positionwhile

PositionWhile is a term used in motion control and robotics to describe a control-flow pattern in which a block of commands is executed while a positional predicate remains true. It is not a standard keyword in most programming languages, but many vendors describe similar behavior as a position-based loop or position-driven loop. The construct relies on real-time position feedback from sensors such as encoders or potentiometers to gate the execution of motion commands.

In practice, PositionWhile ties the progress of a motion task to its physical position. The condition typically involves the current position, target position, and a tolerance, ensuring the loop continues while the system has not yet reached the desired region. This allows a machine to approach a target and hold, scan along a path while staying within a specified corridor, or perform actions at points along a trajectory as long as the position remains within defined bounds.

Example in pseudo-code:

PositionWhile (distanceToTarget() > tolerance) {

commandTowards(targetPosition);

currentPosition = readPosition();

}

The loop terminates when the position enters the tolerance region or when an external timeout or stop command is issued. Potential caveats include sensor noise, dead zones, and the risk of an infinite loop if the position cannot be updated; robust implementations use rate limiting, timeouts, and watchdogs.

PositionWhile sits alongside general while loops but is distinguished by its emphasis on the coupling between position state and control flow, and by its frequent use in real-time systems that require position-aware decision making. See also: while loop, position control, servo, CNC.