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Dealing with noisy motor: Traditional compensation

Posted: 11 Feb 2016 ?? ?Print Version ?Bookmark and Share

Keywords:vibration? noise? tuning?

Tuning challenges with the traditional approach
There are several items to take into consideration when tuning using the traditional approach. In this section we will talk through those tasks before implementing the traditional approach.

Load changes
When the load changes, the dynamics of the pulsating load might also change. This is due to the properties of the load itself. For example, in a compressor application, the pressure applied to the gas being compressed changes depending on how long the compressor has been running. As a result of that change, the compensation table might also change. In other words, the profile that we are trying to compensate for is non-linear. We will have several compensation tables for each operating point of the compressor, which consumes a lot of memory in a microcontroller. In order to show the impact of a change in load, consider figure 8, showing a compensated load. At 0.75 seconds, the torque pulsations are removed and the feed-forward term of the speed controller will introduce a vibration:

Figure 8: Compensated load showing a change in load (Source: Texas Instruments)

Speed changes
In a typical system, the load profile will change with the speed of the motor. As expected, a single compensation table is not enough to remove all compensation across the speed range of a motor. Figure 9 shows a single compensation table being applied, and at 200 rpm, the load profile changes to half the value.

Figure 9: A single compensation table being applied at 200 rpm (Source: Texas Instruments)

Zeroing the angle of the compensation table
Typically, a torque vs. angle load curve is contingent on the mechanical angle of the system since the load is a characteristic of the mechanical load attached to the motor shaft. Systems such as compressor applications do not have a mechanical sensor attached to the shaft, so all the system detects is the electrical angle of the motor. It is also standard to have multiple rotor poles, which create a ratio between the electrical and mechanical angle. If we apply a compensation table based on the mechanical angle, then the algorithm has to synchronise the compensation table angle with the mechanical load angle. This turns out to be challenging since there is no absolute position sensor that can be used for zeroing the compensation table.

In figure 10, we use a four pole-pair motor, with a torque vs. mechanical angle table. Keep in mind that in an FOC system, the electrical angle is the one used. An electrical angle of zero can translate into a mechanical angle of 0, 90, 180 or 270 degrees in a four pole-pair motor. If the electrical angle started at a 0 mechanical angle, then the table works great, but if it started at 180 degrees for example, then the table does not compensate well, as shown in figure 10. In fact, a difference of 180 degrees doubles the vibration:

Figure 10: A four pole-pair motor with a torque vs. mechanical angle table (Source: Texas Instruments)

At this point, it is apparent that a fixed-table approach is not a solution to this problem; instead, a dynamic solution that adapts to changes in the load profile will solve this problem. The second article in this two-part series will cover a new algorithm that studies the profile and applies its learning to the FOC system to compensate for a pulsating load.

About the author
Jorge Zambada is a Sr. Motor Control Systems Engineer at Texas Instruments. He has over 15 years of experience working on motor control applications for the semiconductor industry, including Freescale, Microchip and Texas Instruments. He has developed applications for refrigerator compressor controllers, air conditioning compressors, washing machines and automotive motor controllers. He is also the author of several articles and application notes related to motor control. Jorge holds a BSEE from ITESM Guadalajara, Mexico.

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