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What is the Difference Between P-Command and Open-Loop Settings for Stepper Loop Mode?

Hardware: Motion Control

What is the difference between P-Command (Position-Command) and Open-Loop settings for Stepper Loop Mode with NI motion controllers?

The P-Command mode is normally chosen for stepper and/or servo drives which accept step and direction inputs (P-Command drives), while Open-Loop mode is normally used with drives which only control steppers. To better understand the differences, it helps to analyze both modes from the perspective of the drive (amplifier) and from the perspective of the National Instruments motion controller.

Drive (Amplifier) Perspective:

A P-Command drive is very different from a typical stepper drive. A stepper drive only works with stepper motors, while a P-Command drive is a smart/intelligent drive which can control servo motors.

With a P-Command drive, the position control loop runs on the drive, not on the NI motion controller. The motion controller simply commands the new position that the drive should move the motor to, hence the name "Position Command" or "P-Command". The P-Command drive usually has built-in circuitry which checks for position error and corrects for it as necessary.

In most cases, these drives work with brushed and/or brushless servo motors, allowing you to control them as if they were stepper motors with step and direction signals. The motion controller sends a digital pulse to the P-Command drive, which then moves the servo motor by a single encoder pulse.

So, although the complexity and internal makeup of P-Command drives are completely different than those of a stepper drive, the signals which control both types of drives are very similar. There are four main differences between the control signals of P-Command drives and stepper drives, which will be discussed in the following section.

National Instruments Motion Controller Perspective:

A motion controller in P-Command mode and a motion controller in Open-Loop Stepper mode are almost exactly the same. The hardware to produce both control signals is almost identical, the NI motion controllers are configured in stepper mode for a P-Command drive, even though the end goal might be to position a servo. Although they are similar, there are four main differences in the way motion controllers are used with P-Command drives compared to how they are used with stepper drives:
  • In Position Signal: This signal is used by a P-Command drive to signal to the motion controller that the motor has reached its desired position.
  • Servo Ready Signal: This signal is used by a P-Command drive to signal to the motion controller that the motor is ready to be moved. The servo motors connected to P-Command drives usually have a longer startup time than most stepper motors. In order to make sure the motors do not miss any pulses, the P-Command drives wait for the Servo Ready Signal before sending commands to the motor.
  • Feedback: When using a P-Command drive, there is no need to connect any feedback (quadrature encoder or analog) to the motion control board because the P-Command drive is responsible for ensuring position movement accuracy. The feedback is redundant, but is often connected to the motion control as a way of monitoring the position of the motor.
  • Pull-In Moves: Motion controllers do not execute pull-in moves when connected to P-Command drives because P-Command drives themselves are responsible for correcting for position error in motion systems.

Related Links:
KnowledgeBase 42HHJIS4: Using a Non-7390 Motion Controller with a P-Command Drive
KnowledgeBase 41EGJUBE: What Is P-Command Mode?
Product Manuals: NI PCI-7390 User Manual


Report Date: 12/07/2006
Last Updated: 08/20/2009
Document ID: 446INTAJ

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