Due to their design and construction, motion control systems can accumulate positioning errors that affect accuracy and can alter the quality of industrial processes. This phenomenon tends to be gradual, as the individual errors of each consecutive positioning add up, increasing the total accumulated error over production time.
These errors can manifest themselves in a variety of ways, such as slow, cumulative variation in the position of an image or product on a package, the amount of product per unit, or even changes in the overlap of a shielding mesh on a cable.
In situations where the problem is not resolved in the design, if the cumulative error reaches an unacceptable level, it may be necessary to stop production and restart the machine from its initial position to override the errors. However, this solution would only restart the error accumulation from the new starting point.
There are several sources and causes of positioning errors, among which the most common are:
Errors in the mathematical model or in the algorithms used to control the system:
If these models do not accurately represent the actual behavior of the system, deviations between the desired position and the actual position can be generated. A common example is the impact of rounding on final position calculations, which can result in minute errors, sometimes imperceptible to the human eye, but cumulative over time.
Friction:
Friction in mechanisms can introduce positioning errors, especially at low speeds. For example, depending on the torque applied when stopping the system, friction could prevent the exact final position from being reached, generating an error. These errors can accumulate if the positions are successive, affecting each new movement.
Mechanical backlash:
In some motion control systems, the actual position is measured indirectly, for example, through the feedback device of the drive servomotor located at the beginning of the mechanical transmission of motion. This can introduce errors due to mechanical play in the components or wear over time. These backlashes are not recorded in the control system and therefore can accumulate errors in the actual position of the moving element.
Position sensors and references:
Devices used to determine actual position (such as encoders, resolvers, etc.) may have inaccuracies of their own or low resolution. Environmental factors or electrical noise can also influence the accuracy of these sensors, leading to incorrect information of the actual position and, consequently, to inadequate corrections and errors.
System response times:
Insufficient response time in the control system can cause oscillations and deviations from the desired position.
To reduce the accumulation of errors, it is crucial to select suitable components according to the system requirements and to perform a mechanical clearance analysis on the drive train including gearboxes or couplings. In addition, periodic maintenance and verification of the constituent parts will help to minimize these errors. It is common to employ dynamic position error correction techniques in motion control software, such as adjusting, based on the measured positioning error, the distance/speed profiles or adding algorithms to compensate for irregularities in the system.
It is essential to note that different correction techniques can be effective for each type of error accumulation, and the choice of the specific technique will depend on the type of motion control system, accuracy requirements and operating conditions. In these cases, the designer’s preferences play an important role.
Autor – Jorge Leonardo Gorbato – Independent Motion Control Consultant
eurojlg@gmail.com