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What is the Difference Between P-Command and Stepper when Using National Instruments Motion Products?

Primary Software: Driver Software>>NI-Motion/FlexMotion
Primary Software Version: 8.0
Primary Software Fixed Version: N/A
Secondary Software: N/A
Hardware: Motion Control

Problem:
I have heard that Position Command (P-Command) is similar to stepper control and I want to know what the difference is between the two. So, what is the difference between P-Command and Stepper when using National Instruments Motion Products?

Solution:
To answer this question, it is necessary to analyze the question from two different perspectives: 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 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 is contained on the drive, not on the NI Motion Controller. The NI Motion Controller simply commands the position, hence the name "Position Command" or "P-Command". The P-Command drive usually has built-in circuitry which checks for position error.

In most cases, these drives work with brushed and/or brushless servo motors, allowing you to control them as if they were stepper motors. The motion controller sends a digital pulse to the P-Command drive, which then moves the stepper 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 five 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 stepper mode are almost exactly the same. The hardware needed to produce the control signals is almost identical, the settings are almost identical, and the motion controller is actually configured in stepper mode.  This is done by setting the axis configuration to "Open Loop Stepper" and enabling the "In Position" signal and the "Servo Ready" signal.

Although they are similar, there are five main differences in the way motion controllers are used with P-Command drives compared to how they are used with stepper drives:
  1. 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. This signal is not created by stepper drives as the motion controller does the position error correcting when in closed loop mode.
  2. 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.
  3. 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.
  4. 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.
  5. Command Signal Types: P-Command drives usually only use "Step and Direction" mode, while it is common for stepper drives to also have a Clockwise/Counterclockwise mode.


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

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Report Date: 12/07/2006
Last Updated: 02/01/2008
Document ID: 446INTAJ

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