What is Accuracy & Repeatability:
In order to understand “repeatability” and “accuracy” we first need to define what these two terms mean.
Repeatability refers to the consistency of measurements taken under identical conditions. Using the classic bow, arrow, and target example, it describes how tightly grouped the arrows are on the target. If all your shots are close to one another, you are considered very repeatable.
Accuracy refers to the freedom from mistake or error and the conformity to a standard or truth. In the same bow, arrow, and target example, it describes how close your shot is to the bullseye. The closer your shot is to the bullseye, the more accurate you are considered to be.
When we apply the terms repeatability and accuracy to motion control, we are referring to the ability of an actuator or stage to position itself. This applies to both linear and rotary motion. In motion control, repeatability is paramount; if your system is repeatable, it can achieve accuracy. However, without repeatability, achieving accuracy is impossible.
How to Measure Repeatability & Accuracy:
To measure the repeatability and accuracy of a motion control system, one must compare the system’s theoretical position (where it is commanded to be) to its actual position (where it physically ends up). Accuracy is determined by measuring the deviation between the commanded position and the actual position using precise instruments like laser interferometers. Repeatability, on the other hand, is assessed by repeatedly moving the system to the same position and measuring the variation in the actual positions achieved. This involves using a known reference point or datum to ensure consistency. By analyzing these measurements, one can quantify how closely the system adheres to its intended path and how consistently it can return to a specific position.
When making measurements, we need to compare our systems position to some sort of measured reference point. In high precision motion control applications, we typically use a CMM (Coordinate Measure Machine) or a Laser Interferometer. These devices are calibrated to precise standards and known benchmarks; as a result, users can have greater confidence in the reliability of their measurements.
Figure 1 shows an example of a linear stage is being evaluated with a linear interferometer. To start the test, we homed the linear stage (moved to one end of travel) and then zeroed out the laser. We then commanded 1mm moves forward, measured each position and then repeated this over the entire range of travel. Once we reached the end of travel, we retracted 1mm all the way back to home and then repeated this process over several iterations. By measuring position both forwards and backlash, we can determine if there is any backlash in the system; backlash refers to the slight movement or play between mechanical components when the direction of motion is reversed, leading to positioning errors and reduced precision.
Accuracy: In this example, we can can see that as our linear system translates further from home, our system becomes less accurate. That is to say that our commanded position (where we think we are) grows further from where we actually in real space. In this example, when command a 10mm we are 35um from accurate.
Repeatability: In this example we can see that as we move through travel, we can determine how consistently we return to any given point is greater. Our repeatability is determined by the greatest range in measurements across all points.
How Repeatability can drive Accuracy:
Evaluating and quantifying a motion control system’s repeatability and accuracy is crucial for enhancing its performance. Repeatability, the ability to consistently return to a specific position, is a key factor in determining overall system performance. When a motion control system demonstrates high repeatability, it means that any deviations from the desired position are consistent and predictable.
These consistent deviations can be systematically corrected through calibration or software adjustments. By mapping out these deviations, engineers can implement compensatory measures that improve the system’s precision. This process involves fine-tuning the control algorithms and making necessary adjustments to the hardware, ensuring that the system can reliably achieve the desired outcomes.
Understanding and improving repeatability provides a solid foundation for achieving greater accuracy in motion control systems. By correcting known errors, the system can deliver more precise and consistent performance. This not only enhances the system’s reliability but also its efficiency, making it better suited for applications requiring high precision and accuracy.