Detect hard taps anywhere on iPhone through accelerometer

Question

I am trying to detect the taps which could be anywhere on iPhone not just iPhone screen. Here is a link which shows that it is possible.

Basically what i want to do is send an alert if user taps 3 times on iPhone while the Phone is in his pocket. What i have achieved is that i can detect the 3 taps but i also get the false alerts as well in these cases. 1) if user walking, 2) waving his phone 3) running. I need to just check if user has hit his iPhone 3 times.

Here is my code.

- (void)accelerometer:(UIAccelerometer *)accelerometer
        didAccelerate:(UIAcceleration *)acceleration
{
    if (handModeOn == NO)
    {
        if(pocketFlag == NO)
            return;
    }

    float accelZ = 0.0;
    float accelX = 0.0;
    float accelY = 0.0;

    accelX = (acceleration.x * kFilteringFactor) + (accelX * (1.0 - kFilteringFactor));
    accelY = (acceleration.y * kFilteringFactor) + (accelY * (1.0 - kFilteringFactor));
    accelZ = (acceleration.z * kFilteringFactor) + (accelZ * (1.0 - kFilteringFactor));

        self.z.text = [NSString stringWithFormat:@"%0.1f", -accelZ];

        if((-accelZ >= [senstivity floatValue] && timerFlag) || (-accelZ <= -[senstivity floatValue] && timerFlag)|| (-accelX >= [senstivity floatValue] && timerFlag) || (-accelX <= -[senstivity floatValue] && timerFlag) || (-accelY >= [senstivity floatValue] && timerFlag) || (-accelY <= -[senstivity floatValue] && timerFlag))
        {
            timerFlag = false;
            addValueFlag = true;
            timer = [NSTimer scheduledTimerWithTimeInterval:1.5 target:self selector:@selector(timerTick:) userInfo:nil repeats:YES];
        }

        if(addValueFlag)
        {
            if (self.xSwitch.on)
            {
                NSLog(@"X sWitch is on");
                [self.accArray addObject:[NSNumber numberWithFloat:-accelX]];
            }
            if (self.ySwitch.on)
            {
                NSLog(@"Y Switch is on");
                [self.accArray addObject:[NSNumber numberWithFloat:-accelY]];
            }
            if (self.zSwitch.on)
            {
                NSLog(@"Z Switch is on");
                [self.accArray addObject:[NSNumber numberWithFloat:-accelZ]];
            }

        }
    //}
}

- (void)timerTick:(NSTimer *)timer1
{
    [timer1 invalidate];
    addValueFlag = false;
    int count = 0;

    for(int i = 0; i < self.accArray.count; i++)
    {
        if(([[self.accArray objectAtIndex:i] floatValue] >= [senstivity floatValue]) || ([[self.accArray objectAtIndex:i] floatValue] <= -[senstivity floatValue]))
        {
            count++;
            [self playAlarm:@"beep-1" FileType:@"mp3"];
        }

        if(count >= 3)
        {
            [self playAlarm:@"06_Alarm___Auto___Rapid_Beeping_1" FileType:@"caf"];
            [self showAlert];
            timerFlag = true;
            [self.accArray removeAllObjects];
            return;
        }
    }
    [self.accArray removeAllObjects];
    timerFlag = true;
}

Any help will be really appreciated.

Thanks

Haywoodhayyim

This is exactly what I am looking for as well! Did you ever find a solution? – Basin

@sajoo did you ever find a solution to this! Would love to talk about this. My email should be in my profile. Good luck! – Basin

@Basin Yes i have achieved it. I,ll try posting my solution when i get back to my system. or if you want to talk to me then here is my email Id. [email protected]. – Haywoodhayyim

Answer 1

Here is how i achieved it.

- (void)accelerometer:(UIAccelerometer *)accelerometer
        didAccelerate:(UIAcceleration *)acceleration
{
    if (pause)
    {
        return;
    }
    if (handModeOn == NO)
    {
        if(pocketFlag == NO)
            return;
    }

