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Dealing with noisy motor: Dynamic compensation

Posted: 23 Feb 2016 ?? ?Print Version ?Bookmark and Share

Keywords:Field-Oriented Control? FOC? speed controller? pulse width modulator? pwm?

Part 1 of this two-part series discussed vibration of pulsating loads and offered an introduction to a traditional method to remove the vibration. Here in Part 2, a dynamic method is introduced, showing its performance in a simulation environment as well as running an actual motor performed on a bench using tools that can be purchased by any user.

Adaptive vibration compensation
The following diagram shows an entire Field-Oriented Control (FOC) system using the dynamic vibration compensation block.

Figure 1: Dynamic vibration compensation block (Source: Texas Instruments).

As can be seen in figure 1, a new block was added, called "dynamic vibration compensation". This block is used to learn the torque load and provide an output to be used by the feed-forward input of the speed controller. This algorithm requires four main blocks:
1. A speed controller with feed-forward input
2. A table to hold a learning curve
3. A way of extracting a specific member from that table based on an index
4. Calculation of an index to update the learning curve and an advanced index to extract a value from that table

Feed-forward speed controller
The first component to this algorithm is to have a speed controller that can accept a feed-forward term as a third input. Figure 2 is taken from a SciLab simulation, which shows how a speed controller with feed-forward is implemented:

Figure 2: A speed controller with feed-forward implemented (Source: Texas Instruments).

As can be seen, the input Ffwd is taken all the way to the final summation point, providing extra help to the speed controller, independent of the speed reference and the speed feedback.

Automatic load learning
The algorithm is able to dynamically learn a load profile based on two inputs:

1. Electrical angle information: Even though the mechanical angle is needed internally, it does not have to be aligned with respect to an absolute mechanical position as was the case with the traditional approach outlined in Part 1. The only requirement of the mechanical angle range is that it needs to cover an entire mechanical rotation regardless of its alignment with respect to the mechanical shaft. In this particular implementation of the algorithm, the mechanical angle is calculated within the module, using the electrical angle as the input and taking into account the number of poles in the motor. This is assuming a synchronous motor is present. If an asynchronous motor is being controlled (i.e. induction motor), then the mechanical angle calculation completed inside the module needs to take slip into account. An alternate method for induction motors is to use a mechanical sensor such as an encoder to find the mechanical angle.

2. A measured current value: In the case of FOC, Iq, is responsible for the torque of the motor.

Figure 3 shows the block's inputs:

Figure 3: Dynamic vibration compensation block inputs (Source: Texas Instruments).

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