How to Control Motor Speed Using a PID Loop, Pulse Width Modulation and a High Speed Counter

Go online with the PLC

Connect to the PLC and place it in RUN mode. The PLC must be online before the HSC configuration window can be opened and edited.

You cannot configure the HSC module offline.

Open the HSC module configuration window

Under the local I/O chassis in MapleLogic, double-click the IO-SHSC02 module to open the High-Speed Counter configuration GUI.

Confirm slot assignment and counter mode

Verify that the slot is automatically detected correctly. In this example, the HSC module is installed in Slot 2. Then select the counter mode. This example uses Linear mode so the counter tracks pulses in one direction.

To learn more about Linear and Ring counter modes, review the high-speed counter tutorial linked earlier.

Set the counter function and sampling method

Select Periodic Pulse Count in the Counter Function Setup. Then set the sampling time used to periodically capture encoder counts.

If no counter function is selected, the module will not return a usable count.

Configure encoder input type

In Input Pulse Setup, select 2-Phase, Multiple of 4 for the quadrature encoder used in this project.

This setting matches the encoder feedback method used in the demo.

Write settings and view live status

Click Write to save the module settings, then click Status to confirm live values are being read from the encoder.

Open the PID configuration window

Open the PID configuration window in MapleLogic for the control loop used in this project.

Review the Maple Systems PID tutorial if you need a refresher on each parameter.

Set a low sampling time

Configure a low sampling time for this demo so the PID loop reacts quickly to changes in motor speed.

Tune Kp and Ki values

Set low Kp and Ki values. In this sample project, those values were determined through trial and error and may differ for your application.

Your final PID values will depend on the motor, encoder, mechanics, and load.

Set MV limits

Set the Manipulative Value low limit to 15 and the high limit to 60. Even though the PWM module can output 0–100% duty cycle, the motor in this example only responds meaningfully between 15% and 60%. Set the MV change rate limit to update every 5 seconds.

These limits were determined experimentally for the motor used in the demo.

Leave the initial set value at zero

Set the initial set value to 0. In this project, the actual target speed is assigned later by ladder logic automation.

Enable PWM output

Enable PWM output channel 4, initialize the PWM output frequency, and set the duty cycle ramp time. In this project, rung 2 enables the PWM module output CH4, rung 4 initializes the PWM frequency, and rung 6 sets a 2-second duty cycle ramp time.

For more information, review the PWM tutorial linked earlier.

Enable and start the high-speed counter

Use HSC module internal memory bits to enable the count and start the high-speed counter. In this example, Y44 and Y46 are enabled because the HSC module is installed in Slot 2 of the I/O chassis.

These registers must be enabled from the HSC module internal memory.

Read encoder counts from HSC shared memory

Use DFRO instructions to retrieve the current count, previous pulse count, and current pulse count from HSC shared memory. These are 32-bit values, so double-word instructions are required.

Reading buffer memory 0 also reads 1, buffer 8 also reads 9, and buffer 10 also reads 11.

Calculate RPM from encoder pulses

Subtract the current pulse count from the previous pulse count, divide the result by 40, and multiply by 60 to calculate RPM. The final RPM value is stored in D225.

The value 40 represents 40 pulses over 10 milliseconds based on the configured sampling time.

Move RPM into the PID process value

Send the calculated RPM value in D225 into the PID Process Value register D101 so the PID loop can compare actual speed to the target speed.

Create an automated setpoint example

Use ladder logic to move different RPM set values into D100 based on timing and conditions. In this sample, the motor starts at 4500 RPM, then moves to 6000 RPM, then drops to 5000 RPM after timed conditions are met.

This simulates a conveyor application responding to changing load conditions.

Move PID output to PWM duty cycle

Send the PWM duty cycle register for CH4 into the PID Manipulative Value so the PID loop directly controls motor speed through PWM duty cycle changes.

Review the PWM logic

Open the PWM ladder logic in MapleLogic and confirm the output channel used in this project is CH4.

