Are flux-vector servos worth it?

It's time for us to upgrade servo motor performance from our old and tired trapezoidal commutation amps. We have to get better torque control. We hear conflicting stories about whether brushless sin/cos commutation will perform just as well as, and maybe cheaper than, the newer flux-vector solutions. We'd like to hear some user point-counterpoint on this.

                                                            --- from May 2004 Control Design

Lack of Standards Hinders Comparison

Sin/cos commutation or flux-vector control of brushless permanent magnet motors can provide a number of performance advantages over trapezoidal commutation. The main sinusoidal benefits are smoother low-speed torque, higher peak torques at speed, and slightly higher continuous shaft torques. The smoother low-speed torque comes from less electromagnetic torque ripple and from eliminating the step changes in currents used in trapezoidal commutation. The higher peak torques at speed provide a squarer torque speed curve that allows for faster position move times.

The difficulty in comparing sin/cos commutation and flux-vector solutions is that there is no industry standard implementation of either method, and thus the amount of benefit either solution provides over trapezoidal commutation varies depending on implementation details. Even more confusing is that some products with very advanced current control loops have all the benefits of flux-vector technology, and in fact, are very similar in details, yet are advertised simply as sinusoidal controllers.

The real way to see if you get all the benefit of these advanced technologies is to carefully examine a torque speed curve. Effective implementations will have a fairly square peak-torque speed curve and the system top speed should be very close the maximum possible without phase advance/constant power, which is the AC line voltage in rms divided by the motor line-line constant K_E in V_rms/kRPM. The other way to tell is to look at the details of the current loop implementation and make sure it is described as being d-q control, stationary frame regulator or a rotate, de-rotate current controller.

To see the full performance benefit of these technologies, avoid products that have so-called stator frame current regulators,that is, products that do not mix the current regulator and the commutation into one block but instead generate a command signal (which is the desired sinusoidal current waveform in one block) and directly regulate the current in the motor to that command with a single, simple current loop. 

In terms of cost, these very simple implementations of sinusoidal current control are theoretically cheaper than the higher performance ones, but, in practice, modern servo amps have been cost optimized with all digital controls and there is essentially no cost penalty between doing it right and doing it the simpler lower performance way.

George Yundt, Advanced Development Manager--Drives

Danaher Motion, www.DanaherMotion.com

Play the Field-Oriented Control

Trapezoidal commutation is a six-step method, i.e., you create six magnetic vectors in a 360? rotation. This means that the torque varies significantly as the motor turns. The downside is that you end up with torque ripple. To the user, this means that the servo gain changes from point to point and it is harder to use in high-performance systems.

What is happening under the hood is that the commutation amplifier is presenting the speed and position control loops (typical outer loops) with a angle-varying gain, which is a non-linear function. This means that to avoid problems, you must back off the controller setting.

Using a less aggressive outer position or speed regulator means that you sacrifice performance, simply because the commutation scheme is not using the motor to its maximum potential.

A first approximation to solving this issue is to create a magnetic vector that moves continuously along with the rotor. (as opposed to once every 60 degrees). This produces less torque ripple, and, since the rotor and stator magnetic flux vectors are optimally aligned, produces the most torque per ampere input in to the motor.

A simple sine modulation scheme does not achieve this: When the motor moves rapidly, the currents will increasingly lag the applied voltages. This means that the flux alignment is non-optimal, and results in significant performance deterioration at higher speeds (Figure 1).

Figure 1: Sine Modulation