Canadian Space Agency algorithm makes motor control more efficient

February 19, 2014

Bernard Cole-February 19, 2014

I’ve just across a development that - if true - will likely catch the eye of motor control engineers and software developers in everything from industrial automation and robotics to automobile engine and drive chain applications.

Specifically, a small Canadian start-up called Alizem has released motor control software IP that it claims can improve torque and speed by up to 20 per cent. It is a commercialized version of algorithms invented and used by the Canadian Space Agency (CSA), the company said.

According to Dr. Marc Perron, President at Alizem, the algorithms incorporated into the company’s software Intellectual Property are meant to work with brushless direct current (BLDC) and permanent magnet synchronous (PMSM) motor control designs where energy-efficiency and reliability are critical.

He said this software can be ported to and run on any new or existing electric motor drive systems. Among the benefits he claims it brings to inverter-fed electric motor drive system designers are:

1) Optimal management of phase current distribution lowering losses and enhancing maximum achievable torque and speed by 20% leading to lower motor rating, weight, space and cooling requirements

2) Ripple-free torque leading to reduced mechanical stress, velocity fluctuation, noise and higher reliability and tracking accuracy in servo applications

3) Automatic fault recovery from a phase winding failure and/or phase voltage/current saturation

4) the ability to provide critical information regarding motor health to master systems in such application environments such as industrial control networks using many-to-many machine to machine connectivity

5) an application programming interface wrapper around the algorithms that allow for easier development as well as safe operation and which includes multiple debug modes for power stage, transducer and system protection verification for early bug detection.

Current uses of the algorithms, he said, are in critical applications ranging from airplane brake systems, robotics and electric vehicles to HVAC systems installed in a nuclear plant.

"In many of those applications, electric motors play a central role and failure may lead to catastrophic human and financial consequences,” said Perron. “This is why the motor control software installed in those motor drive systems must not only deliver excellent operational performance but must also be robust to motor damage in order to reduce the risk of failure and downtime.

"This also includes consuming as low energy as possible to maximize operation time when systems are operating from a limited energy source such as batteries or gas tank."

The software IP the company has commercialized appears to make use of -among other things - a novel back BLDC waveform that minimizes power dissipation, subject to current and voltage limits of the motor's drivers. When one or more phases reach their voltage and/or current saturation levels, the algorithms reshape stator currents of the remaining phases for continuing accurate, ripple-free torque production.

It automatically reshapes the excitation currents in such a way that the motor continues to generate the torque that is requested. For optimal phase currents at given angular position, the control algorithm allows the motor to operate above the rated speed and torque that would be achieved without current reshaping.

The company claims this optimal management of the motor's excitation currents can significantly increase the rated speed or torque of the motor in the face of voltage and current limits of the drivers. Perron said experiments show that the maximum torque capability is boosted by 20% when the phase saturation is considered in the phase current shape function.

In addition, the torque controller can be used as a remedial strategy to compensate for a phase failure by optimally reshaping the currents of the remaining healthy phases for accurate torque production. The control algorithm allows the motor to optimally generate precise torque, even when operating under a phase failure.

Traditional approaches
By comparison, in traditional BLDC motor designs, electric power is distributed by an electronically controlled commutation system using a feedback from the rotor angular position into a control system, which excites the stator coils of the motor in a specific order, in order to rotate the magnetic field generated by the coils to be followed along by the rotor. He said these conventional drivers of BLDC motors produce sinusoidal (or, alternatively, trapezoidal) current waveforms for smooth motor operation.

In practice, however, said Perron, non-ideal motors do not have a perfect sinusoidal distribution of magneto-motive force, and hence the sinusoidal commutation can result in torque ripple. Suppressing the torque ripple of the motor drive of the servo system, the company claims, can significantly improve system performance by reducing speed fluctuations.

The limitation of this approach is that the motor's drivers have fixed rated current and voltage limits, and some of them may not be able to deliver the current inputs dictated by the electronic commutator which may occur when the motor operates at high torque or speed.

Consequently, the performance of the torque production may significantly deteriorate as a result of the phase current distortions caused by the voltage or current saturation of the amplifiers. When there is a fixed inverter voltage and current, flux weakening allows a BLDC motor to operate above the base speed in constant-power, high-speed regions.

Below the rated speed, all of the stator currents can be used to produce torque. Above the rated speed, a part of the stator current must be used to oppose the permanent magnet flux while the remaining portion is used to produce torque.

Using optimal excitation currents
In the CSA algorithms the company uses, a closed-form solution for optimal excitation currents is used for accurate torque control of brushless motors that minimizes power dissipation, subject to current and voltage limits of a motor's drivers.

When the motor terminal voltages and/or phase currents reach their saturation levels, the controller automatically reshapes the excitation currents in such a way that the motor generates torque as requested. This optimal management of the motor's excitation currents, the company claims, can significantly increase the rated speed or torque of the motor in the face of the voltage and current limits of the drivers.

A novel aspect of this CSA technology, said the company, is the efficient use of non-linear feedback from the rotor's angular position and angular rate (speed) that makes accurate torque production possible when the voltage or current of one or more phases reach their saturation level, or when phase failure occurs. The CSA control algorithm permits torque sharing among phases when some phases saturate which results in a considerable increase in the attainable maximum motor torque.

In order to evaluate the performance of the optimal torque controller, experiments were conducted on a three-phase synchronous motor with nine pole pairs.

Perron said experiments show that the maximum torque capability is boosted by 20% when the phase saturation is considered in the phase current shape function. Experimental results obtained from a brushless servomotor under the proposed torque controller demonstrated accurate torque production under voltage/current saturation of the motor's drivers or failure of one phase.

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