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

There are several common misunderstandings when it comes to adjusting PID parameters in motion control servo systems. One of the biggest misconceptions is that simply tuning the P, I, and D values will automatically improve system performance.

However, the response time of a motion control system to a setpoint isn't just about adjusting those parameters. It's also influenced by factors like the actuator's rated power and torque, as well as the load conditions, such as the inertia ratio of the system.

Blindly tweaking PID values without understanding the underlying mechanics can lead to instability, overshoot, or even system failure. So, it's important to consider all aspects of the system before making any adjustments.

Let me give you a simple example that illustrates this clearly:

I once designed a closed-loop PID system for controlling water pressure in a heating boiler. After the initial setup, the real-time pressure was always below the target value and rarely reached it.

When we increased the P parameter, the pump started running at 50Hz constantly. The pressure would sometimes exceed the setpoint and trigger the protection system, causing the pump to shut down. Then, the pressure dropped, and the pump restarted again.

So, what went wrong here? Let's break it down:

1. The pump was rated for a flow rate of 10 square meters per second.

2. The PID output voltage ranged from 0 to 10V, which corresponded to an inverter frequency of 10 to 50Hz.

3. The system pressure was set to P Pa.

4. At different times (t, t+1, t+2, ..., t+60), there was a loss of water in the system, causing the pressure to deviate from P by ΔP.

5. As the water loss increased, the feedback from the sensor caused the PID to output voltages ranging from 1V to 10V accordingly.

6. This led to the inverter frequency increasing from 5Hz to 50Hz, and the pump’s flow rate followed suit, rising from 1 to 10 m²/s.

7. The time it took for the pump to restore the pressure back to the setpoint varied, starting at 1 second and increasing over time.

If we replaced the pump with one that had a flow rate of 100 m²/s, the response time dramatically improved. For instance, it could reach the desired pressure in just 0.1 seconds under similar conditions.

Even more interesting: if we also expanded the pressure sensor’s range by 10 times, the response time remained consistent, regardless of the water loss. This showed that the actuator’s capacity—its power and flow rate—is the key factor in ensuring fast and stable control.

This means that no matter how well the PID parameters are tuned, if the actuator lacks sufficient power or torque, the system’s performance will be limited. That’s why many servo systems struggle with large loads or varying conditions.

In conclusion, when working with closed-loop PID systems, it’s crucial to understand that the actuator’s capabilities play a major role in determining the system’s response speed and stability. Without proper actuator support, even the best-tuned PID won’t deliver consistent results.