//  float accelZ = 0.0;
//  float accelX = 0.0;
//  float accelY = 0.0;

    rollingX = (acceleration.x * kFilteringFactor) + (rollingX * (1.0 - kFilteringFactor));
    rollingY = (acceleration.y * kFilteringFactor) + (rollingY * (1.0 - kFilteringFactor));
    rollingZ = (acceleration.z * kFilteringFactor) + (rollingZ * (1.0 - kFilteringFactor));

    float accelX = acceleration.x - rollingX;
    float accelY = acceleration.y - rollingY;
    float accelZ = acceleration.z - rollingZ;

    if((-accelZ >= [senstivity floatValue] && timerFlag) || (-accelZ <= -[senstivity floatValue] && timerFlag)|| (-accelX >= [senstivity floatValue] && timerFlag) || (-accelX <= -[senstivity floatValue] && timerFlag) || (-accelY >= [senstivity floatValue] && timerFlag) || (-accelY <= -[senstivity floatValue] && timerFlag))
    {
        timerFlag = false;
        addValueFlag = true;
        timer = [NSTimer scheduledTimerWithTimeInterval:1.5 target:self selector:@selector(timerTick:) userInfo:nil repeats:YES];
    }

    if(addValueFlag)
    {
        [self.accArray addObject:[NSNumber numberWithFloat:-accelX]];
        [self.accArray addObject:[NSNumber numberWithFloat:-accelY]];
        [self.accArray addObject:[NSNumber numberWithFloat:-accelZ]];
    }
}

Haywoodhayyim

Description of that Code is given below. I have added a sensitivity settings in my settings, where user can set the accelerometer sensitivity. So if i get the value equals to or greater then that sensitivity. i start a timer for 1.5 seconds and then i check other values in between that 1.5 seconds. Once my timer is over i now have the values of accelerometer in the array, which are equals or greater then the sensitivity. and then in timerTick method i check from the array if there are 3 to 4 values are in the array which i needed. I trigger Alert method that the hard tap is detected. – Haywoodhayyim

Answer 2

You should apply a high pass filter to the accelerometer data. That will give you just the spikes in the signal - sharp taps.

I did a quick search on "UIAccelerometer high pass filter" and found several hits. The simplest code takes a rolling average of the accelerometer input, then subtracts that average from the instantaneous reading to find sudden changes. There are no doubt more sophisticated methods as well.

Once you have code that recognizes sharp taps, you'll need to craft code that detects 3 sharp taps in a row.

Phocaea

Answer 3

This is, as suggested by another answer, all to do with filtering the taps from the stream of accelerometer data. The impulse-like tap's will have a characteristic spectrogram (combination of frequencies) that can be detected when the response from a proper filter is higher than a threshold.

This is a very common operation on iPhone, I would suggest you look at official documentation such as here

The sample code I have linked to gives you two important things: official example code for high-pass filter AND a sample app that will graph the accelerometer data. This you can use to visual your taps, steps and jumps, to better understand why your filter responds falsely.

Furthermore, the internet is a huge source of literature on filter design - if you need to make a very high quality filter, you may need to consult the literature. I think however that a suitable second order filter would likely be sufficient.

@implementation HighpassFilter

- (id)initWithSampleRate:(double)rate cutoffFrequency:(double)freq
{
    self = [super init];

    if (self != nil)
    {
        double dt = 1.0 / rate;
        double RC = 1.0 / freq;
        filterConstant = RC / (dt + RC);
    }

    return self;    
}

- (void)addAcceleration:(UIAcceleration *)accel
{
    double alpha = filterConstant;   

    if (adaptive)
    {
        double d = Clamp(fabs(Norm(x, y, z) - Norm(accel.x, accel.y, accel.z)) / kAccelerometerMinStep - 1.0, 0.0, 1.0);
        alpha = d * filterConstant / kAccelerometerNoiseAttenuation + (1.0 - d) * filterConstant;
    }

    x = alpha * (x + accel.x - lastX);
    y = alpha * (y + accel.y - lastY);
    z = alpha * (z + accel.z - lastZ);

    lastX = accel.x;
    lastY = accel.y;
    lastZ = accel.z;
}

- (NSString *)name
{
    return adaptive ? @"Adaptive Highpass Filter" : @"Highpass Filter";
}

@end

Importantly this filter is direction agnostic, since only the magnitude of the acceleration is filtered. This is crucial to make the response seem normal. Otherwise users may feel like the have to tap from different angles to find a sweetspot.

On another note, if this task is proving too difficult and fiddly, I strongly suggest capturing your data ( in a WAV file for example) and using one of any common signal anaylsing program to get a better idea of where it is going wrong. See Baudline

Mapes

Answer 4

Some good answers. Here's some working code. I implemented this as a subclass of UIGestureRecognizer so that you can just drop it in and attach it to a UIView or UIButton. Once triggered, it will have the "pressure" set to a float between 0.0f and 2.0f. You can optionally set the minimum and maximum pressures required to recognize. Enjoy.

