Behind the scenes : Hall sensors vs resistive potentiometers

[SimWare 730]

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Behind the scenes : Hall sensors vs resistive potentiometers

To measure values, for instance pitch, bank, throttle settings, we need to measure

the displacement or position of a physical device. (Yoke handle, throttle lever, etc.)

There are to commonly used systems to do this : The resistive potentiometers and the Hall effect sensors.

1 - RESISTIVE POTENTIOMETERS :

A potentiometer is actually a resistance bridge (R1 & R2)  with a variable (displaceable) middle-point.
(For info, this assembly is also called a Voltage Divider)

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The simplified formula says that the middle point voltage is proportional
to R2 / (R1 + R2)
So, if the middle point is displaced so that is “gets into” R1 or R2, the value of R1 and R2 will change, so will the the middle point voltage.
à Therefore the middle point voltage is an image
     of the middle point position.

The next picture shows the schematic of a potentiometer.
The “wiper” is actually the shaft of the potentiometer sliding on the “Resistance wire” wich is actually R1 & R2.

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à Therefore the middle point voltage will change according to the wiper angular position
     (shaft position)
 

ADVANTAGES :
- Not complex ad easy to use
- Easy to find and to replace
- Cheap
 

DISADVANTAGES

- Will wear in time due to friction of the wiper on the resistance wire
- Linearity depends on the resistance quality

 

2 - HALL EFFECT SENSORS :
 

A Hall effect sensor is made of two separate parts :

- A permanent magnet
- A semi-conductor (just consider it as a wire) in wich a current flows.

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The magnetic field of the permanent magnet will “disturb” the magnetic field induced by the current flowing in the semi-conductor.
This “disturbance” is actually a variation of the current easly translatable into voltage.
This “disturbance” is proportional to the distance of the permanent magnet to the semi-conductor.
àTherefore the measured output or Hall voltage is proportional to the
    distance of the magnet, in other words, the position of the magnet.

This is very obviously visible in the pitch sensor of the Honeycomb XPC Alpha yoke :

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The permanent magnet is a fixed non-moving bar, and the semi-conductor is mounter on the carriage sliding on the sliding rod.

We can see that the distance of the magnet to the sliding rod varies : D1 > D2
So, when the carriage moves on sliding rod, the distance between the semi-conductor and the permanent magnet varies accordingly.
à Therefore the measured output voltage varies according to the position
     of the sliding carriage, and the sliding carriage is connected to the yoke handle.
 

ADVANTAGES :

- More accurate as the linearity only depends on the distance
- Less prone to wear as there is no contact between then magnet and the semi-  

conductor (no friction)

DISADVANTAGES
More expensive
It may be affected by external interfering magnetic field.

 

 

IMPORTANT :

 

The resolution does not depend on the sensor type (Potentiometer or Hall sensor)
The resolution depends on the digital-to-analog converter to wich the potentiometer or the the Hall sensor is connected.
So by essence, a Hall sensor does not have a greater resolution than a potentiometer.
Resolution must not be mistaken to accuracy or precision !