When we have hall sensors inside a BLDC to use as encoders for the processor, I saw some diagrams on web showing 3 output signals (open collector outputs, need external pull-ups to the VCC of processor).
It is ok to use only 2 sensors and be able to control the motor properly? I have a situation where a have only 2 inputs… 2 signals of encoder is enough?
Electronic – Brushless DC motor – number of hall sensors
brushless-dc-motorencoderhall-effect
Related Solutions
can FOC even work for BLDC without an encoder but just with Hall sensors?
You have an encoder, it just doesn't have very high resolution. But how to increase it?
Once you know how fast the rotor is spinning (time between Hall sensor signal changes) you can predict intermediate angles. If motor speed is changing then the prediction will be off, but you can compensate for this too by measuring the acceleration and factoring it in.
A "raw" Hall effect sensor will give an output voltage proportional to the current and orthagonal magnetic field.
Figure 1. The principle of the Hall effect. Source: Wikipedia Hall effect.
- In your case it seems as though you have two output terminals per sensor. These would correspond to \$ V_H+ \$ and \$ V_H- \$ in Figure 1.
- Since the Hall effect produces quite low voltages it seems reasonable to assume that some amplification of the signal takes place in the chip.
- Since the chip has single rail supply it also makes sense that the voltage would vary around mid-supply. This is 5.8 V in your case.
- It also seems plausible that there are three possible states for the sensor:
- North pole at sensor.
- Neither or balanced / cancelling / opposing poles at sensor.
- South pole at sensor.
- These three options would explain your three output voltages. More than likely you should find that when A+ is high that A- is low and vice versa. You should also find that when A+ is neutral that A- is too.
To "decode" these signals you will need three analog comparators.
Figure 2. The only similar circuit found on a web search shows the three sensors in series rather than parallel. This is an efficient approach where possible as the series current reduces overall current consumption. The TLC3704 contains four comparitors. Image source: Flickr.
Related Topic
- Electronic – Dealing with coupling of PWM noise into Hall-Sensors as spikes in BLDC driver circuit
- Electrical – Pull-up scheme for Hall effect sensors on a BLDC
- Electronic – Hall effect sensor as 2 wires switch
- Electronic – Help on translating 5V rotary signal output to interface with lathe’s cnc controller!
Best Answer
You need all three hall sensors.
You can infer the third phase by measuring the other two phases in a 3-phase sinusoid but that is because there are continuously varying so you can distinguish exact points in the cycle based on value, but the square wave of hall encoder outputs have flats (since it is a rectangular wave) so you cannot distinguish where the third phase is in the cycle by using the other two phases.