Understanding Loop Times

What loop time is, what makes it slow, and how to measure it.

Loop time is how long one pass through while (opModeIsActive()) takes. A slower loop means more delay between a stick input or a sensor change and the robot reacting to it. For PID and other feedback control, the loop rate limits how well the controller can hold a target.

What slows a loop

  • Blocking calls. sleep(500) stops everything for half a second. Nothing is read and nothing is updated.
  • Hardware reads. Every encoder read, sensor read, and setPower() call is a command to the hub. I2C sensors like the color and distance sensors are slower than encoder reads. Reading the same device twice in one loop costs twice.
  • Telemetry. telemetry.update() sends data to the Driver Station. Calling it more then once per loop, or sending a lot of lines, adds up.
  • Object creation in the loop. Creating new objects every pass triggers garbage collection pauses.

Measuring

ElapsedTime loopTimer = new ElapsedTime();

while (opModeIsActive()) {
    // rest of the loop

    telemetry.addData("Loop Time (ms)", loopTimer.milliseconds());
    loopTimer.reset();
    telemetry.update();
}

Watch the number as you add or remove code to see what is expensive.

Why it matters for controllers

PID and motion profiles use dt, the time since the last update. Measure it with ElapsedTime instead of assuming a fixed value, so the controller stays correct when the loop time changes. A measured dt does not fix a loop that is simply too slow to react, it only keeps the math consistent.

Keeping loops fast

  • Use bulk reads so all encoder and digital reads on a hub happen in one command.
  • Read each sensor once per loop and store the value.
  • Replace sleep() with ElapsedTime checks or a state machine.
  • Call telemetry.update() once, at the end of the loop.
  • Only write to a motor or servo when the value changed. The SDK already skips repeated identical writes for motors, but skipping the call still saves time.

Mistakes

  • Never measuring. Add the loop time line to telemetry during development so slowdowns are visible.
  • Adding sleep() to fix timing. It blocks the whole OpMode. Use a timer or a state.
  • Assuming it only matters for autonomous. Driver response and sensor safety checks depend on the loop too.