Enhancing Motor Performance in Demanding Systems with Advanced Current Sensing

­One of the best ways to address motor-control circuit optimization is with advanced current-sensing solutions.

We live in a time of disruptive growth, as new technologies are both benefiting and challenging society, with advances ranging from alternate energy systems to wide-bandgap semiconductors. One aspect of our increasingly more technological society is the growing proliferation of powered systems, like electric bicycles, cars, and buses. The demand for economy, efficiency, power, and functionality is placing a great deal of pressure on the engineering community to address these needs.

These increasing demands for higher levels of efficiency, precision, reliability, and safety in electronic systems, especially motor-control and power conversion require optimized control electronics. These solutions must not only address the performance demands of the customer and the application in question, they must also address the regulatory environment of the region they will be used in.

Driving brushless DC motors

Among the various types of electric motors, the brushless DC (BLDC) motor is used in a large variety of industrial, automotive, medical, and consumer products. This is due to the design’s high reliability over a long operational lifetime. When it comes to this, or actually any type of electric motor, the driver electronics are critical. Any inefficiencies in the circuit will not only result in sub-par motor performance, they will create cascading issues in areas like thermal management as well.

There is no precision without feedback, and this also applies to powered systems. Using fast and accurate current sensing in the power conversion and driver circuits provides the precise feedback for optimum motor performance (Figure 1). BLDC motors are synchronous devices, which means the stator flux and the motor phase currents must be kept synchronous with the rotor. This means the magnetic flux in the stator must be monitored, with fast and accurate current feedback to precisely control the motor.

One of the most important reasons to monitor the motor-control circuit is because if the drive current and the counter-electromotive force (also known as counter EMF, CEMF, back EMF) are not kept in phase, the motor will not run well, and could even stop. Circuits using field-oriented control, also known as vector control, back-EMF often means monitoring the voltage generated in order to determine the speed of the motor. Used for decoupled control of the flux and torque in a three-phase system, such a solution requires that the motor currents be sensed accurately in real time for optimal motor control.

In order to provide the accurate feedback with the required signal integrity for the needed levels of system control and stability, especially when it comes to managing motor-drive factors like speed and torque, advanced current sensors are the best solution. In addition to providing increased system stability, an advanced current-sensing solution will make measurements that reject common-mode transients made by PWM cycling, ensuring precise feedback and motor control.

AMR sensor technology

Sensing current is an old approach, and it has always been a part of any high-performance motor-control system, used to help determine the current, position, and speed of the rotating motor. In the last few years, advancements in Anisotropic Magneto-Resistive (AMR) sensor technology have improved the accuracy and reliability of current sensing. 

Providing a cost-effective solution, the latest AMR sensors integrate the signal-conditioning circuitry into the same package to address a range of power applications (Figure 1). To obtain the most accurate information in a circuit, multiple sensors can be used in the control loop, which can also improve the circuit protection by detecting fault conditions that may damage the motor.

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Figure 2: Current sensors are critical power management components in a wide range of applications – from appliances and tools to EV car charging infrastructure, to server farms and telecom systems

Each of the most popular ways to measure current, high-side, low-side, and inline, has advantages. Multiple approaches can be used in situations like when a pulse-width modulated (PWM) signal is used to drive the motor. In such a circuit, it can be hard to obtain accurate measurements due to common-mode transients (dV/dt).

In a three-phase motor, for example, PWM polyphase signals drive the load, making current measurement both necessary and challenging. Advanced current-sensing solutions can address these demanding requirements.

A resistive method with good dynamic performance and linearity, a legacy way to monitor motor performance involves using shunt current sensors to measure the voltage drop in the circuit to determine the current.  Although limitations at both high and low currents can be addressed by active compensation, at high currents the power dissipation in the shunt itself becomes a growing thermal management problem. Since shunt sensors are contact-based, there are also additional system complexity and circuit failure issues involved.

AMR sensing leverages the magnetic field generated when current passes through a wire, measuring the magnitude of the field parallel to the sensing direction. Usually configured to measure the magnetic field with a U-shaped conductor positioned over the AMR material, the AMR sensors sit on top of opposite current-carrying conductors with equal distance from a symmetry axis of the sensors. The output signal is based on the magnitude of the magnetic field parallel to the sensing direction of the AMR sensor (Figure 2).

