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Position-Pressure and Position-Force Control

This topic describes position-pressure and position-force control. For information on pressure control only or force control only, see the Pressure/Force Limit topic.

Pressure/Force Control or Pressure/Force Limit can be used on position-pressure or position-force control axes. Position-torque is identical to  or position-force with a single-input force, or position-pressure.

Required Hardware

A position-pressure or position-force axis requires the following RMC hardware:

• Pressure-Enabled Controller

  This is the term for an RMC that supports dual-loop control, such as position-pressure or position-force.

  • RMC75: Requires the AP2 module.

  • RMC150E: Requires the pressure option (denoted as RMC151). This must be specified when ordering the RMC.

  • RMC200: Requires two Feature Key control loops per dual-loop control axis.

• One Control Output

• One Position Input
  This can be any type of position feedback supported by the RMC.

• Pressure or Force Input, which may be:

• One Analog Input (±10 V, 0-10 V, 4-20 mA, 0-20 mA)
  A pressure sensor input or a load cell signal conditioner.

• Two Analog Inputs  (±10 V, 0-10 V, 4-20 mA, 0-20 mA)
  Two pressure sensor inputs for measuring the pressure on each end of a cylinder for resultant force.

• Load Cell Input (mV/V)
  A directly connected mV/V load cell.

Pressure or Force feedback requires analog feedback transducers (voltage or current) or load cells with mV/V signals. The input range from the transducer must fall within the RMC supported ranges of ±10V or 4-20mA, or 5mV/V. For the best resolution, the feedback should use a large portion of the input range.

Define the Axis

The first step for pressure/force control is to define the axis. This means assigning the physical inputs and analog Control Output to an axis in RMCTools.

1. Open the Axis Definitions dialog to define the axes.

2. In order to define the axes as you want them, you may need to first remove the existing axes and add new axes. Or, you may be able to change an existing axis.

3. Whether you choose to change an existing axis or add a new axis, set the axis to the following:

   • Control Axis

   • 2 Inputs

   • First Input: Select your position input type.

   • Input Type: Pressure, Force (single-input), or  Force (dual-input)
     If you are using a single pressure transducer or load cell for the feedback, choose Pressure or Force (single-input). If you are using two pressure transducers for differential force feedback, choose Force (dual-input, diff.).

4. The Axis Definitions dialog will show you exactly which Control Output and analog input(s) on the RMC will be used for the axis. You can change them if you wish.

5. Click OK on the Axis Definitions dialog. If you are online, these changes will be applied to the controller. Make sure to update Flash and save your project.

Set Up and Tune the Position

Set up and tune the position control first. Refer to the Startup Procedure for details.

While tuning the position, you may need to set some of the pressure/force AutoStops to Status Only to prevent any pressure/force errors from halting the axis.

Set Up the Pressure/Force Transducer Parameters

Make sure to set the following pressure/force parameters:

Analog Inputs (±10 V, 0-10 V, 4-20 mA, 0-20 mA)

• Input Type: Choose Voltage or Current.

• Positive Pressure/Force Limit and Negative Pressure/Force Limit: These specify the pressure/force range within which the axis will be allowed to control. If the Target Pressure/Force exceeds these values, an error bit will be set and will halt the axis if the AutoStops are set to do so.

Load Cell Inputs (mV/V)

• Wire Sense
  The LC8 offers two methods of compensating for the voltage drop in in the Exciter+ and Exciter- wires:

• Adaptive:
  The LC8 periodically measures the voltage at the Sense pin and determines the voltage drop. It assumes the voltage drop of the Exc+ and Exc- wires is the same. Therefore, the wire length and gauge of the Exc+ and Exc- wires must be identical.

Available only for 6-wire load cells.

To choose this option, set the Exciter Mode axis parameter to Adaptive.

• Fixed Value:
  The user can use a voltmeter to measure the voltage at the load cell and enter the value into the Fixed Exciter Voltage axis parameter.

This option is available for 4-wire and 6-wire load cells.

To choose this option, set the Exciter Mode axis parameter to Fixed Value.

