Differential force control uses two separate analog channels as pressure inputs to generate one force reading. Both inputs are scaled to forces, and then one is subtracted from the other, giving a final differential force.
Notice that force and pressure are not interchangeable terms. This is because it is only acceptable to subtract pressures if the surface areas acted on by the pressure are equal. In a typical hydraulic cylinder with a single-ended rod, this is never the case because the rod takes up part of the surface area on one side of the cylinder.
Analog channels may be configured to be used as one of two differential force input types. Both require the Pressure Control firmware option:
Differential Force Control
Two pressure inputs are used with the corresponding drive output (in this mode, analog modules without drive outputs cannot be used for pressure control) for closed-loop, differential-force control. See Controlling Solely Position, Pressure, or Force for details.
Auxiliary Differential Force
The inputs do not have their own drive output. However, differential force control is achieved by sharing a drive output of another input. This is used in applications where an axis switches between closed-loop position control and closed-loop differential-force control. See Transitioning from Position to Auxiliary Pressure/Force Control for details.
The following general steps are necessary for setting up differential force inputs:
Step 1: Assign Analog Channels to be an Differential Force Input
To assign a pair of analog channels to be a differential-force input, do the following:
On the Tools menu, click Module Configuration.
In the Slots list, click an analog module.
Click Slot Options.
In the Analog Channel Assignment dialog box, click the tab of the channel pair (0-1 or 2-3) you wish to assign.
Click the option button of a channel assignment of either differential force control or auxiliary differential force.
Click Update RMC.
The Update Module Configuration dialog box will be displayed to indicate the progress. If the RMC could not be reset automatically, you may be prompted to reset it manually.
In the RMC Configuration dialog box, click Close.
Step 2: Configure the Analog Transducer Type for the Differential Force Input
Refer to Configuring the Analog Transducer Type for details on selecting a voltage or current input type. Notice that both channels will be affected by this configuration.
Step 3: Set the Scale A, Offset A, Scale B, and Offset B Parameters for the Differential Force Input
Refer to the individual field sections for details on setting these parameters. The first channel in the pair is channel A, and the second channel is channel B.
Analog Input Ranges
See the COUNTS topic for detailed information on the maximum and minimum limits of the various analog input ranges.
See also:
Analog Transducer Configuration
Using Analog Channels as Position Inputs
Using Analog Channels as Velocity Inputs
Using Analog Channels as Pressure Inputs
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