How to Measure Motor Speed Using a High Speed Counter with a Quadrature Encoder

Review the main wiring diagram

Use the wiring diagram below to connect the encoder, motor, and PLC for this example setup.

Verify the PLC input terminals

Use the terminal diagram below to confirm the correct PLC input connections for the High Speed Counter signals.

Select the counter mode

In this sample, select a Linear Counter to count in one direction.

Select the input pulse type

Because this example uses a 2 Phase Quadrature Encoder, select “2 Phase, 4 Multiplication” as the Input Pulse Type.

Set Unit Time

Enter 1000 for Unit Time based on the number of pulses the encoder outputs in one revolution.

Set Pulse per 1 Cycle

Enter 1000 for Pulse per 1 Cycle, since this example uses 1000 pulses per rotation.

Set RPM or PPS mode

Set RPM to 1. If you want to read pulses per second instead, enter 0 for PPS.

Review the ladder diagram

Use the following ladder diagram as the basis for the High Speed Counter logic.

Enable the RPM flag

Use an ALWAYS_ON bit on the first rung to turn on the RPM Flag. This must be enabled to use RPM in the High Speed Counter configuration.

The instruction is [SET D116.5] because the HSC configuration starts at D100.

Locate the RPM status flag in MapleLogic Help

In the MapleLogic Help file, navigate to Built In High Speed Counter, then go to Parameters and scroll to the control bits. Look for register 16, Bit 5 for the RPM Status Flag.

Calculate the correct address

Because the starting address is D100, add 16 to get D116. Bit 5 makes the full address D116.5, so the application instruction is [SET D116.5].

Add the alarm compare instruction

On the second rung, use a compare instruction to turn on alarm output Y10 when motor speed is greater than or equal to 1800 RPM.

Go online and download the project

Download the project to the PLC and open the ladder diagram online view.

Enable the counter

Check “Enable Count” to initiate the High Speed Counter. This must be checked for the counter to operate.

Verify the RPM bit is enabled

Confirm that F10 is ON and turns on [SET D116.5] to enable the “ON : RPM” bit.

Monitor the High Speed Counter

Click “Monitor” to view the High Speed Counter while online.

Confirm counter status

After pressing Monitor, verify that both the “Enable Count” flag and the “ON : RPM” flag are turned on.

Observe encoder inputs with speed below the alarm threshold

In the live camera view, X00 and X01 are ON for the encoder Phase A and Phase B wires. The motor speed is below 1800 RPM, so alarm Y10 remains OFF.

Observe alarm activation above 1800 RPM

When the motor speed exceeds 1800 RPM, alarm Y10 turns ON. In the live camera feed, the red LED indicates the alarm output is active.

Observe alarm reset below 1800 RPM

When the motor speed drops back below 1800 RPM, alarm Y10 turns OFF again.

Observe reverse rotation and down count

A 2 Phase Quadrature encoder can count in both forward and reverse directions. When the motor runs in reverse, a negative RPM value appears in the ladder logic and the Down Count status flag turns ON.