• Positive Pressure/Force Limit and Negative Pressure/Force Limit: These specify the pressure/force range within which the axis will be allowed to control. If the Target Pressure/Force exceeds these values, an error bit will be set and will halt the axis if the AutoStops are set to do so.

• The following axis parameters may also need to be set, especially if external excitation is used:

• Exciter Mode

• Load Cell Overflow Limit

• Load Cell Underflow Limit

Scale the Pressure/Force

Scaling the pressure or force feedback converts it from volts or current or mV/V to useful units such as pounds, newtons, etc. To scale the feedback, use the Pressure/Force Scale/Offset Wizard.

1. In the Axis Tools, in the Axis Parameters Pane, on the Setup tab, in the Tools and Wizards section, in the axis column you are using, click Launch to open the Pressure/Force Scale/Offset Wizard.

2. Complete the wizard, then download the parameters. Make sure to update Flash and save your project.

3. After scaling, move the axis and apply pressure or force and verify that the correct Actual Pressure or Actual Force is displayed in the Axis Status Registers in Axis Tools.

Tune the Pressure/Force

See Tuning a Position-Pressure System.

Before tuning the pressure or force, you should read the rest of this topic to become familiar with the pressure/force commands that will be used during tuning.

If the pressure/force is scaled in small units, such as psi, pounds, Pascal, or Newtons, the gains will probably be very small. For example, you may want to start with a value of 0.01 for the Proportional and Integral Gain, and even smaller for the Differential Gain.

You can tune the pressure or force in either pressure/force control or pressure/force limit, and it will control in either mode. For many applications, it is easiest to first tune the axis in pressure control, even if you will be using pressure/force limit in your application.

Choose Pressure/Force Control or Pressure/Force Limit

At this point you should decide whether you will be using pressure/force control or Pressure/Force Limit. With pressure/force control, the axis transitions from position control to pressure/force control, and the pressure/force control is then independent of the position control.

Pressure/force limit is a special type of pressure or force control. With pressure or force limit, the pressure or force is limited during the position or velocity motion of an axis.

Pressure/force limit cannot be used simultaneously with pressure/force control. If the axis is in pressure/force limit, and a command is issued that puts the axis in pressure/force control, the pressure/force limit will no longer be active and the axis will start controlling the pressure or force. However, the Pressure/Force Limit Enabled status bit will still be set, and if the axis exits pressure/force control, it will resume pressure/force limit.

Determining Whether Pressure/Force Limit is Required

• If you want to transition between position and pressure or force control, and each control mode does not depend on the other, then you probably do not need pressure or force control.

• If you want to transition between position and pressure or force control, but you want to make sure the axis will not exceed a certain position while controlling pressure/force, you probably need pressure/force limit. However, you can use pressure/force control and monitor the other values from a user program to handle error conditions.

• If you want to perform position or velocity motion, but limit the pressure or force during the motion, then you probably need pressure or force limit.

If you will be using pressure/force limit, see the Pressure/Force Limit topic. If you will be using pressure/force control, continue reading this topic.

Enter Pressure or Force Control

To enter pressure or force control, issue one of the following commands. Notice that these commands will enter pressure/force control, not pressure/force limit.

• Hold Current Pressure/Force (19)
  This command will enter pressure or force control and hold the current pressure or force. This command is a good choice if your axis is stopped or moving slowly.

• Enter Pressure/Force Control (Time) (45)
  This command will enter pressure or force control and then ramp to the requested pressure or force within the specified time.

• Enter Pressure/Force Control (Auto) (44)
  This command will enter pressure or force control and ramp to the requested pressure or force. The ramp is automatically calculated based on the current rate of change of the Actual Pressure or Force. This command is designed to be used when the system is moving and the Actual Pressure or Force is rising quickly, which indicates the axis is encountering the resisting pressure or force. This command can provide a very smooth transition (bumpless transfer) from position to pressure control while the system is moving.

• Enter Pressure/Force Control (Rate) (46)
  This command will enter pressure/force control and then ramp to the requested pressure/force at the requested rate and acceleration.

