Selection Criteria of Hall-Effect Sensors for Brushless DC Motors

Energy efficiency is quickly becoming one of the most significant drivers in the design of electronic equipment – including miniature motors! Brushless DC motor manufacturers can increase motor efficiency by selecting the ideal latching Hall effect sensor for electronic commutation. In this blog, we will explore the basic concept of a hall sensor, the two basic types of hall sensors, and what specifications we need to consider when selecting the right Hall sensor for your device.

What is a Hall Effect Sensor?

A Hall effect sensor, or Hall sensor, is a type of sensor that uses the Hall effect to detect the presence and amplitude of a magnetic field. When a magnet is placed perpendicular to a current-carrying conductor, the electrons in the conductor are pushed to one side; this results in a potential difference in charge (i.e. voltage). The Hall sensor’s output voltage is proportional to the strength of the field.

The majority of brushless DC (BLDC) motors have three Hall sensors embedded in the stator on the non-driving end of the motor (Figure 1), which are used to sense the rotor’s position. When the rotor’s magnetic poles pass near the Hall sensors, they deliver either a high or low signal; this indicates that the North (N) or South (S) pole is passing near the sensors. The exact sequence of commutation is determined based on the combination of these three Hall sensor signals. Keep in mind that the hall sensor should be aligned with phase back emf voltage to ensure proper commutation of the motor (Figure 2).

                               

                                         Figure 1: Placement of Hall Sensor in Commutation PCB                           Figure 2: Hall Sensor Waveform w.r.t. Phase Back EMF Voltage

For a detailed overview of how BLDC motors work, see our related blog posts An Overview of Drivers for Brushless DC Motors and An Overview of Commonly Used Control Techniques for Brushless DC Motors.

Types of Hall Effect Sensors

Hall effect sensors are typically divided into two categories: digital and analog, with the former being digital further divided into unipolar and bipolar sensors. Unipolar sensors can be activated by either the north or south pole of the magnet and switches to OFF when the applied magnetic field diminishes or is removed. Bipolar sensors, however, are switched ON when in proximity to one magnetic pole and switched OFF when in proximity to the opposite pole. In the absence of a magnetic field, the sensor remains in its present ON or OFF state.

Selection Criteria for Hall Effect Sensors

Selecting the appropriate Hall sensor for BLDC commutation is based on the following specifications:

• Sensitivity. This is based on the sensor placement in relation to the magnet, air gap, and magnet strength. Sensor datasheets indicate the BOP (the magnetic operating point) and BRP (the magnetic release point) values that cause the output of the latch to switch states when exposed to the magnet. High-sensitivity latches have low BOP and BRP, which helps enable the motor to use smaller magnets to create a more compact motor design. A Hall effect sensor with high sensitivity, or a lower magnetic switch point, delivers more efficient motor performance.
• Repeatability. Referring to a Hall effect sensor’s latching time, repeatability goes hand-in-hand with high sensitivity, as it allows the sensor to be more repeatable. A highly repeatable sensor has a consistent response time, which will maintain all the angular measurements that are very close to the same value. To produce the maximum amount of torque on the shaft, the timing between the current flowing through the coil and the position of the shaft must be as accurate as possible. If there is a delay in the sensor’s response to changes in the magnetic field, the slower response can lead to lower bandwidth and accuracy errors. Any error in the switching point of the Hall-effect sensor will reduce the torque of the motor, which results in decreased motor efficiency.
• Stability. Similar to repeatability, stability refers to how much the angular position changes, but in relation to temperature or voltage. The stability-over-temperature, coupled with high sensitivity, is needed for precise position detection. Magnetic stability also helps improve jitter performance, which is critical for BLDC efficiency and results in less speed variation.
• Response Time. Response time is the time that it takes for the output of the sensor to change state. For example, if a sensor has an operating point of 30 Gauss, and a 30 Gauss magnetic field level is applied to the sensor, the response time is measured from the point when the 30 Gauss field is applied to when the output changes state. A faster response time to a change in the magnetic field delivers greater efficiency in commutating a BLDC motor.
• Current Consumption. If the application is battery-powered, the current consumption of the Hall position sensor should be low to maximize the lifetime of the battery.
• Operating Voltage Range. Different systems feature different supply voltages, so it’s critical to select a sensor that operates within the voltage range of the system.
• Open-Drain vs Push-Pull Output. Open-drain outputs are selected when the logic-high output voltage needs to be at a different voltage level than the VCC voltage of the Hall position sensor. Compared to open drain outputs, push-pull outputs do not require a pull-up resistor, thereby reducing solution size.
• Frequency Bandwidth. The device frequency bandwidth is the fastest-changing magnetic field that can be detected and translated to the output, thereby determining how fast a motor can spin and still be detected by the sensor.
• Jitter. The jitter of a Hall latch is the variation seen in the output pulse width if the motor is spinning at a constant speed. Jitter introduces angle error in the measurement of the rotor position.
• Output Delay. The output delay is defined as the time between the magnetic field crossing the BOP or BRP threshold and the time it takes for the output to reflect its new value.
• Refresh Period. This is the amount of time the device takes before a new magnetic sample is taken and used to update the output as necessary.
• Temperature. Hall sensors must operate at high temperatures if they are implemented in high-temperature motor applications.
• Size. The physical size of the device is key when designing compact motors. Battery-powered, medical hand tools are an example of a system where a compact motor is highly beneficial. For this reason, smaller Hall position sensors become extremely advantageous, as these can be placed strategically inside the motor casing without impacting the overall diameter of the motor design.

Conclusion

Though tiny in size, the impact of Hall effect sensors can’t be overlooked – especially during the device design process! Hall effect devices make excellent sensing elements, as they are completely non-contact and have no moving parts; this gives them a long operating life. These sensors also significantly impact the efficiency, reliability, performance, and life cycle of many critical industrial and medical applications, ranging from robotics and portable medical equipment to HVAC fans and machine tools. The above parameter can also be considered while designing a hall sensor-based encoder. Have questions? Reach out to our engineers here – they’ll be happy to help!