For additional background, review the Pulse Width Modulation tutorial before continuing.

Enable the PWM output channel

Use rung 2 to enable PWM module output CH4 so the PLC can begin driving the motor control output.

Initialize the PWM frequency

Use rung 4 to initialize the PWM output frequency in hertz for CH4.

Set the PWM duty cycle ramp time

Use rung 6 to initialize the duty cycle ramp time to 2 seconds for smoother motor response.

Review HSC internal memory

Open the MapleLogic help files and review the HSC module internal memory required to enable and start counting.

The required memory bits depend on the installed slot position of the HSC module.

Enable the HSC count

Use rung 8 to enable the HSC count and start the module.

Use the correct internal memory bits

Enable Y44 and Y46 because the HSC module in this project is installed in Slot 2 of the I/O chassis.

These internal memory registers must be enabled for the counter to run correctly.

Review HSC shared memory

Open the MapleLogic help files and identify the HSC shared memory addresses for Current Count, Periodic Pulse Count, and Current Pulse Count.

Read encoder counts with DFRO

Use the first three lines of rung 10 to execute DFRO instructions that read the current count, previous pulse count, and current pulse count from the encoder.

DFRO is a double-word FROM instruction and must be used because these values are 32-bit.

Verify double-word reads

Confirm that buffer memory 0 also reads 1, buffer memory 8 also reads 9, and buffer memory 10 also reads 11 when retrieving encoder data.

A normal single-word read will not return the correct encoder value.

Subtract the pulse counts

Subtract the current pulse double word D210 from the previous pulse double word D205 and store the result in D215.

Scale the pulse count

Divide D215 by 40 and store the result in D220. In this project, 40 represents 40 pulses over 10 milliseconds based on the configured HSC sampling time.

Convert the result to RPM

Multiply D220 by 60 to convert the scaled value into rotations per minute and store the final RPM result in D225.

Review the PV transfer logic

Open the rung that moves the calculated RPM into the PID Process Value register.

Write RPM into the PID PV register

Use rung 12 to move the RPM value in D225 into the PID Process Value register D101.

Set the initial target speed

Use rung 16 to move 4500 RPM into the set value register D100 when M00 is enabled.

Start the first timer condition

Use rung 17 to trigger a 15-second ON timer when D100 is greater than or equal to 4000.

Increase the setpoint to 6000 RPM

Use rung 18 to move 6000 RPM into D100 after the first 15-second timer completes.

Start the second timer condition

Use rung 19 to trigger another 15-second ON timer when D100 is greater than or equal to 5500.

Decrease the setpoint to 5000 RPM

Use rung 20 to move 5000 RPM into D100 after the second 15-second timer completes.

Review the final PID output rung

Open the rung that sends the PID output to the PWM duty cycle register.

Write the PID MV to the PWM duty cycle

Use rung 22 to send the CH4 PWM duty cycle register into the PID Manipulative Value so the PID loop can automatically control motor speed.

Establish online communication

Connect to the PLC using Online > Link+Download+Monitor so the project is running and available for live monitoring.

Open Mon-View

Click Mon-View to monitor the initial PID values before starting the logic.

Enable the motor logic

Highlight M00 and press Shift + Enter to initiate the motor and enable the HSC count, RPM calculation, PID process value updates, and PWM duty cycle control.

Monitor the 4500 RPM stage

When M00 is enabled, the process value begins oscillating toward the initial set value of 4500 RPM. Once the RPM value reaches 4000 or greater, a 15-second ON timer is triggered.

Monitor the 6000 RPM stage

After the first timer completes, the ladder logic moves the set value to 6000 RPM. The process value then oscillates toward the new target speed. When the RPM value reaches 5500 or greater, another 15-second ON timer is triggered.

Monitor the 5000 RPM stage

After the second timer completes, the ladder logic moves the set value to 5000 RPM. The process value then adjusts and oscillates toward the final target speed.