Capture Objects

Introduction

A Capture object manages a single position capture logic block. It represents the physical hardware capture logic and data. When configured and armed, the capture logic block can latch a motor's position based on one or more source input triggers.

The Capture object's number, motor input trigger sources, edge, type, feedback source, and capture index are all configurable. There are two capture types: Position-based and Time-based. The capture methodologies for these types differ and can affect performance. The table below summarizes Position-based and Time-based methodolgies and their best use.

Capture Type	Methodology	Best Use
Position-based	Position counters are latched in the FPGA and are read directly by the controller.	Works well for incremental quadrature encoders.
Time-based	FPGA latches the clock and the controller reads the clock value and position value for that sample period. The position value is interpolated by the controller from the sample's present and previous position, and the clock data.	Works very well for any node with Drive Feedback (cyclic feedback data that is digitally transmitted from the drive to the FPGA).

Capture Type

Methodology

Best Use

Position-based

Position counters are latched in the FPGA and are read directly by the controller.

Works well for incremental quadrature encoders.

Time-based

FPGA latches the clock and the controller reads the clock value and position value for that sample period.

The position value is interpolated by the controller from the sample's present and previous position, and the clock data.

Works very well for any node with Drive Feedback
(cyclic feedback data that is digitally transmitted from the drive to the FPGA).

Note: Many drives have a proprietary serial encoder that decodes the encoder position and sends the position information to the FPGA once per sample. In these cases, time-based capture is more accurate than position-based capture.

For the Position type, the motor number for the input sources and the feedback motor number must be the same.

For the Time type, the motor number and feedback motor number can be different. This makes it possible to use inputs from one node to capture positions on another node.

When using captures, the controller must have enough enabled captures to process the specified capture number. The controller will process the enabled captures (captureCount) every sample period. Since each capture object is configurable, use the minimum number of captures possible for best controller performance. For example, if you want to use 2 captures for motor 0 and motor 3, set the capture count to 2 and use capture number 0 and 1.

NOTE: Time-based capture will only work correctly if the speed of an axis is less than 344 million counts per second.

For an overview of the Capture Engine, see diagram below.

See Also: Overview of Capture

| Error Messages |

Methods

Create, Delete, Validate Methods
	mpiCaptureCreate	Create Capture object
	mpiCaptureDelete	Delete Capture object
	mpiCaptureValidate	Validate Capture object

Configuration and Information Methods
	mpiCaptureConfigGet	Get Capture configuration
	mpiCaptureConfigSet	Set Capture configuration
	mpiCaptureStatus	Get status of Capture
	mpiCaptureConfigReset

Action Methods
	mpiCaptureArm	Arm capture object

Memory Methods
	mpiCaptureMemory	Set address to Capture memory
	mpiCaptureMemoryGet	Copy Capture memory to application memory
	mpiCaptureMemorySet	Copy application memory to Capture memory

Relational Methods
	mpiCaptureControl
	mpiCaptureNumber	Get index of Capture (for Control list)

Data Types

	MPICaptureConfig
	MPICaptureEdge
	MPICaptureMessage / MEICaptureMessage
	MPICaptureSource
	MPICaptureState
	MPICaptureStatus
	MPICaptureTrigger
	MPICaptureTriggerGlobal
	MPICaptureType

Constants

MPICaptureNOT_MAPPED

Capture Engine Diagram