= Speed measurement with IR sensor GP2Y0A02YKOF (Python) =

This tutorial guides you through calculating a relative velocity by using the distance measurement of the GP2Y0A02YKOF infrared sensor over the time.

In order to be able to measure a speed at all, we must first be able to measure a distance to an object. Ultimately, the infrared distance sensor can do no more. Therefore, first read [[Drivers/Tutorials/DistanceMeasurementWithIRSensorGP2Y0A02YKOFPython|Speed measurement with IR sensor GP2Y0A02YKOF (Python)]].

== Calculating the speed ==

Let's take another closer look at the triangle from the previous chapter:

{{https://lastminuteengineers.b-cdn.net/wp-content/uploads/arduino/Distance-Speed-Time-Formula-Triangle.png}}

So we need

{{{
Speed = Distance / Time
}}}

However, we do not want to go to the speed of sound now and check whether there is a deviation here.

I will show you how to measure the speed of movement of an object using this sensor. For that purpose we need to take two distance measurements in a short time apart and we have:

{{{
distance2 - distance1 = distance speed at a given time
}}}

If we make the measurements in a time period of 1 second, then we get the speed of movement of the object in cm/s.

When the object is moving in the opposite direction, the speed represented on the display has a negative sign.

== Programming the speed measurement ==

We change our code from the previous example as follows:

{{{
import spidev
import time
 
spi = spidev.SpiDev()
spi.open(0,0)
spi.max_speed_hz=1000000

def readChannel(channel):
    val = spi.xfer2([1,(8+channel)<<4,0])
    data = ((val[1]&3) << 8) + val[2]

    return data
  
if __name__ == "__main__":
    v1 = (readChannel(0)/1023.0)*3.3
    dist1 = (16.2537 * v1**4 - 129.893 * v1**3 + 382.268 * v1**2 - 512.611 * v1 + 301.439) * 0.01

    time.sleep(1)

    v2 = (readChannel(0)/1023.0)*3.3
    dist2 = (16.2537 * v2**4 - 129.893 * v2**3 + 382.268 * v2**2 - 512.611 * v2 + 301.439) * 0.01

    speed (dist2 - dist1) / 1.0

    print(speed) # m/s
}}}

Now that we know what we can measure with the GP2Y0A02YKOF and how, we can write our driver and wrap it in ROS in the next chapter. Please read for this [[Drivers/Tutorials/WritingYourGP2Y0A02YKOFDriverPython|Writing your GP2Y0A02YKOF driver (Python)]].