#import <UIKit/UIKit.h>

#define CPBPressureNone         0.0f
#define CPBPressureLight        0.1f
#define CPBPressureMedium       0.3f
#define CPBPressureHard         0.8f
#define CPBPressureInfinite     2.0f

@interface CPBPressureTouchGestureRecognizer : UIGestureRecognizer <UIAccelerometerDelegate> {
@public
float pressure;
float minimumPressureRequired;
float maximumPressureRequired;

@private
float pressureValues[30];
uint currentPressureValueIndex;
uint setNextPressureValue;
}

@property (readonly, assign) float pressure;
@property (readwrite, assign) float minimumPressureRequired;
@property (readwrite, assign) float maximumPressureRequired;

@end


//
//  CPBPressureTouchGestureRecognizer.h
//  PressureSensitiveButton
//
//  Created by Anthony Picciano on 3/21/11.
//  Copyright 2011 Anthony Picciano. All rights reserved.
//
//  Redistribution and use in source and binary forms, with or without
//  modification, are permitted provided that the following conditions
//  are met:
//  1. Redistributions of source code must retain the above copyright
//     notice, this list of conditions and the following disclaimer.
//  2. Redistributions in binary form must reproduce the above copyright
//     notice, this list of conditions and the following disclaimer in the
//     documentation and/or other materials provided with the distribution.
//  
//  THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
//  IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
//  OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
//  IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
//  INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
//  NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
//  DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
//  THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
//  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
//  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
//

#import <UIKit/UIGestureRecognizerSubclass.h>
#import "CPBPressureTouchGestureRecognizer.h"

#define kUpdateFrequency            60.0f
#define KNumberOfPressureSamples    3

@interface CPBPressureTouchGestureRecognizer (private)
- (void)setup;
@end

@implementation CPBPressureTouchGestureRecognizer
@synthesize pressure, minimumPressureRequired, maximumPressureRequired;

 - (id)initWithTarget:(id)target action:(SEL)action {
self = [super initWithTarget:target action:action];
if (self != nil) {
   [self setup]; 
}
return self;
}

 - (id)init {
self = [super init];
if (self != nil) {
    [self setup];
}
return self;
}

- (void)setup {
minimumPressureRequired = CPBPressureNone;
maximumPressureRequired = CPBPressureInfinite;
pressure = CPBPressureNone;

[[UIAccelerometer sharedAccelerometer] setUpdateInterval:1.0f / kUpdateFrequency];
[[UIAccelerometer sharedAccelerometer] setDelegate:self];
}

#pragma -
#pragma UIAccelerometerDelegate methods

-(void)accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration {
int sz = (sizeof pressureValues) / (sizeof pressureValues[0]);

// set current pressure value
pressureValues[currentPressureValueIndex%sz] = acceleration.z;

if (setNextPressureValue > 0) {

    // calculate average pressure
    float total = 0.0f;
    for (int loop=0; loop<sz; loop++) total += pressureValues[loop]; 
    float average = total / sz;

    // start with most recent past pressure sample
    if (setNextPressureValue == KNumberOfPressureSamples) {
        float mostRecent = pressureValues[(currentPressureValueIndex-1)%sz];
        pressure = fabsf(average - mostRecent);
    }

    // caluculate pressure as difference between average and current acceleration
    float diff = fabsf(average - acceleration.z);
    if (pressure < diff) pressure = diff;
    setNextPressureValue--;

    if (setNextPressureValue == 0) {
        if (pressure >= minimumPressureRequired && pressure <= maximumPressureRequired)
            self.state = UIGestureRecognizerStateRecognized;
        else
            self.state = UIGestureRecognizerStateFailed;
    }
}

currentPressureValueIndex++;
 }

 #pragma -
 #pragma UIGestureRecognizer subclass methods

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
setNextPressureValue = KNumberOfPressureSamples;
self.state = UIGestureRecognizerStatePossible;
}

- (void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event {
self.state = UIGestureRecognizerStateFailed;
}

- (void)reset {
pressure = CPBPressureNone;
setNextPressureValue = 0;
currentPressureValueIndex = 0;
}

 @end

Greeting

have not you read the original question and problem at all...? how can it be a possible solution for that? – Lashawna

@Lashawna Apologies even now i am wondering who post it from my account . update the answer – Greeting