A robot control loop reads sensor and hardware state, computes commands and sends them to the actuators. Compare the time assigned to synchronous work with the period set by your loop rate, using a timing calculator alongside the ros2_control read-update-write sequence.
Control-Loop Timing Calculator
Enter a loop rate and four declared timing buckets. The worksheet compares their arithmetic sum with one selected period; the buckets are user-defined accounting categories, not a prescribed ROS 2 architecture or measured latency.
Equation Used
- Loop rate is positive and defines one nominal period.
- The four entered values are nonnegative, mutually exclusive declared timing buckets.
- All four categories are assumed synchronous in this calculation; any actual asynchronous execution must be measured and modeled separately.
- Jitter, scheduler policy, operating system, middleware, transport, hardware, controller dynamics, sensor age, deadline misses, safety and stability are excluded.
The read-update-write sequence illustrates the control loop. Entered time categories show how much of one period is used and whether their sum exceeds it; they are not measured execution times.
The controller manager reads, updates and writes
The official ros2_control controller-manager documentation describes a real-time loop that reads hardware state, updates active controllers and writes commands. Its update_rate parameter sets the loop frequency.
The four colored timing bars are user-defined categories for the work being timed. They are not required ros2_control stages. Use measurements from the target system when assessing an actual implementation.
Use this as a timing-accounting worksheet
The worksheet can compare a selected period with a declared sum of synchronous work. It can expose an obvious arithmetic overrun before implementation.
It does not select hardware, middleware, a ROS distribution, controller, robot, actuator, safety architecture or deployment configuration.
Nominal period and declared timing sum
Times are in microseconds and f is in hertz. A negative margin is an arithmetic overrun. A nonnegative margin is not a proof of closed-loop stability, deadline reliability or safety.
Declared timing example
At 1,000 Hz, one nominal period is 1,000 microseconds. Declared buckets of 200, 400, 200 and 200 microseconds total exactly 1,000 microseconds: 100% utilization, zero margin and zero arithmetic overrun.
That edge case leaves no allowance in this calculation for additional synchronous work or timing variation.
Timing variation and controller behavior
Real execution depends on the operating system, scheduler policy, CPU isolation, priorities, hardware interfaces, middleware, transport, controller design, asynchronous work, memory behavior and measurement method. Closed-loop stability also depends on plant dynamics, delays and the controller, not merely whether a time sum fits.
A timing sum is therefore one part of system analysis. It cannot establish controller accuracy, deadline reliability or deployment readiness.
Control-loop timing questions
Does a positive margin prove the robot is stable?
No.
Are the four buckets required ROS 2 stages?
No. They are user-declared accounting categories. The official ros2_control loop is described as read, update and write.
Does zero margin mean the system is deployable?
No. This calculation omits jitter and additional work not included in the entered times.
Can this select a processor or network?
No. Use measurements and requirements from the actual target system.
ROS 2 control-loop references
- ros2_control Kilted, Controller Manager: official documentation for update_rate and the read-update-write loop.
- ros2_control Rolling, Running Controllers Asynchronously: official timing example and synchronous-period constraint.
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