Capture

Capture is the process of capturing a precise position of an axis or feedback at the edge of a digital input. In addition to capturing a position, the time of the edge is also stored.

The AKD2G drive supports two user capture channels, CAP1 and CAP 2. Each channel can capture one position and one time on the transition of a trigger input or the index pulse from some position feedback devices. Each capture channel can be used with any feedback or axis and any digital input.

Optional pre-conditions allow filtering of which input edge will be used to capture the position. For example, you can trigger the first rising edge of DIN3 after another input DIN4 goes high.

Each capture channel is arranged as follows:

The position is a sampled with dedicated hardware in the drive. When using the inputs that use fast optocouplers, DIN1 or DIN2, or the DIO channels, then the accuracy of the captured position will be better than 0.0038°. When using the other digital inputs that have slow optocouplers, DIN3 to DIN2, then the added latency on these inputs increases the worst-case error to 0.24°. These numbers assume the filter turned off (DIN#.FILTER is 0).

For a typical case where the axis is at a constant speed of 2000rpm and using DIN1 as the trigger the accuracy would be of the order of 0.00048° which is significantly better than the accuracy of most feedbacks.

It is recommenced to set the input filter, DIN#.FILTER or DIO#.FILTER, for the trigger source to zero to disable the input filter. If you configure an input filter, this additional delay to the trigger will make the captured position less accurate.

When using auto-rearm the smallest time between consecutive triggers is 125µs.

The following keywords can be used to configure and then retrieve the captured positions.

Keyword Summary
CAP#.ARM Arms or disarms the capture channel.
CAP#.COUNT

The number of capture events that have happened

CAP#.EDGE Selects which edge of the trigger input to use.
CAP#.P The captured position.
CAP#.PREEDGE Selects the edge used by precondition
CAP#.PREMODE Selects the type of precondition. 0 is no precondition.
CAP#.PRESELECT Selects the digital input is used by the precondition.
CAP#.REARM Selects auto rearm instead of single shot.
CAP#.SOURCE Selects what will be captured on the trigger edge.
CAP#.STATE Reports if the channel is waiting for the trigger or is triggered.
CAP#.TRIGGER Which digital input will trigger the capture.

Typical Uses of Capture

Position capture is used in precision environments where the motor may be moving at a very high velocity, and you must know exactly where the motor was when an input changes state.

Motion Tasking - Registration Moves

See Registration Moves.

Measure Pulse Width

You can use the captured times to measure the width of a digital input pulse.

To measure the width of a pulse, or the time between two pulses, you can use both sets of CAP# keywords. Set one to capture the first edge and the other to capture the second edge and the width of the pulse is the difference between the two captured times.

CANopen (EtherCAT or CAN bus) Touch Probes

AKD2G does not use CAP1 or CAP2 to provide the CANopen touch probes. Two touch probes per axis are fully supported over EtherCAT or CAN bus with their own dedicated hardware in the drive.

Reference

Element Property Image Description Parameter
Source for Capture # Configuration Source Select the source of the captured position. CAP#.SOURCE
Trigger Input for Capture # Configuration Input Select which digital input will trigger the capture. CAP#.TRIGGER
Pre-Condition Input for Capture # Configuration Input Select the digital input that is used by the precondition. CAP#.PRESELECT
Pre-Condition for Capture # Configuration Pre-Condition Mode Select pre-condition for the capture.
  1. No precondition
  2. Trigger after precondition edge
  3. Trigger while precondition is 1
  4. Trigger while precondition is 0
CAP#.PREMODE
Pre-Condition Edge Select the edge used by the precondition.
  1. Rising edge
  2. Falling edge
  3. Rising or Falling edge
CAP#.PREEDGE
Arm button   Arm or Disarm the capture channel CAP#.ARM
Capture block   This item performs the logic necessary to calculate the capture state. This item provides four pieces of information.
  • Arming Mode icon (Single Shot or Continuous)
  • Arming Edge icon (Rising, Falling, or either)
  • Armed status icon (on or off)
  • Current capture state
CAP#.STATE
Capture # Configuration Arming Mode Select how the channel is armed.
  1. Single shot
  2. Continuous (automatically re-armed after capture)
CAP#.REARM
Arming Edge Select which edge of the trigger input to use.
  1. Rising edge
  2. Falling edge
  3. Rising or Falling edge
CAP#.EDGE
Capture Position   This output shows the capture position. CAP#.P
Capture Time   This output shows the drive's local time at capture. CAP#.T
Capture Count   This output shows the capture count. CAP#.COUNT