Doing More With Analog Input (Data Acquisition Toolbox)

Data Acquisition Toolbox

Engineering Unit Conversion

The Data Acquisition Toolbox provides you with a method for converting analog input signals into values which represent specific engineering units (Newtons, degrees Celsius, etc.). When specifying engineering units (EU), there are three aspects to consider:

The sensor range
The A/D range
The engineering units range.

You can set engineering unit conversions for each channel of data. Therefore, the properties related to EU conversions can be set on a per-channel basis. These properties are given below.

**Table 4-10: Engineering Units Properties**
Property	Description
`SensorRange`	Expected range of data from sensor
`InputRange`	Range of A/D converter
`Units`	Engineering units name
`UnitsRange`	Range of data in MATLAB workspace

Note:
Only linear engineering unit conversions are supported. You can perform nonlinear conversions by creating the appropriate M-file function.

The InputRange property is often expressed in terms of a device's gain and polarity. If you set InputRange to a particular value, then gain and/or polarity will change accordingly. For example, if an analog input subsystem has a built-in voltage range of 10 volts, and if InputRange is set to [0 10], then polarity is automatically set to unipolar, but gain is unchanged.

Example: Performing a Linear Conversion

Suppose your task is to measure acceleration with an accelerometer. The sensor can measure acceleration up to +-10g (1 g = 9.80 m/s/s) and this range produces an output voltage from -3 to 3 volts. In addition, assume that you will use the full range of the accelerometer and the gain and polarity of your data acquisition hardware can be configured to match the sensor's output range. Furthermore, you want to display the acquired data in units of g's. This application is illustrated in the figure below.

This configuration is summarized below.

**Table 4-11: Engineering Units Configuration**
Name	Property	Property Value
Sensor range	`SensorRange`	`[-3 3]`
A/D range	`InputRange`	`[-5 5]`
Units range	`UnitsRange`	`[-10 10]`

The figure below shows the conversion between the input range of the A/D converter and the units range.

The code snippet below illustrates how you might configure the engineering units properties for a National Instruments board.

AI = analoginput('nidaq',1)
addchannel(AI, 1);
set(AI, 'SampleRate', 10000);
set(AI.Channel,'SensorRange', [-5 5]);
set(AI.Channel,'InputRange', [-5 5]);
set(AI.Channel,'UnitsRange', [-10 10]);
set(AI.Channel,'Units', 'g''s (1g = 9.80 m/s/s)');

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