Motion control servo closed loop PID parameter adjustment misunderstanding - Database & Sql Blog Articles

There is a common misunderstanding when adjusting PID parameters in motion control servo systems. Many people believe that simply tuning the P, I, and D values will improve performance, but this isn’t always the case.

The response time of a motion control system to a given value depends on more than just the PID settings. It also relates to the actuator's rated power and torque, as well as the load it has to handle—especially the inertia ratio.

Blindly adjusting PID parameters without considering these factors can lead to unstable or inefficient performance.

Let me give you a simple example that’s easy to understand:

I once designed a closed-loop PID control system for maintaining constant pressure in a heating boiler’s water supply. After tuning, the real-time pressure always stayed below the setpoint and rarely reached it.

When I increased the P parameter, the pump ran at 50Hz constantly, causing the pressure to exceed the setpoint and trigger the protection mechanism. Then the pressure dropped, and the pump would restart, creating an oscillation.

Let’s break down what happened:

1. The pump was rated for 10 square meters per second.

2. The PID output ranged from 0–10V, corresponding to 10–50Hz on the inverter.

3. The system pressure was set to P Pa.

4. At time t, the system lost 1, 2, 3, ..., up to 70 units of water.

5. Due to the water loss, the pressure deviated by ΔP, which caused the PID to output increasing voltages (1V, 2V, ..., 10V), leading to higher pump speeds.

6. The pump flow rate increased accordingly (1, 2, 3, ..., 10 m³/s), but the pressure still took longer to stabilize.

7. When the pump was upgraded to 100 m³/s, the response time significantly decreased, showing how critical the actuator's capacity is.

8. Even with the same PID settings, changing the pump flow and sensor range allowed for faster and more stable control over a wider range.

This demonstrates that the actuator's capabilities are crucial for consistent performance in a PID system. Without sufficient power and torque, even the best PID settings won't guarantee good results.

In many cases, servo systems suffer from limited rated power and torque, which restricts their ability to adapt to different loads and causes inconsistent response times.

Therefore, when designing a servo control system, it's essential to match the actuator’s specifications with the expected load conditions. Only then can the PID loop perform effectively across a wide range of operations.