- Published: Saturday, 04 April 2009
- Written by Digital DIY
- Hits: 6598
Hall Effect sensors allow the detection of a magnetic field. They can be extremely small, and very accurate, and often used in applications that require some sort of speed/RPM feedback. For example, you wanted to make a digital speedometer for your car, and don't want to impede on the pre-existing circuit, you could use a Hall Effect sensor to determine how many revolutions/second your wheel is doing, and calculate your speed from there.
In this example, the A1102 made by Allegro is used. Other models (A110x) allow for different sensitivities and hysteresis properties. Hysteresis is important as you don't want false readings from ambient magnetic fields, as the devices react very quickly (400nS switching time).
The A1102 is a Unipolar Hall-Effect Digital Switch, this means that the device can detect a south polarized magnetic field when its perpendicular to the face of the device. As shown on the device list, the A1102 has an open collector output, this means when the device detects a southern magnetic field, the output is tied to earth through a transistor. We will need to add a pull-up resistor to bring the output up to 5V when it the transistor is off, perhaps the circuit diagram will explain it more,
The A1102 shown above is grossly oversized compared to real life in the circuit diagram - its just an representation of the curved edges on the front that I was trying to portray. Its actually smaller than the picture displayed at the top of the topic.
Now for our program. I thought an RPM type display would be ideal for this device. There's no way you would sample for a whole minute to determine what the RPM of a device is (well you could - but simply wouldn't). So instead its much more efficient to sample over a known time period and then scale the result back up. The sample register is also known as a PreScaler.
For my example, I will sample for 500mS, and then scale it too RPM. The longer you sample for, the more accurate your results - but the slower the refresh rate. It's a trade off, but I have found 500mS to be a happy median.
Notice that I cap the value of PreScaler to 84, this is to ensure that my program leaves the loop if the RPM > 10000, as 84 * 2 * 60 = 10080. The value of PreScaler does not need to be capped, however if it rolls over 65535 during any math routine then your result/accuracy goes out the window. An easy fix would be defining PreScaler as a different variable type (such as LongInt for values up to 2,147,483,647).
Just to go one bit further on sample times, I have used 500mS (1/2 a second), so the slowest recordable rate = 1 sample, which equals 1 * 2 * 60 (RPS to RPM) = 120. That is also my error rate (120 RPM), as I could miss one sample each loop. If my sample time was 100mS, then my slowest sample speed would be 1 * 10 * 60 (RPS to RPM) = 600. As you can see, the error rate increases as the sample time decreases... but its a matter of how fast you need the data/want to display it that accounts for what sample speed your happy with.