Real-Time Robotics: Before you start

• Slip refers to the difference between the measured (or expected) position or orientation of a robot and its actual position or orientation. Slip can be caused by a variety of factors, but most commonly internal to the robot due to wear on mechanical components and external to the robot due to reduced friction in grasping an object or driving a wheel.

• Drift refers to a gradual, persistent change in the output of a sensor over time. Drift can be caused by a variety of factors, such as temperature changes, aging of the sensor, or in case of sensor fusion, a bad calibration.
• Noise refers to random, unwanted variations in the output of a sensor. Noise can be caused by a variety of factors, such as electrical interference, mechanical vibrations, or random fluctuations in the sensor's environment.

I wish that was it, but there’s a fourth factor that we first need to control before we can address the above three correctly…

Example: Imagine you are dribbling a basket ball. Each time the ball is about to hit its peak, you need to push it down again. The timing of pushing down depends heavily on your eyes seeing and hands feeling the ball.

Imagine now that your hands and eyes sense a different time for the basketball’s peak. Even worse, sometimes your eyes see the peak first and then the hands feel it, and next the other way around. Since you can’t get the timing right, the ball will quickly loose height and come down to rest.

These jumps in timing is what we call jitter, and even with perfect eyes and perfect touch, jitter will make you loose the ball. Exactly the same will happen in real-time control systems, the controller will fail to keep the system running.

To address jitter directly, we use synchronization techniques to align the timing of sensor readings with a common reference signal. The most common synchronization techniques in robotics are:

1. Common clock signals: In this technique, multiple sensors sample their inputs at regular intervals, using a common electrical pulse signal to coordinate their operation. So you are guaranteed that every measurement was done at the same time.
2. Precision timing protocols: Protocols such as IEEE 1588 (Precision Time Protocol) can be used to synchronize the operation of multiple sensors and the control system with high accuracy, up to microseconds, using a combination of hardware and software techniques.

If jitter can not be addressed directly, so measurements are made out-of-cadance, one can revert to measuring the time at which a sensor reading was taken. This requires a setup with three important features:

1. Taking Timestamps: Many sensors have built-in real-time clocks or timestamps that can be used to record the time at which a reading was taken.
2. Synchronizing clocks: An external clock signal, such as a GPS signal or the IEEE 1588 protocol above, can be used to synchronize the clocks of the sensors and the control system, allowing to record the exact time at which each reading was taken.
3. Re-alignment: A means to re-align the measurements to a single time instant, by interpolating or extrapolating each sensor reading to that time instant.

Eliminating jitter does not eliminate slip, drift and noise in a robotic control system. It only allows us to start working on the real-time data readings. Now let’s explore the open source libraries available to implement real-time data readings.