truAPM | Advanced Panel Meters by truAPM
truAPM | Advanced Panel Meters by truAPM

Application Notes

Date: 9th March 2015
Revision: 1st

The APM-FREQ meter is capable of measuring speed of a rotating wheel by using an inductive sensor looking at pulses from the sensor generated by non-magnetic teeth on a wheel. Such as the systems used in the automotive ABS sensor type applications


The APM Frequency Meter detects and measures the frequency of the output from the inductive sensor. The maximum frequency is 400Hz therefor consideration must be made when designing the system to ensure that the maximum frequency from the system will not exceed this.

To enable the APM Frequency Meter to accurately measure the frequency the threshold voltages must be correctly set and the following sections discuss each configuration in more detail.

Using the free APM configurator software the Frequency Meter needs be set to measure the pulses coming from the inductive pick up sensor, by setting the threshold voltages to trigger the counting circuitry such as in the diagram below:
Sensor Output Waveform


The upper threshold should be at least -10% of the maximum ­output voltage whilst the lower threshold should be at least 10% of the maximum output voltage, The period calculation is carried out over a 30mS sample period and an average is then calculated. Therefore any noise will lead to inaccurate calculations

The input impedance of the APM Frequency Meter is approximately 1.5MΩ

Showing RPM
In order to show a meaningful value on the display of the APM such as revolutions per minute

You must first calculate one over the number of pulses from the wheel in one revolution this figure must then be entered into the display scalar section of the APM Configurator software

For instance in the application below the wheel has 12 teeth

Therefore the scale required is 1 ÷ 12 = 0.083 this is then entered into the scale box which will then mean the scale on the APM can read true RPM
Using the Outputs
We can also use the APM-FREQ-AXO to transmit a 4-20 mA signal to indicate RPM to a PLC by setting one of the Alarm outputs to Analogue Output and then by setting the upper and lower calibration set points for the speed required as above