In electromechanical linear motion systems, accuracy and repeatability are key to measuring the performance of an actuator. These two factors define the proper production techniques that are key for the standardization and consistency required when automating manufacturing processes.
Accuracy is the ability of the system to move to its intended position within some measurable tolerance. Repeatability is the ability to achieve that specified level of accuracy each time it is called to action. There are a number of variables that can affect accuracy and repeatability. An electromechanical actuator (EMA) is better able to mitigate these variables to improve accuracy and repeatability than pneumatic or hydraulic actuators. The factors are explained below.
Factors Affecting Accuracy and Repeatability
For both ram-style and rodless screw driven units, accuracy and repeatability are mainly a function of the screw and its mating nut. In most cases, the accuracy is driven by the precision of the manufacturing processes that are used to form the threads on the screw. The screw the nut travels upon needs to be dimensionally consistent while maintaining the specified tolerances for critical variables. Maintaining tolerance control is critical because each screw rotation translates into a corresponding amount of travel distance. If there are any manufacturing deviations along the length of the screw, the distance traveled for each rotation of the input shaft can vary. This could lead to variations in the position of the load.
Two notable screw manufacturing processes are used for screws implemented in EMAs. Many ball screws have threads that are rolled (a form of cold forming in which threads are forged onto a round steel blank). This process yields highly consistent thread forms along the total length of the drive element. An added benefit of thread rolling is that the thread surface is much smoother than those of
other screw production processes. This reduces friction and helps mitigate wear. Thread forms can also be cut or ground, as is typical of Acme, Trapezoidal, and roller screws.
The amount of “play” or freedom of movement between the mating surfaces of the screw and the nut is known as backlash. Backlash can be present in all screw forms. When that allowable freedom is reduced, either through more precise manufacturing processes or the ability to pre-load nuts (where two ball nuts are pre-tensioned against each other with a spring or spacer), the associated backlash can be reduced or eliminated entirely. This ultimately improves the repeatability of the actuator.
Belt driven EMAs can offer appreciable accuracy and reliability, albeit with some limitations that are inherent to these drive types. As with screw driven EMAs the consistency in the belt drive mechanism, namely the pulleys and drive belt, is crucial to the resulting accuracy. Quality of manufacturing and design control are paramount for these components. The more precisely the components are manufactured, the greater the improvement will be in the accuracy and repeatability of the system.
Belt tension is also a factor and should be regularly checked to ensure the levels of accuracy and repeatability are maintained. This is especially true for systems that experience high force loads (because the drive belt can stretch) and for longer stroke length systems (because there is a greater tendency for drive belts to sag and disengage the pulley).
It should be noted that the accuracy and repeatability of all EMAs rely upon the ability of the motor to provide a consistent output. If the motor is equipped with an encoder capable of supplying positional feedback, then precision can be further controlled. In some cases, the control system logic can be used to account for variances in travel respective to tolerance stack-ups. This allows accuracy and repeatability to be controlled and accounted for in the system over the course of the life of the actuator. This affords the user the ability to compensate for wear over time, which is not possible in any other type of linear motion system.
The mechanical relationships among an EMAs system components are easily definable and calculable which makes its performance more predictable. Learn more about the accuracy and repeatability advantages of EMAs and how variabilities in hydraulic and pneumatics can affect consistent performance with our new free guide. Download Accuracy and Repeatability in Linear Actuators: Why Electromechanical Systems Outperform Pneumatics and Hydraulics.
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