When the axis is in pressure or force control, the Pressure/Force Control status bit will be set.

Controlling Pressure or Force

Once the axis is in pressure or force control, you can issue any of the pressure/force commands, including the commands that enter pressure/force control. For example, you can ramp the pressure up or down, or perform a sinusoidal profile, or follow a spline.

For details on each command, see the respective help topics.

• Ramp Pressure/Force (Rate) (18)

• Hold Current Pressure/Force (19)

• Ramp Pressure/Force (S-Curve) (41)

• Ramp Pressure/Force (Linear) (42)

• Stop Pressure/Force (43)

• Enter Pressure/Force Control (Auto) (44)

• Enter Pressure/Force Control (Time) (45)

• Sine Start (Prs/Frc) (76)

• Sine Stop (Prs/Frc) (77)

• Change Target Parameter (Prs/Frc) (81)

• Curve Start (Prs/Frc) (87)

• Curve Start Advanced (Prs/Frc) (89)

• Trapezoid Start (Prs/Frc) (133)

• Trapezoid Stop (Prs/Frc) (134)

• Trapezoid Change Shape (Prs/Frc) (135)

Exiting Pressure or Force Control

To exit pressure or force control, issue an open-loop or closed-loop motion command. An Open Loop Halt or Direct Output Halt will also cause the axis to exit pressure or force control. A Closed Loop Halt will not exit pressure or force control.

Releasing a Pressure or Force

Exiting pressure or force control when the actuator is exerting a force against a dead stop may require careful consideration, especially if it is critical that the force does not overshoot.

One method is to issue the Hold Current Position (5) command. However, the axis will now be holding position, and since the Actual Position typically jumps around a bit, it may cause an undesired increase in the force. Therefore, this may not be a good choice.

Another option is to issue a Move Absolute (20) to move back from the current position. However, the Actual Velocity typical jumps around a bit, and if it moving in the direction of increasing force at the instant the Move Absolute command is issued, then the Target Position must first turn around, meaning that the force may actually increase before decreasing.

Sometimes, the Open Loop Rate (10) command may work best because it alleviates both of the aforementioned problems. The Quick Move Absolute (15) will also work well, with the added benefit that it moves to a known position.

Pressure/Force Control Status Bits

The following status bits give the status of the pressure or force control. These tell you what the control is doing, and can also be very useful in user programs.

At Pressure/Force

When the axis is in Pressure/Force control or Pressure/Force Limit and the Target Pressure/Force reaches the Requested Pressure/Force and the Actual Pressure/Force is within the At Pressure/Force Tolerance window from the Target Pressure/Force, the At Pressure/Force Status bit will be set. This bit indicates that the ramp is complete and the axis is at the pressure or force.

Pressure/Force Control

The Pressure/Force Control Status bit indicates that the axis is in closed-loop pressure or force control.

Pressure/Force Limit Enabled

This bit indicates that pressure/force limit is enabled. If an axis is in pressure or force control, pressure/force limit will not be active. If the axis exits pressure/force control to open loop or closed-loop position or velocity control, it will enter pressure/force limit if the Pressure/Force Limit Enabled bit is on.

Pressure/Force Limited

This bit indicates that pressure/force limit is enabled and the axis is limiting the pressure/force. If an axis is in pressure or force control, pressure/force limit will not be active and this bit will not be set.

Pressure/Force Target Generator Bits

The Pressure/Force Target Generator bits in the Status Bits register indicate which portion of the move the axis is currently in. These bits are useful when programming complex motion sequences.

Pressure/Force Target Generator Done bit

This bit indicates that the Target Pressure or Target Force has reached the Requested Pressure/Force. If the ramp is interrupted, e.g. due to a halt, the done bit will not be set because the commanded motion was not completed. Notice that this bit does not indicate whether the Actual Pressure or Actual Force has reached the Requested Pressure/Force.

Pressure/Force Target Generator State A and B bits

See Also

Pressure/Force Control Overview

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