Many applications would benefit from a brushless motor without a sensor. A method developed by maxon is now setting new standards for precision and reliability.
Driving a brushless motor requires control electronics for precise commutation. However, this is possible only if the control electronics ‘know’ the exact position of the rotor at all times. Traditionally, this information was provided by sensors installed inside the motor. But, it can be done differently. Sensorless control methods use current and voltage information from the motor to determine the rotor position. The motor speed can then be derived from changes in the rotor position and this information can be used for speed control. More advanced sensorless control methods can even control the torque and the position. Leaving out the sensors has a range of benefits, such as lower cost and space saving, because cables, connectors and sensitive electronic circuits become unnecessary.
Sensorless controllers by maxon use three basic principles that are adapted specifically to maxon BLDC motors.
Principle 1: EMF method with zero crossing
The EMF method with determination of the zero crossing uses induced voltage (or EMF) in the non-powered phase during block commutation. The zero crossing happens in the middle of the commutation interval. The time delay to the next commutation point can be estimated from the preceding commutation steps.
The EMF method with zero crossing works only when the speed is high enough, because EMF becomes zero at standstill. Starting up the motor requires a special process, similar to step motor control, and must be configured separately. True sensorless commutation is possible only with motor speeds of 500 to 1000 rpm and up. The EMF method works for all brushless motor models, and it is robust and cost-effective. This approach is used in many standard products, such as the maxon ESCON Module 50/4 EC-S.
Principle 2: Observer-based EMF method
Observer or model-based EMF methods use information about the motor current to determine rotor position and speed. The model-based approach yields a much higher resolution of the rotor position. This enables sinusoidal commutation (or field-oriented control), with all its benefits − higher efficiency, lower heat generation, less vibration and noise.
Principle 3: Magnetic anisotropy methods
Methods based on magnetic anisotropy deduce the rotor position from the motor inductance, which is minimal when the magnetic flows of the rotor and the stator are in parallel in the magnetic return. Measurement is achieved by means of brief current pulses, which do not cause the motor to move. Unlike EMF-based methods, this method also works at standstill or very low speeds and it permits sinusoidal commutation. The measured signals are highly dependent on the motor type. The rotor position is determined in a model of the motor, which needs to be parameterised and adapted for each motor.
Why sensorless control?
In price-sensitive applications the use of sensorless motors may reduce the cost. Hall sensors, encoders, cables and connectors become unnecessary. Typical applications in this field are fans, pumps, mills and other fast-turning applications with a relatively modest control performance that do not require a tightly controlled startup.
Cost optimisation for high control performance
Cost savings aren’t the only reason to choose sensorless control. Applications like door drives or bike drives require high controller performance. Jerk-free motor control from zero rpm is important, as are high dynamics and sinusoidal commutation for noise avoidance. All this needs to be realised without using an expensive encoder. Over the last few years, high-quality sensorless controllers based on the anisotropy method have become established, including maxon’s new High Performance Sensorless HPSC Control. However, the engineering effort required for adapting the model parameters can only be justified for quantities upward of a few hundred.
Rough ambient conditions
Sensorless control may also be required in situations where vulnerable sensor electronics need to be avoided in a motor. Examples include applications in very high or low ambient temperatures, cleaning and sterilisation in medical technology, and ionising radiation in space, nuclear facilities or medical settings. The lower number of motor connectors also makes integration easier if space is limited.
Summary
There are three main reasons for choosing sensorless control: Cost savings, space savings and operation in environments unfavourable to sensors. The EMF method with zero crossing determination is widespread in cost-sensitive applications that run at high speeds. Sensorless control from standstill and at low speeds requires more advanced methods. The implementation effort is greater and includes modelling and parameterisation. Cost savings are secondary. Field-oriented control yields a higher efficiency, less heat buildup and a lower vibration and noise level.
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