By Jerry Lin, Aerotech Taiwan
When a user finds that the accuracy of a motion control application is low, such as unacceptable following error due to friction, low frequency disturbance, etc., they will normally first increase the servo loop gains to reduce the following error. Unfortunately, this “quick fix” can result in excess gains that make the system unstable.
In general, autotuning tools do not fully optimize the system performance since they usually can only increase or decrease servo loop gains, but cannot automatically apply filters at resonant frequencies to ensure the system is stable and maximize the bandwidth. For this reason, control engineers use Bode plots to analyze system performance, stability, and to reduce following error – and this leads to our second conundrum. In order to analyze the system to resolve stability problems, control engineers need to have substantial control theory and control system experience. Normally equipment manufacturers will have control engineers on staff and they can tune systems in the factory before shipment. However, after machines are shipped, service engineers may take over and may face substantial tuning issues, but not have the vast experience of a long-time control engineer. Here’s where EasyTune® can help.
EasyTune includes the following functionality: (1) automatically adjust servo loop gains, (2) automatically adjust servo filters, and (3) automatically analyze system stability. EasyTune puts the capabilities of an experienced control engineer at your fingertips. It was extensively refined on many different motion systems including those with different bearings, drive technologies, and mechanical configurations. To get the benefit of all of these features, the user need only press one button. EasyTune then automatically analyzes the system, applies the same remedies as an experienced control engineer, and completes the task in just a few minutes.
The motion parameters in this example are as follows.
Stroke: 50 mm
Velocity: 500 mm/s
Acceleration: 9800 mm/s2
In-Position Distance: ±5 µm
Before using the EasyTune tool, move time is 153 ms, settling time is 210 ms, and the total move and settling time is 363 ms. The red line in Figure 1 shows us that the in-position threshold is 5 µm.
Figure 1. Example showing system before the application of EasyTune.
Figure 2. Move time is 153 ms.
Now it is time to apply EasyTune. First, as shown in Figure 3, the user selects the performance target: Aggressive, Normal, or Conservative. We will apply the Aggressive performance target.
Figure 3. We apply EasyTune’s Aggressive performance target.
Following the EasyTune prompts (Figures 4 and 5), we set travel distance and speed, identify the axis on our controller, and then press the “Start” button.
Figure 4. Setting travel distance and speed.
EasyTune then begins optimizing performance (Figures 6 and 7) by optimizing at different frequencies. When this is complete, the controller saves the new servo loop gains and filter parameters.
After using EasyTune, we then run the same motion parameter. Settling time has now been reduced from 210 ms to 58 ms (Figure 8). Move and settling time is 211 ms.
Figure 8. After using EasyTune tools, move and settling time is 211 ms.
How does EasyTune work? Let’s overlap two loop transmission plots. Looking at the plots, we find that crossover frequency is increased from 32 Hz to 105 Hz. Because the bandwidth increased, the following error has been reduced.
Figure 9. Overlap of two loop transmission plots.
EasyTune was extensively tested and refined on different motion systems. This tool can reduce settling in order to enhanced your machine’s throughput. It allows non-control system users to complete system tuning quickly and easily.
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