Accuracy and reliability in linear motion systems

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In this technical article Paul Seale, Regional Sales Manager for the Electromechanical Division of Parker Hannifin, discusses the question of accuracy and reliability in linear motion technology.

Accuracy and reliability in linear motion systemsThere are many options open to the designers and specifiers of automation systems based on linear actuator technology. OEM machine builders or end users preferring to design their own bespoke systems can choose from simple slide systems driven indirectly from stepper or servo motors, or they can opt for the higher performance offered by direct-drive linear motors. But before the final choice is made, there are some fundamental factors that have to be taken into consideration when selecting the appropriate system for each application.

With most positioning systems the prime concern is usually repeatability, or the ease with which a linear motion device can consistently return a work-piece or tool unit to the same position time after time. Repeatability measurements are expressed as plus-or-minus values and are generally statistical rather than absolute measurements; in statistical terms, repeatability is the width of the dispersion about the mean value for a large number of positioning trials.

Accuracy versus repeatability

In comparison, accuracy is defined as the maximum allowable error between the target position and the actual, or achieved, position of the linear motion system and is again expressed as a plus-or-minus figure.

However, repeatability and accuracy figures are often confused. There are many analogies to illustrate the difference between these figures, but the classic 'bulls eye' is perhaps the best image to visualise. Accuracy is hitting the target, or positioning the actuator exactly to where it is required, while repeatability is hitting the same position consistently, time after time (even if it is not in the centre of the 'bulls eye'). In terms of positioning motors, it is the ability of the motor to return to the same position from the same direction, with that position being defined by the accuracy of the system.

Stepper motor specifications provided by manufacturers may list other performance criteria along with accuracy and repeatability, such as relative accuracy and hysteresis, for example. Relative accuracy, also sometimes known as step-to-step accuracy, relates to micro-stepped motors that are more suited to low-speed operation.

On closed-loop stepper systems and those driven by servo motors, repeatability, accuracy and relative accuracy are as much functions of the feedback systems as they are of the inherent characteristics of the motor and drive. Some use resolver feedback while others have an encoder providing the feedback for the control loop. In both cases, it is the resolution of the sensing device – resolver or encoder – that determines the accuracy of the system.

Other characteristics

So, armed with a knowledge of the desired accuracy and repeatability of a new system, the designer can now set about specifying the system in detail. First, though, there are a few more factors to be considered and fully understood. For example, load characteristics, performance requirements and coupling techniques all contribute to the final decision-making process. The designer must take into account the torque required from the motor, ensuring that the torque capacity of the motor exceeds the load. This 'torque margin' is necessary to accommodate mechanical wear, lubricant hardening, and other unexpected causes of friction in the system that, over time, can increase the torque demand on the motor.

Even under normal operation, all mechanical systems are subject to frictional forces and, again, these need to be taken into account when sizing the motor. A small degree of friction is, however, generally desirable, since it can reduce the settling time of the system and help to improve performance.

The positioning resolution demanded of the application will also influence the type of mechanical transmission selected – gears, lead screws, belts and so on – and the motor resolution. For example, a 5mm lead ballscrew driven by a 25,000 step/rev motor drive is equivalent to 5000 steps/mm, with each step translating to a theoretical movement of 0.0002mm.

Inertia and velocity

Other parameters that need consideration include inertia and velocity. Inertia is basically a measure of an object's resistance to a change in velocity, and both the motor's own inertia and that of the load can affect the final choice of system. For a high-performance, relatively fast system, for instance, the load inertia reflected back to the motor should not generally exceed the motor's own rotational inertia by more than a factor of ten, as this can cause the system to become unstable. This figure can, however, be improved considerably by the use of devices such as Parker's Compax3 servo drive, which can accurately handle mismatches by factors in excess of 100:1.

The design and specification of a linear motion system is clearly not to be taken lightly, as acknowledged by many leading system and component manufacturers, such as Parker Hannifin, who now offer pre-assembled multi-axis systems built around their standard products. This approach lessens the design load on the OEM or end user, but retains the flexibility for them to arrive at the optimum solution for each application.

Case study: capsule filling machine

This application was for a machine to dispense radioactive fluid into capsules. After dispensing, the capsules are inspected and the data stored on a PC. The main objective was to increase throughput without any danger of spillage. The motion control requirements were for rapid, accurate moves from a multi-axis controller fitted with a high-speed interface. The drive was to be via an open-loop stepper motor with a high-resolution, microstepping motor/drive.

The system offered by Parker Hannifin consisted of a multi-axis indexer to control and synchronise both axes of motion on one card using Ethernet communications; integral I/O was also required to activate the filling process.

Each tray of capsules to be filled is carried on a linear motor driven along the horizontal axis to the filling station. The simple mechanical construction of the motor makes for easy installation and guarantees maintenance-free operation.

A vertical axis raises and lowers the filling head and is driven by a microstepping motor and a leadscrew assembly. A linear motor was deemed not suitable for this axis, as the fill head could drop onto the tray in the event of a loss of power to the motor. However, friction in the leadscrew and residual torque in the stepper motor prevent this from happening in the final design – which, by using a combination of linear actuator technologies, fulfilled the customer's detailed requirements.

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