Before you buy any data logging system there are a number of key elements that must be taken into consideration.
One of the major considerations is what you are going to measure and how you are going to go about it. In this months tech article, we're going to investigate some of the options available and how to install them.
Analogue input single ended
This input type is by far the most common and for motor sport purposes and typically has arange of 0>5v, which suits the majority of sensors having a variable voltage output. This input voltage is always referenced to the Analogue Ground of the system to determine the potential difference.
Analogue differential input
Similar to the single ended type but with the sensor ground side isolated from the system ground, The voltage potential is determined as the difference between the Signal positive and the Signal negative. This input type is preferable to isolate the measurement from ground fluctuations or ‘noise’ within the structure. This input type is commonly used for thermocouples and strain gauges. It should however be noted that both of these sensor types require additional signal conditioning to achieve the correct output.
NTC temperature measurement
NTC stands for Negative Temperature Coefficient. This sensor type creates a variable resistance proportional to change in temperature. This output ratio is non linear and has a reducing resolution as the temperature gets higher. For this reason there are many different types of NTC with output characteristics that give good resolution over the range they will typically be used. A standard automotive NTC sensor will perform well down to –25 and up to 125 but can be inaccurate beyond these limits.
For the data logger to use this variable resistance it is necessary to convert it to a variable voltage and measure using an analogue input. This process uses a circuit known as a potential divider. (or voltage divider).
In Diagram 1.1 you will see a single ended 0-5v circuit with 2 equal values at resistance 1 and 2. Because the resistors create an equal ratio between the ground (0v) and the 5v supply, the output voltage is exactly 50% or 2.5v To make this circuit work for the NTC, resistance 1 is replaced by the NTC (variable) and resistance 2 is a fixed value chosen to work well with the variation seen by the NTC. The resulting variable output voltage is proportional to the change in temperature of the NTC.
This same measurement principal is used with sensors such as damper potentiometers or throttle potentiometers. The main difference being that both R1 and R2 change at an equal rate (one increasing and one decreasing) to create a linear voltage change proportional to movement.
This special device uses relies on the interaction (connection) of two different metals at the point of measurement to create a very small voltage. The amount of voltage created depends on the type of metals used and the temperature at this point. This point is known as the ‘hot junction’.
Because of the minute voltage created and the fact that it is non-linear, extra signal conditioning is required to amplify (gain) and convert the voltage to engineering units. This conversion and gain is normally carried out close to the point of measurement to reduce voltage drop over long distances. The measurement circuit type is also bi-polar to remove ground interference and permit negative values. The majority of automotive systems rely on the Type K thermocouple.
Digital input, speed & RPM
This section does not go into detail about the sensors themselves but more about the measurement types and how this is achieved. Regardless of sensor design there are 2 main principles used. Type 1 is a sensor that produces a wave from signal with both a negative and positive wave aspect; type 2 is a square signal, which alternates between 2 positive voltage ranges.
Inputs for type 1 sensors measure the time taken from the point the signal crosses the dotted line to the next crossover point. This crossing point is typically 0v
Inputs for type 2 sensors typically measure the time taken from the point the signal drops below the dotted line to the next crossing point. This crossing point is typically in the range of 1 > 3v to avoid errant signals caused by ground fluctuation. It may also be triggered on either the rising or falling edge, but not both. For the circuit to use the type 2 sensor it must state ‘pull up’
Speed RPM - Type 1 sensor
This sensor type is usually low cost, is referred to as magnetic or inductive, has two wires and is the firm favourite for measuring engine crankshaft speed. There are both advantages and disadvantages to its use.
It requires no power supply
Reverse polarity does not damage the sensor (only invert the output)
In a fixed situation it works very well
Good temperature range
The amplitude of the signal increases with the speed of the passing trigger tooth
The amplitude of the signal increases with the size of the passing trigger tooth
The amplitude of the signal reduces with the distance from the passing trigger tooth
The voltages produced can be very high.
The problems arise when using this sensor for measuring low speed rotation using a small diameter trigger wheel with a gap capable of avoiding contact. Under these poor conditions the waveform amplitude is very low and the crossover point is hard to detect exactly from any background noise. It may also be highly variable making it difficult for the measuring system software to accept the crossover as a valid signal.
Speed RPM - Type 2 sensor
This sensor type is usually higher cost, is normally Hall effect, has 3 two wires and is the firm favourite for measuring anything at lower speeds or where the trigger wheel design is limited. There are both advantages and disadvantages to its use. For wheel speed or Cam position this design is the favourite.
Identical signal regardless of trigger wheel speed
Identical signal regardless of sensor gap
Very large gaps possible
Requires a voltage supply