Data Acquisition Toolbox

Linearly Scaling the Data: Engineering Units

The Data Acquisition Toolbox provides you with a way to linearly scale analog input signals from your sensor. You can associate this scaling with specific engineering units such as volts or Newtons that you might want to apply to your data. When specifying engineering units, there are three important considerations:

• The expected data range produced by your sensor. This range depends on the physical phenomena you are measuring and the maximum output range of the sensor.
• The range of your analog input hardware. For many devices, this range is specified by the gain and polarity. You can return valid input ranges with the daqhwinfo function.
• The engineering units associated with your acquisition. By default, most analog input hardware converts data to voltage values. However, after the data is digitized, you might want to define a linear scaling that represents specific engineering units when data is returned to MATLAB.

The properties associated with engineering units and linearly scaling acquired data are given below.

Table 5-13: Analog Input Engineering Units Properties

Property Name	Description
SensorRange	Specify the range of data you expect from your sensor.
InputRange	Specify the range of the analog input subsystem.
Units	Specify the engineering units label.
UnitsRange	Specify the range of data as engineering units.

Note    If supported by the hardware, you can set the engineering units properties on a per-channel basis. Therefore, you can configure different engineering unit conversions for each hardware channel.

Linearly scaled acquired data is given by the formula

scaled value = (A/D value)(units range)/(sensor range)

Note    The above formula assumes you are using symmetric units range and sensor range values, and represents the simplest scenario. If your units range or sensor range values are asymmetric, then you must also include the appropriate offset in the formula.

The A/D value is constrained by the InputRange property, which reflects the gain and polarity of your hardware channels, and is usually returned as a voltage value. You should choose an input range that utilizes the maximum dynamic range of your A/D subsystem. The best input range is the one that most closely encompasses the expected sensor range. If the sensor signal is larger than the input range, then the hardware will usually clip (saturate) the signal.

The units range is given by the UnitsRange property, while the sensor range is given by the SensorRange property. SensorRange is specified as a voltage value, while UnitsRange is specified as an engineering unit such as Newtons or g's (1 g = 9.80 m/s/s). These property values control the scaling of data when it is extracted from the engine with the getdata function. You can find the appropriate units range and sensor range from your sensor's specification sheet.

For example, suppose SensorRange is [-1 1] and UnitsRange is [-10 10]. If an A/D value is 2.5, then the scaled value is (2.5)(20/2) or 25, in the appropriate units.

Examples: Using Callback Properties and Functions

Example: Performing a Linear Conversion