sensor circuits

One common question is why use an aftermarket temperature sensor - can't the on board diagnostic system be used? In a lot of cases, the OBD system provides a great way to get access to sensor data on the systems in your car. There are a few limitations to this system though - first, you are limited to sensing data in the locations that the OEM set. Second, and more importantly, is that the update rates of these sensors are variable and may not be useful under a performance context like racing. Third, the reliability and tolerance of the OBD system is often less than a good aftermarket alternative - although this is not always the case.

The most common reason is more simple - on a lot of race cars, there is no on board diagnostic system!

What sensing elements are out there?

The most common are one wire, resistive-based sensor elements. The most common types of these are thermistors, which vary their resistances according to changes in sensed temperature, and diaphram pressure sensors that vary their resistance in response to a change in pressure at a sensing port. Resistance based sensors are inexpensive, but have one problem - they are nonlinear, and are typically challenging to get good readings from. They also degrade over time, as anyone who has had to replace the coolant temperature sensor on a engine has found out.

Digital sensors are the other type, and they exploit other physical or micromachined changes to sense environmental parameters. There are digital versions of the more common pressure and temperature senders, and more exotic digital sensors, like the accelerometers found in vehicle airbag or data acquisition systems. 

 

  sender1.png sender2.png

Typical nonlinear curves for resistance based senders (Autometer 2258/2242)


Hardware Design & Signal Conditioning 

 

Sensors require conditioning circuits to get meaningful information out. Most microcontrollers require a proportional voltage signal back to indicate what the signal is. Digital sensors will provide this analog voltage directly, or provide 

In the case of a resistance-based measurement, a pull up resistor is selected that will generate a variable voltage. In this configuration, a matched resistance value corresponding to the full scale resistance of the sensor acts as a constant reference. As the sensor changes resistance, the output voltage fluctuates accordingly.

 

-- stay tuned for more

 

 

 

 

 

 
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