END_CRITERION = 5

Implementation

Beep sound feedback

22 Different beep sounds in the minimum and maximum peak points

-20-15-10-505

101520

18 132

242

342

466

562

662

765

862

972

1071

1181

1283

1412

1542

1672

1792

1882

2018

2142

2242

2352

2472

2603

2703

2813

2932

Acc

eler

atio

n(㎨

)

swing_0523125001

X Y

-15

-10

-5

0

5

10

15

20

85 152

214

282

332

392

466

522

576

623

686

742

792

846

913

972

1025

1092

1151

1202

1262

1322

1404

1472

1542

1612

1683

1755

1812

Acc

eler

atio

n(㎨

)

swing_0523125001

X Y

time [0] [1] [2] [3] [4] [5] [6] [7] [8] [9]

Minimum Peak point

Maximum Peak point

Minimum Peak point

Maximum Peak point

Implementation

Visual feedback

23

Swing duration time The maximum and minimum

acceleration graph

An acceleration graph

• AChartEngine version1.0 [4] is used for drawing graphs.

Implementation

The method of evaluation Field evaluations (two times) – Functional verification and performance evaluation – Collected around 80 golf swing data – Collection time was 10 or 20 seconds according to the setting

menu – Threshold values: 5㎨ and -5㎨

24

Evaluation & Result

The detection rate The detection rate of critical points

25

• The reasons of detection failure

Detectable swings

Undetectable swings

Total

Number 59 23 82

Percentage(%) 71.9 % 28.04 % 100 %

Reasons Number Percentage

Weak swing (No data less than negative threshold (-5㎨)) 12 52.2 %

Weak swing (less than positive threshold (5㎨)) 6 26.1 %

Undistinguishable swing 5 21.7 %

Total 23 100 %

Evaluation & Result

Examples of undetectable swings

26

-30-25-20-15-10-505

1015202530

124

454

280

3 95 353

603

893

1142

1376

1646

1946

2222

2474

2743

3003

3293

3553

3814

4072

4362

4635

4893

Acc

eler

atio

n(㎨

)

second(ms)

X

Y

-20

-15

-10

-5

0

5

10

15

20

40 832

1721

2553

3353

4181

5023

5873

6712

7573

8413

9242

1007

310

943

1177

312

593

1342

214

292

1519

215

973

1685

217

702

1855

419

376

Acc

eler

atio

n(㎨

)

second(ms)

X

Y

-20

-15

-10

-5

0

5

10

15

20

20 242

492

743

972

1233

1462

1682

1912

2142

2336

2533

2733

2933

3132

3353

3593

3833

4062

4273

4483

4692

Acc

eler

atio

n(㎨

)

second(ms)

X

Y

No data less than the negative threshold (-5㎨)

Less than the positive threshold (5㎨)

Undistinguishable swing

Evaluation & Result

The detection rate of the backswing

Detectable swing data (total 59 swing data)

Backswing Downswing Follow-through

Number 37 59 59

Percentage 62.7 % 100 % 100 %

27

• Identifying the backswing is problematic - The movement is very small - The successful detection rate was 62.7 %.

Evaluation & Result

Example of distinctive backswing

28

-15

-10

-5

0

5

10

15

19 151

311

462

641

824

1015

1181

1333

1482

1642

1781

1941

2091

2265

2462

2633

2773

2923

Acc

eler

atio

n(㎨

)

second(ms)

rounddown(X,0)

-15

-10

-5

0

5

10

15

19 131

262

402

532

691

843

985

1151

1272

1402

1534

1662

1781

1915

2051

2183

2352

2503

2643

2763

2893

Acc

eler

atio

n(㎨

)

second(ms)

X

Y

• Original acceleration graph

• Round-down graph of the x-axis

532 msec (382 ~ 914)

Evaluation & Result

Detection failure of the backswing

29

-15

-10

-5

0

5

10

15

1 9 17 25 33 41 49 57 65 73 81 89 97 105

113

121

129

137

145

153

161

169

177

185

193

201

209

217

225

233

241

249

Acc

eler

atio

n(㎨

)

second(ms)

Rounddown(X,0)

Too short non-zero area (only two non-zero values)

-15

-10

-5

0

5

10

15

20 172

342

522

702

876

1071

1257

1446

1622

1832

2042

2272

2462

2643

2833

Acc

eler

atio

n(㎨

)

second(ms)

No non-zero area

Evaluation & Result

-30

-20

-10

0

10

20

3022

1212

2396

3553

4753

5953

7043

8142

9232

1039

311

512

1259

313

653

1473

315

833

1696

318

072

1914

6

Acc

eler

atio

n(㎨

)

second(ms)

No zero area

The problem of choosing threshold Detected different part of a swing according to threshold values

30

-15

-10

-5

0

5

10

15

3910

6421

4132

7343

6155

2267

2278

8290

3210

142

1124

212

422

1350

214

652

1578

316

874

1800

219

132

Acc

eler

atio

n(㎨

)

second(ms)

X

Y

-15

-10

-5

0

5

10

15

3910

6421

4132

7343

6155

2267

2278

8290

3210

142

1124

212

422

1350

214

652

1578

316

874

1800

219

132

Acc

eler

atio

n(㎨

)

second(ms)

X

Y

Threshold = 5㎨ and -5㎨ Threshold = 10㎨ and -10㎨

Evaluation & Result

Conclusion and Future work Conclusion – This study has showed the design and implementation of an

Android application. • measured golfers’ movement and identified critical points of the golf

swing using an accelerometer – This thesis can provide simple and convenient way

• without expensive and complicated devices • useful to individuals and amateurs

Future work – Selecting threshold values according to golfers’ skill

• A heuristic algorithm is required to select golfers’ suitable threshold values

– New approaches to improve identifying the backswing movement • measuring other parts of the body • using other sensors together

31

References [1] Hiroshi Negoro, Masahiko Ueda, Kajiro Watanabe, Kazuyuki Kobayashi, and Yosuke

Kurihara, “Measurement and Analysis of Golf Swing Using 3D Acceleration and Gyroscopic Sensors”, SICE Annual Conference 2011, pp 1111-1114.

