Open Loop Command

Character: O
Decimal: 79
Hexadecimal: 0x4F
Command Value: Millivolts of Drive

CAUTION: Use this command with care! Open Loop operation disables all safety features on the RMC!

Note: The open loop command will not affect drive output while the Simulate bit is set in the Config word. Drive output is always 0 volts in simulate mode.

Note: For pressure/force axes, this command will take effect on the analog module’s drive output, if one is available. Only 16-bit analog cards have drive outputs, therefore this command will have no affect on 12-bit analog cards. Also, because drive outputs are only assigned to input channels 0 and 2 on 16-bit cards, only analog or pressure axes using those input channels will be able to issue this command. Otherwise, this command behaves normally on pressure/force axes, except that no status registers other than the Drive field are modified by this command.

The Open Loop command allows the Controller to directly specify values for the analog output. The output range is -10000 to 10000 and is given in millivolts. However, the drive outputs are not precision outputs. You cannot rely on the output being exactly the value you specify. Look up specifications in the online help index for the tolerance on your particular interface module’s drive outputs.

The O command uses the following parameters from the last open loop profile specified:

The Command Value field specifies the desired drive in millivolts to output. The current null drive is added to this value.

The Speed field is not used.

The Acceleration and Deceleration fields control the rate at which the drive output ramps to the requested value. Acceleration is used when the drive output is moving away from 0 and deceleration is used when drive output is moving toward 0. The actual meaning of the values depends on the Acceleration/Deceleration mode bits in the Mode word. Notice that only modes 1, 2 and 3 are used:

Mode 1
Acceleration and deceleration are given in millivolts per millisecond. For example, if the current drive output is 0 millivolts, and the Command value is 1000 millivolts, and the Acceleration is 50, then it will take 20 milliseconds for the drive output to reach 1000 millivolts.

Mode 2
Acceleration and deceleration are given as distances. That is, the drive will be ramped over the distance specified. This mode is most useful when stopping on poor performing hydraulics. This mode cannot be used for starting from a stop because no drive will be applied since the position does not move, and conversely, the position does not move because there is no drive. For example, if the current drive output is 1000 millivolts, and the Command value is 0 millivolts, and the Deceleration is 4000, then the axis will ramp the drive down to 0 in the time it takes the axis to move 4000 position units.

Mode 3
Acceleration and deceleration are given in milliseconds. For example, if the current drive output is 0 millivolts, the Command value is 1000 millivolts, and the Acceleration is 50, then it will take 50 milliseconds for the drive output to reach 1000 millivolts, as the drive will increase 20 millivolts each millisecond.

Note: If a move crosses 0 drive, then Deceleration will be used until the drive reaches 0, and then Acceleration will be used until the final drive is reached.

Tip: If you are going to be changing from closed loop to open loop, you must be aware of the following. While in closed loop and holding a position, the drive may be switching back and forth between small positive and negative drives. If an open loop command is given to go to a non-zero drive, the starting drive may have a sign opposite of the requested drive. In this case, the drive will decelerate down to zero millivolts and then accelerate up to the new drive. For example, if the current drive is -10 millivolts and an open loop command requests a drive of 2000 millivolts (2V), the drive will ramp from -10 to 0 and then accelerate from 0 to 2000mV. To ensure that this deceleration happens quickly, the deceleration must be set properly (by setting it to 0 in Mode 3, and to a high number (1000 or greater) in Mode 1).

Stepper Axes (QST Modules)

Stepper axes do not have an analog output so the Open Loop command generates pulses on the Step+ and Step- Outputs. It also sets the polarity on the Direction+ and Direction- signals.

The pulse frequency is specified in the Command Value register as the number of pulses per millisecond.

The Acceleration and Deceleration parameters specify the rate at which the Open Loop output value is changed. See the discussion above for a description of the modes available.