Measure Pulse Width Using NI Devices

This example shows how to measure the width of an active high pulse. A sensor is used to measure distance from a surface. The width of the pulse is correlated with the measured distance.

Create a Counter Input Channel

Create a DataAcquisition object, and add a counter input channel with PulseWidth measurement type. For this example, use CompactDAQ chassis NI c9179 and module NI 9402 with ID cDAQ1Mod5.

dq = daq("ni");
ch = addinput(dq, "cDAQ1Mod5", "ctr0", "PulseWidth");

Determine the Terminal of the Counter Input Channel

To connect the input signal to the correct terminal, examine the Terminal property of the channel. The terminal is determined by the hardware.

ch.Terminal

ans =

    'PFI1'

Measure Distance

To determine if the counter is operational, acquire a single scan. This example uses a sensor that generates a high pulse of width 0.0010 seconds when the corresponding distance measured is one meter. To obtain the distance measured in meters, multiply the pulse width by 1000.

1000*read(dq, "OutputFormat", "Matrix")

ans =

     0.5

Measure Distance over Time

Use the hardware clock to acquire multiple counter measurements over time. NI counter devices require an external clock. By adding an analog input channel for a module on the same chassis, the internal clock is shared with both modules.

addinput(dq, "cDAQ1Mod5", "ctr0", "PulseWidth");

dq.Rate = 1;
data = read(dq, seconds(10));
plot(data.Time, 1000*data.cDAQ1Mod5_ctr0);
xlabel("Time (s)");
ylabel("Distance (m)");