About Lookup Tables
The standard signal conditioner will produce a linear output in response to a linear input. This means that when plotting output against input on a line chart the result would be a straight line.
The USC Configuration program plots output on the vertical axis and input on the horizontal axis.
Many measurement applications require the input output response to be tailored to correct errors in the linearity of the measured signal as in the following 'Shaft Angle Example'.
Shaft Angle Example
A potentiometer is used to measure an aperture opening from 0 to 100%. As the potentiometer is connected using long wires to the signal-conditioning module in a noisy factory environment the current flowing through the potentiometer (connected as a two-wire series resistance) is measured instead of the voltage.
PROBLEM
- This method of measurement is highly immune to noise but has a non-linear response.
- The aperture measurement of 0 to 100% does not use the full range of the potentiometer.
Steps to Solving the Problem
- Run the 'New Module Wizard' ('dc Signal', 'Isolator', CH1 '30mA', Output 1 '0-22mA', Relay 1 'Not Set', 'System') and configure an input channel to measure input current and set the auxiliary voltage output to a level suitable for driving the potentiometer. (I used a 5k ohm potentiometer with an extra 150 ohm series resistor the auxiliary set to 8Vdc and CH1 set to the 30mAdc range).
- Set the USC to display the input channel (CH1) on the ACCESS module. On the USC Properties Screen change the program name from 'dc Signal Isolator' to 'Shaft Angle Example' and program the USC.
- Connect the USC into the system to be linearised.
- Move the aperture to different points and record the measurement displayed on the ACCESS module display. These measurements are shown in Figure 20.
- Create a lookup table from measured points to linearise the measurement
- Update the engineering unit for the new measured output.
Create a lookup table from measured points
The following procedure will add a lookup table to CH1 where CH1 is currently set up as 30mA input. CH1 is connected to an external device (Aperture position), measurements were taken for different aperture positions as shown.
|
Measured Input
|
Aperture Position %
|
|---|---|
|
1.811
|
0
|
|
2.257
|
25
|
|
2.941
|
50
|
|
3.865
|
75
|
|
6.075
|
100
|
- Open the 'USC Properties Screen' for the module definition to be changed.
- Click the CH1 icon then the 'Configure' button. The 'Modify Wizard' will step through to the current CH1 range setting.
- In the 'Table' list click on 'Add table'. Click the small 'u' button after it appears above the table list.
- On the 'Enter Points for Table' screen click 'Create Table From Points'.
- In the description box enter 'Aperture Position'.
- Enter the known points on the grid.
- After entering the points then press 'Ok' to create the table.
- Two points will create a straight line.
- Entries are sorted by the input value.
- The "Create Table From Points" option will open with the values used to create the last table.
- To add rows enter data in the last (empty) row. Press resort to display the values, as they will be used.
- To delete rows click in the row, then select the whole row and press delete.
- The first row and the last row will contain the maximum and minimum input values or the input range.
- The input range entered will be divided into 101 even divisions for use by the USC. The input range can be changed after the table is created if required.
- The 'Look Up Table' screen will open. I clicked on the chart line causing the data grid to move to E51. This point shows that input 3.98554 will have an output of 76.3647.
- The input range consists of 101 even points on the x-axis the user cannot directly change these 'In' values.
- The output is plotted in the vertical axis these 'Out' values can be directly changed by the user.
- Click the 'Ok' button to accept the currently selected table for use.
- The Min, Max Values and the Engineering Min and Max values are entered and locked by the table.
- The Engineering units can be changed to reflect the input function.
- Change the engineering units (Eng.Units) to 'Apert%' and press 'Next' until the modify wizard is on 'Output 1'.
