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Precise torque control at a lower installed cost

A new technique for field oriented control in drives provides precise brushless torque control with only one shunt resistor.

Sinusoidal permanent magnet motors - or  brushless motors (BLDC) - are becoming popular nowadays and replacing brushed DC in a variety of applications. The main reasons are that they achieve higher efficiencies, with lower inertias and require less maintenance due to the elimination of the brushes.

As brushless motors are not self-commutated, an electronic board for commutation control is always necessary which increases the cost and the size of the overall system. So there is ongoing research the discipline of motor control to try to improve system efficiency while reducing the cost of electronics.

The three most common techniques for controlling the torque generated by a brushless motor are scalar, trapezoidal and field oriented control (FOC). Scalar control does not reach high efficiency and high torque because it just rotates the magnetic field produced by the stator without taking into consideration the position of the rotor.

Trapezoidal control only applies current in two windings of the motor leaving the third one unconnected. When the motor turns, the used windings are switched progressively, generating a total of six different magnetic field vectors. This control strategy works well in many applications but it introduces a torque ripple due to the misaligned from the optimal direction, which also represents a loss in efficiency.

Field oriented control (FOC) provides smooth movement even at low speed as well as high efficiency because it ensures that the generated magnetic field vector is always orthogonal to the position of the rotor, generating the maximum torque. However to implement FOC, the value of the currents passing through the three windings of the motor must be precisely known.

The circuitry to accurately sense the current of three windings of a motor adds a significant size and cost to an electronic board. But a new control algorithm developed to work with only one shunt looks set to redefine standards. For example, Ingenia has designed and successfully implemented field oriented torque control of a brushless motor using a single shunt sensing circuit, delivering an ultra-low cost but high performance device.

Brushless motors normally use a three-phase inverter which allows independent control of the current applied to each coil of the motor. A three-phase inverter is composed of three legs, each one with two electronic switches (MOSFETs or IGBTs) allowing the current to flow from and to the legs. The electronic switches normally work only in saturation zone to improve the system efficiency, and use Pulse-Width Modulation (PWM) techniques to control the amount of energy applied to the motor. The most common techniques used are sinusoidal modulation, third-harmonic injection and Space Vector Pulse Width Modulation (SVPWM). In Ingenia's new development, the SVPWM technique implemented increases the voltage utilisation by about 15% compared to sinusoidal modulation and reduces Total Harmonic Distortion (THD) compared with third-harmonic injection.

To perform field oriented control in a three phase motor, the current passing through the winding must be known. Normally, using Kirchhoff's law, sensing two phases is enough to know the value of the three winding currents. There are three common methods to sense the current passing through the windings of the motor:
  1. Direct measurement on the phase line by means of transformers, Hall sensors or amplifiers with high common-mode rejection ratio. This approach offers easy implementation from a hardware and firmware point of view but leads to a high product cost due to the components used.
  2. Measurement on the low side of the half-bridge path. Putting a shunt resistor on the low side path generates a voltage drop proportional to the circulating current which could be amplified by standard operational amplifiers. The normal approach is to use three shunt resistors, one for each phase. However using Kirchhoff's current law (the sum of currents in a network is zero), reading just two motor coils would be enough and the third one could be inferred. This approach reduces the price of the system as it needs only two low-cost sensing circuits, but it still needs an ADC with simultaneous sampling capabilities.
  3. Measurement on the bus voltage path. This offers the potential to reduce the number of current sensors and therefore the used space and overall cost of the design. The three-phase stator currents could measured by means of a single DC-Link current shunt sensor. This method has many advantages but needs a complex current reconstruction process. As the same conditioning system is used to sense the three phases, the gain and offset will be exactly the same, eliminating the need of a specific calibration process for each phase.

The single-shunt algorithm senses the current two times per PWM cycle. Both samplings are executed in the middle of the space between PWM transitions. Depending on the active sector, each sampling will correspond to a different motor coil. The third current could be obtained using Kirchhoff's law. The ADC senses the currents during the first half of the PWM period while, and can sense some extra analogue inputs (temperature, voltage monitoring, user analogue input, etc.) during the second half. This algorithm works well but cannot read the current in the following cases:
  • When the voltage vector is crossing a sector border. In this situation the length of the duties of the two phases are similar (or the current is not still stable) and therefore there is no space to sample the current between transitions.
  • When the modulation index is low. In this situation the length of the duties of the three phases are similar and therefore there is no space to sample the current between transitions.

To overcome this problem, Ingenia has modified the algorithm to generate an asymmetrical PWM ensuring that there is always enough time to perform the correct current sensing. However this improvement has some limitations. A minimum voltage vector is always needed to perform a correct current sensing, and the maximum bus utilisation cannot be reached due to the asymmetrical PWM update. This will limit the maximum voltage utilization of the system.

These limitations aside, however, there is no doubt that Ingenia has designed and successfully implemented a useful field oriented torque control strategy for brushless motors using single-shunt current sensing circuitry, delivering an ultra-low cost but high performances device. Velocity and position control loops have been finally incorporated bringing advanced control features to the system. Moreover, the measurement of other analogue inputs has been added giving high-end protection to the board, such as over-temperature, over-voltage and under-voltage. The key benefits of this new development include:
  • Precise torque control, high efficiency and low noise over a wide speed range thanks to the use of field oriented control
  • Large cost reduction because only one sensing circuitry is used
  • Same gain and offset for three current sensing, eliminating the need of a specific calibration process for each phase during production
  • Reduced physical space
  • Popular power-stage with common ground normally used for trapezoidal commutation could be used, simplifying the design and reducing the cost.
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