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Figure 3: ACEINNA’s current sensor includes an AMR sensor die that detects the magnetic field generated by current through a U-shaped copper trace and generates a voltage signal

AMR sensors use permalloy, an alloy of nickel and iron, whose resistance changes proportionally when presented with a magnetic field, turning that measurement into a voltage. An AMR chip is also electrically isolated, as there is no contact between the sensor and the motor driver circuit being measured, providing galvanic isolation with no power dissipation.

AMR sensors also enable faster readout while correcting offsets via active feedback loops, enabling the circuit to adjust gain parameters and actively compensate for sensor offsets. An integrated sensing solution, advanced AMR devices have a significantly smaller footprint than legacy board-level solutions using shunts, or an op-amp and comparator, among other advantages (Figure 3).

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Figure 4: An integrated solution, AMR-based current sensing provides multiple advantages over legacy current sensing technologies like Hall Effect, Current and Resistors

Protection is important

The ability to optimize the circuit protection of the motor drive is another force-multiplier; an AMR sensing solution enables a motion-control system to also optimize its circuit protection. The speeds, power levels, and long operational times of next-generation motorized systems means traditional “blowing” fuses are inadequate, as the higher switching speeds and power levels demand real-time operation.

More than just a way to manage demanding high-power applications like motor drives, overcurrent sensing is also critical to protect the other circuits in a system. Subsystems like position-sensing and angle sensors are very susceptible to interference and power irregularities, and in critical applications, a fuse doesn’t give you any data, preventing engineers from troubleshooting problems.

AMR current-sensing solutions like those from ACEINNA improve overcurrent detection because of their very fast response and wide current-measurement range.

Inherently-isolated AMR current sensors can be used on both the high and low sides of a circuit, improving performance as well as safety and reliability. ACEINNA’s contactless current sensors correct offsets via active feedback loops to adjust gain parameters and actively compensate for the sensor offset.

Using AMR sensors can also address performance and safety issues like user error and minor damage to cables and connectors, intended or unintended, while optimizing performance. Using ACEINNA AMR current sensors in the high side, for example, can detect ground faults of the phase current, which could be due to wrong wiring, PCB aging, or other wear factors, protecting the entire system.

Any high-performance motion system has thermal management issues, from power conversion to motor-management inefficiencies, and advanced current sensing enables a circuit to be as efficient as possible. Close monitoring of the circuit reduces internal waste heat, making it easier to address any external heat that can add to the thermal load. Motor driver circuitry can also generate heat that can impact the performance of co-packaged electronics, particularly sensitive analog components.

Performance advantages

A very important part of efficient operation in advanced motorized products today is power quality, and a circuit’s power factor correction (PFC) improves the power factor ratio, reducing grid stress while increasing the circuit’s energy efficiency (Figure 4). Advanced current sensing on the low-voltage side of a circuit improves the available power, for example.

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Figure 5: Using Powerfactor correction (PFC) to modify the power factor ratio can reduce the cost of electricity while at the same time improve overall power quality, reduce grid stress, and maybe most importantly, increase the device’s energy efficiency

An AMR chip also has a wide operating bandwidth, with a higher sampling rate than Hall-based systems, while being less costly as well. Another advantage is that it makes an absolute measurement and does not just track the changes in the circuit.

Form factor is also an important aspect in today’s products, whether they are used in consumer, industrial, or Mil/Aero applications. The integrated aspect of ACEINNA’s AMR current sensors reduces the board space required. More efficient than a shunt-based solution, it generates less waste heat than legacy approaches, further reducing the space needed for thermal management.

Conclusion

Current measurement is fundamental for providing feedback on the performance of any motor-control application, and several topologies can be used to develop current sensing solutions. There are several legacy approaches that can be pursued, each with their pros and cons. An AMR-based current-sensing solution can address performance, reliability, and safety, as well as help with regulatory compliance for various markets. AMR sensors can also help with circuit protection, overall system cost effectiveness, product form factor, and other important circuit design issues. 

ACEINNA