Overcoming vector control challenges

Dec 5, 2007 10:52 AM

Vector control (also called field-oriented control) combined with DSPs and low-count encoders offer practical solutions to many motion control problems.

How it works

When the tuning procedure begins, the speed controller is bypassed and its function preformed by the tuning block. Based on the amplitude of i_ds and i_qs, motor current amplitude will change in order to maintain constant motor speed, with d and q current amplitudes being equal. In this step, produced torque is controlled by the change in slip frequency gain.

In the next step, slip frequency is changed linearly, such that i_ds and i_qs vary but do not exceed their maximum allowed value. During this change, the minimum value of the i_qs (=i_ds) and corresponding slip gain are recorded.

During these two steps, speed is regulated and remains constant. The slip frequency gain obtained at the second step is the tuned one. By obtaining the tuned slip frequency gain, the tuning block is removed from the control system and the speed controller is placed back in the control loop. Before closing the loop, the output of speed controller must be adjusted to match the current and avoid any sudden jump in the control system.

Besides tuning the slip frequency gain, this procedure can be used to achieve maximum torque per stator ampere whenever the motor is running in a steady state condition. In order to avoid saturation, however, which results in problems such as overheating, i_ds should not exceed its rated value.

Developed torque must remain constant during this procedure. To keep it constant under steady state load, the algorithm regulates the speed at its reference value in the low-speed region and calculates the developed torque in the high-speed region. However, during the transition - or in case of load change at low speed - the torque will change in order to regulate

speed, and the tuning procedure will not be valid.

To detect transients in the load in the low-speed region and avoid running the self-tune procedure, a halt signal is sent to the tuning block if it detects a change of more than 5% in calculated developed torque. In this case, the tuning block stops tuning until the next tuning period. The developed torque can be calculated as:

where

Getting more from field-oriented control

Determining rotor position under all circumstances and at a reasonable cost is an on-going engineering challenge, particularly since the operating environment of induction motors can be both harsh and variable over time. What's most affected by environment are rotor and stator time constants. These typically vary with time and are particularly sensitive to temperature-induced changes in rotor resistance.

Over the full operating temperature range of a typical induction machine, rotor resistance can easily vary by about 50%, corresponding to a 33% change in the rotor time constant. Without employing some form of self-tuning technique, the torque degradation in the "bare bones" field-oriented control can be on the order of 29% or more. From a design perspective, this means that in many applications where vector control is used to save energy, the motor must still be oversized. Despite the advantages of field-oriented control, much remains to be gained in terms of motor size and cost.

By adding self-tuning algorithms, it is possible to limit torque degradation to between 3% and 7%. For most applications, this is an acceptable range, meaning that a typical induction machine application can be designed with a smaller, cheaper motor, thus resulting in cost savings at the same time as improving the performance.

Dialed in for motors

Implementing advanced field-oriented control algorithms demands significant computational bandwidth. Because of this requirement field-oriented control has been overly complex, costly, and restricted to a narrow range of applications. The advent of modern digital signal processors, however, has made field-oriented control the relatively straightforward solution that it is today.

DSP-based controllers incorporate very powerful number crunching cores: up to 32-bit, 150 MIPS for motor control customized DSPs. With such power, DSPs can implement control laws and reference-frame translations at high sample rates, resulting in very high current loop bandwidths. This, in turn, ensures fast servo response times and precise transient control.

The key distinguishing feature of DSP-based controllers is their bus architecture, which allows the efficient movement of data to and from the DSP's single-cycle multiply-and-accumulate processing cores. Lightning fast, DSPs can perform rotor positioning and speed calculations in real time without lookup tables. Some DSPs even incorporate motor control peripherals such as PWM outputs, a/d converters, flash memory, and on-chip CAN bus.

Keeping pace with hardware, and just as important, are software advances. Today's extremely efficient C compilers let developers create object code nearly as compact as native assembly language. The compilers yield excellent performance and C-to-ASM ratios of 1.1, which means that designers are free to think creatively about their systems and how to improve them.

For more information, contact:
Kedar Godbole,
Motion Control Strategist,
Texas Instruments,
via e-mail at controlanswers@list.ti.com