[2] Robert D. Grober, “Measuring Tempo, Rhythm, Timing, and the Torques that Generate Power in the Golf Swing”, arXiV: 1001.1137v1, January, 2010

[3] http://blog.snowlove.co.kr/30056382913 [4] AChartEngine, http://www.achartengine.org/

32

33

Questions?

Analysis of the golf swings The maximum and minimum values of 8 golfers The two threshold values are decided based on the collected data – Threshold values: 5㎨ and -5 ㎨

36

-25

-15

-5

5

15

25

Golfer1 Golfer2 Golfer3 Golfer4 Golfer5 Golfer6 Golfer7 Golfer8 AverageMax of X 19.16016 18.58535 18.24047 13.37379 14.06356 6.82102 14.06356 5.32652 13.704304

Min of X -17.32078 -19.00688 -13.56539 -18.93023 -16.51606 -12.60738 -15.94125 -9.42680 -15.41435

Max of Y 9.55176 9.82 19.62000 9.82 8.89031 8.24887 6.21789 3.80371 9.4965675

Min of Y -6.16957 -21.56434 -12.45410 -14.43676 -13.22051 -14.28348 -12.6357 -3.28555 -12.25625

Acc

eler

atio

n (㎨

)

The maximum and minimum acceleration values

One swing data

37

-20

-15

-10

-5

0

5

10

15

20

6443

764

616

6483

065

136

6532

565

536

6574

665

984

6616

466

350

6654

766

736

6694

6

Acc

eler

atio

n(㎨

)

second(ms)

Player1

X

Y-9.8

Z

-20

-15

-10

-5

0

5

10

15

20

1590

916

079

1624

816

440

1665

116

850

1704

017

220

1739

017

580

1776

017

920

1809

318

300

Acc

eler

atio

n(㎨

)

Player2

X

Y-9.8

Z

second(ms)

-20

-15

-10

-5

0

5

10

15

20

2299

123

201

2341

123

640

2387

924

121

2435

024

581

2478

025

011

2524

225

441

2571

125

920

2615

9

Acc

eler

atio

n(㎨

)

second(ms)

Player 3

X

Y-9.8

Z

-20

-15

-10

-5

0

5

10

15

20

1 28 55 82 109

136

163

190

217

244

271

298

325

Acc

eler

atio

n(㎨

)

second(ms)

Player 4

X

Y-9.8

Z

One swing data (cont’d)

38

-20

-15

-10

-5

0

5

10

15

20

4563

245

901

4619

146

560

4686

147

211

4749

147

835

4812

248

392

4871

249

062

4944

0

Acc

eler

atio

n(㎨

)

second(ms)

Player 6

X

Y-9.8

Z

-20

-15

-10

-5

0

5

10

15

2036

449

3674

037

055

3738

037

681

3796

138

232

3852

138

821

3919

939

531

3986

640

161

4047

140

821

Acc

eler

atio

n(㎨

)

second(ms)

Player 5

X

Y-9.8

Z

-20

-15

-10

-5

0

5

10

15

20

2477

625

005

2530

325

553

2582

526

105

2635

726

605

2685

527

096

2732

527

596

2787

528

148

2846

3

Acc

eler

atio

n(㎨

)

second(ms)

Player 7

X

Y-9.8

Z

-20

-15

-10

-5

0

5

10

15

20

1 30 59 88 117

146

175

204

233

262

291

320

Acc

eler

atio

n(㎨

)

second(ms)

Player 8

X

Y-9.8

Z

Detection failure of the backswing

39

-15

-10

-5

0

5

10

15

20 162

322

492

662

824

1013

1171

1343

1512

1692

1884

2102

2316

2485

2653

2833

3017

swing_0523124421

X Y

-15

-10

-5

0

5

10

15

15 189

350

555

715

876

1059

1226

1398

1556

1726

1906

2106

2297

2486

2657

2837

swing_0523124638

X Y

-15

-10

-5

0

5

10

15

1 16 31 46 61 76 91 106

121

136

151

166

181

196

211

226

241

Rounddown(X,0)

Rounddown(X,0)

Too short non-zero area (only two non-zero values)

-15

-10

-5

0

5

10

15

20 162

322

492

662

824

1013

1171

1343

1512

1692

1884

2102

2316

2485

2653

2833

3017

rounddown(X,0)

No non-zero area

Evaluation & Result

-30

-20

-10

0

10

20

30

2210

6521

5232

3442

3253

6363

8273

5383

3393

1310

383

1135

312

343

1334

414

262

1528

216

261

1730

218

262

1921

2

Rounddown(X)

Rounddown(X)

Detection failure of the backswing

40

-30

-20

-10

0

10

20

30

2212

9225

7237

6350

9263

1374

8386

5398

4311

042

1222

213

403

1453

315

704

1689

318

086

1921

2

swing_0619141321

X Y

-20-15-10-505

101520

16 152

302

456

612

792

958

1131

1327

1511

1691

1874

2072

2242

2422

2612

2813

2983

swing_0523124907

X Y

-20-15-10-505

101520

16 142

282

422

566

732

882

1042

1217

1391

1551

1724

1882

2072

2227

2392

2573

2753

2903

Rounddown(X)

Rounddown(X)

Cannot find the zero area

No non-zero area

Evaluation & Result