Topic Capture

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From RSI - Robotic Systems Integration - SynqNet Systems

Contents 1 General Information 2 Position-based Capture 3 Time-based Capture 4 Sample Applications

General Information

Capture enables high accuracy position latching based on an external electrical input. Typical sources of external electrical inputs are home switches and encoder index pulses. Capture will return the position and sample counter (time in servo samples) for each capture position latch.

See the following methods for information on RapidCode API relevant to Capture:
Axis::CaptureConfigSet()
Axis::CaptureArm()
Axis::CaptureStateGet()

Position-based Capture

For Position-based Capture, the motor number for the input sources and the feedback motor number must be the same.

Quadrature Feedback with Position Capture

• Low 10s of nanoseconds capture latency.
• Most accurate type of position capture.

Drive Feedback with Position Capture

• This type of setup leads to very large position capture errors.
• MEI strongly discourages using position capture with drive feedback.
• Position-based capture is very accurate for systems that have continuous position information. (i.e. Digital quadrature encoders, in most cases.) Any encoder that is sampled less frequently than the 10s of nanoseconds accuracy that is inherent to time-based capture will suffer poor accuracy with position-based capture. Any encoder using drive mode will be sampled at the drive update rate (usually 16 kHz (62.5 microseconds).
• Sin/cos encoders are used in Drive Feedback mode.
• Position-based capture with encoders in drive feedback mode results in WORSE capture accuracy than drive feedback mode with time-based capture.

Time-based Capture

For Time-based Capture, 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.

Time-based Capture

• Position is linearly interpolated.
• Calculated position is truncated to an integer.
• It only works correctly if the speed of an axis is less than 344 million counts per second.
• Calculation is very accurate even in high acceleration systems.
• Time-based capture can be used across SynqNet nodes. (Trigger on one node, feedback position on a different node.)
• Time-based capture is ideal for encoders in drive mode.
• Time-based capture is accurate to ~40 nanoseconds in time. Latched positions are interpolated between adjacent position samples using this highly accurate time stamp. The position latch is subject to how well the encoder approximates a constant velocity trajectory between two adjacent samples.

Sample Applications

• capture1.cpp

Bibliography: MEI