Position-Pressure Setup

This section provides detailed information on how to set up the RMC for position/pressure control.

Read this topic before continuing to Tuning a Position/Pressure System.

The Position/Pressure Overview topic should be read before this topic.

For a practical example of setting up and tuning a system, see Position/Pressure Example.

Steps for Setting Up Position/Pressure Control:

Configure the Axes

Once the RMC is correctly wired to the transducers, the modules must be correctly configured.

Configure the Position Axis
The following procedure is for configuring MDT modules. If you have a different module, look up its configuration procedure in the on-line help.

Set blanking period (if required)

If you have a clevis-mounted transducer, see MDT configuration to change the blanking period from the default 21 ms to 5 ms.

Configuration word

Double-click the Config Word for the MDT axis.
Select the transducer type you have. See MDT configuration for more details on the Configuration word.

Set scale and offset

On the Tools menu, click MDT Scale/Offset Calibration.
In the Desired Position Units field, enter the desired measurement units. Many position/pressure applications use 1000 units per inch.
In the Transducer field, enter the transducer data. This information is available from the transducer specifications.
The Offset field allows you to set your zero point at any point along the transducer. Move the cylinder to where you want your zero point to be. Click the button in the Offset field to automatically enter the counts into the counts box.
If you wish your measurement to be in the opposite direction of the transducer counts, select decreasing position units in the Increasing counts equals field.
Set the desired extend and retract limits in the Extend/Retract Limits field.
Click Apply or Done when you are finished.
Issue a Set Parameters (P) command to initialize the axis with these settings.

Configure the Analog Axis

The following procedure is for configuring the analog axis for pressure control. For further details on configuring the module for pressure control and for your particular transducer type refer to Using Analog Channels as Differential Force Inputs Using Analog Channels as Pressure Inputs and Configuring the Analog Transducer Type.

Module Configuration

On the Tools menu, click Module Configuration.
In the Slots list, select an analog module.
Click Slot options.
Click the Channels 0-1 or Channels 2-3 tab depending on which channel your pressure transducer is connected to.
Click an option button to set the correct channels for either auxiliary pressure if you are using single-ended pressure, or auxiliary force if you are using double-ended (differential) pressure.
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 the RMC manually.
In the RMC Configuration dialog box, click Close.

Configuration word

Double-click the Config Word (in the parameter area of the main screen) for the pressure axis.
In the Input type field, select the type of feedback of your pressure transducer.

Set scale and offset

On the Tools menu, click Scale/Offset Calibration.

If the axis is pressure control:

Enter the desired Actual Pressure for two different pressure counts (the counts are the feedback from the transducer). The scale and offset will be automatically calculated.

If the axis is force control:

Enter the following information:

Maximum gauge reading
Actuator Type
Cylinder Dimensions
Desired Force Units
Desired Force Direction

This information allows the Actual Pressure in the Status area of the main screen to display the net force on the cylinder.

Click Apply or Done when you are finished.

Assign the Pressure Axis to a Position Axis

The RMC performs pressure control using two or more inputs from transducers and one drive output to the hydraulic valve. Any position axis with analog drive output (not stepper output) can be used to provide the drive output and position input. To enable pressure control on this position axis, you must assign a pressure-enabled analog axis to the position axis. In this configuration, the analog inputs—whether using two transducers monitoring differential force, or a single transducer monitoring single-ended pressure—are then used together with the position axis to control both position and pressure.

To assign a pressure axis to a position axis:

Double-click the Config Word for the position axis. This will open the Config Word dialog box.
In the Pressure Axis field, click the desired Pressure Axis.
Click OK.
Issue a Set Parameters (P) command to cause these bits to take effect.
If you want the axis assignment to be remembered after the RMC power is cycled, then issue an Update Flash (U) command $image\ebx_-746503463.gif$ . This needs to be done only once.

Set Up the Event Step Table

Setting up and tuning position/pressure control requires issuing multiple commands numerous times. Doing this from a PLC or by typing commands in RMCWin is very awkward. To simplify the process, Event Steps in RMCWin should be used for setting up and tuning position/pressure control.

