|Oct 29, 2008, 08:32 PM|
Joined Nov 2002
Hall effect sensor notes, in the context of the servo actuator project
This scope capture shows the output of a linear hall effect sensor as a radially magnetized magnet is "spun" next to the sensor, which repeatly moves the magnets south pole closer to the sensor, then the north pole, and so on...
Channel 2 shows the positive rail to be around 2.64 volts, while the output of the hall effect sensor is about .2 volts to 2.4 volts.
From the picture you should realize the following:
1. The position of the arm gives you a sensor readinig that is unambuguous "between" the poles, but is ambiguous on either side of a pole.
2. Over that operating region between the poles the output seems pretty much linear...
3. You may need some gain from the hall effect sensor to the ADC of the CPU to get rail to rail operation if you need it. Another solution to this is to use a
more sensitive sensor, or move the sensor closer to the magnet...
This may be important if your ADC sample width is small...
4. The mechanical design of a servoactuator system needs to make the operating limits of the output arm fall so the magnet causes the hall effect sensors output to be between the poles of the magnet.
5. the physical limitis (mechanical stops) , if any of the output arm of your actuator, may cause the sensor output to be in a certain range, which could be a useful piece of inoformation in your control algoritm.. any readings outside the stops are either noise to be disregarded, or a sign of failure, which may cause you to de-energize the outputs...
6. The speed spec (degrees/second) of the output arm. also gives some information useful for the control algorithm. If the coil when fully energized can only move the arm so far between samples of the position, then if you get a sensor reading to the contrary, you consider that sample to be noisy and ignore it,,,
7. The output voltage characteristics give the actuator a sort of physical orientation. , when the arm is all the way in one directon, the output will read a particular value, and another particular value for the other way. That means if you plug the wires of the coil in backwards accidentally, your control algorithm may not work right anymore......because you are driving the output arm the wrong direction and you wont be able to move it to the desired setpoint.... This notion means that if you get the thing working,
to build another one that will work correctly you need to keep track of which
magnet pole is which, , and that you get the same readings more or less when the actuator arm on each one servo are in the same "orientation.
You also need to connect the coil up so the hbridge driving it puts current through it the same way as the one you have working, so the arm goes the right direction.
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