To perform position/pressure control, the following steps must be set up in the Event Step Table:

Issue a P command so the axis will hold position.
Issue a Set Null Drive to Integral Drive (n) command. This updates the null drive (drive needed to hold position), which is critical for pressure control.
Move the axis to the correct starting position. Normally, the axis should be at a position where the pressure is below the desired entry pressure (Pressure Set A).

Set up the pressure control mode. This sets up the parameters that the pressure axis will use when it enters pressure regulating mode. This is done by issuing the Set Pressure (^) command.
Note: this step does not put the axis into pressure regulating mode.

This step sets up the following parameters:

Mode:	Sets Ramp Type, Ramp Time and Integrator Mode
Pressure Set A:	Pressure at which Pressure control begins.
Pressure Set B:	Pressure at which Pressure control ends.
Ramp Time:	Time to ramp to commanded pressure.
Command Pressure:	The pressure the system will go to when regulating pressure.

Move in the direction of increasing pressure with the monitor pressure bit set (pressure monitoring mode). When the pressure reaches Pressure Set A, the axis enters pressure regulating mode. The axis enters pressure control according to the parameters previously set up by the Set Pressure command.
Control the pressure as desired.

The following sample Event Step Table illustrates these steps:

$image\PosPressSetup1.gif$

Explanation of each step in the Event Step table:

Step 0:
This step is normally linked to after finishing a sequence. If the event control is at step 0, it usually means the event control has stopped. This step does nothing.

Step 1:
Issues a P command so the axis will hold position. Waits for 1/2 sec so the drive output will stabilize for the next step.

Step 2:
Issues a Set Null Drive to Integral Drive. This updates the null drive (drive needed to hold position), which is critical for pressure control.

Step 3:
This step moves to 36.0 in. at 10 in./sec (assuming it is set up for a resolution of 0.001 in., then 36000 = 36 in. and 10000 = 10 in.) It then waits until bit 1 (the In Position bit) is on before continuing to the next step.

Step 4:
This step sets up the pressure parameters that will be used in step 5. The Mode word is set up to Calculate Ramp Time and Integrator Active Only at Pressure. Pressure Set A is 1000, Pressure Set B is 500, and the Command Pressure is 5000. This step waits 0 msec and goes to step 5. Note that the Ramp Time, set to 200, is not used since Calculate Ramp Time has been selected.

Step 5:
This step moves toward 40 in. with the Monitor Pressure bit set in the Mode word. Once the pressure reaches Pressure Set A (as set in step 4), it will begin controlling pressure. It will go to a pressure of 5000 ( as set in step 4). This step waits until bit 0 (At Pressure) is on before continuing to step 6.

Step 6:
This step changes the Command Pressure to 6000, with the same Pressure Set A and B as in step 4 and a Ramp Time of 50. The pressure will ramp to 6000, and then the step waits 1000 msec before continuing to step 7.

Step 7:
This step changes the Command Pressure to 5000, with the same Pressure Set A and B and Ramp Time values as in step 6. The pressure will ramp to 5000, and then the step waits for 1000 msec before continuing to step 5. Steps 5 and 6 will repeat until a Quit Events (Q) command is issued. Ramping the pressure up and down in this manner is very useful when tuning the pressure. It is important that the Ramp Time be fairly short, because parameters suitable for slow changes (long Ramp Times) may make the system unstable for fast changes.

Notes on using event control for position/pressure:
- To begin an Event Step sequence, enter the desired event number in the Command Value field and issue a Start Events (E) command by typing "E" in the Command field on the main screen.
- Before any commands may be issued to an axis, a Set Parameters (P) command must be issued. This initializes the axis.
- Event control is only allowed on position axes. However, when a pressure command is entered in an Event Step on a position axis that has an assigned pressure axis, then the command is issued to the pressure axis instead of the position axis.
- For a list of pressure commands that will be sent to the pressure axis, refer to Command. Notice, however, that the Set Mode (M), Open Loop (O), and Set Parameters (P) commands will not be sent to the pressure axis because they are also valid on the position axis.
- When editing the event table you will have to keep in mind that for pressure commands, you should enter values for Pressure Set A in the Acceleration field, values for Pressure Set B in the Deceleration field, and values for Ramp Time in the Speed field.
Once you have set up your Event Steps table, continue to Tuning a Position/Pressure System.