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MOTION CONTROL & LINEAR MOTION FEATURE


CONSIDERING linear systems


A


number of factors need to be considered before a suitable linear


module for a single-axis, two-axis or three-axis positioning system can be selected. In reality, not all the technical data below will be available, so certain assumptions may need to be agreed.


1. MASS & CENTRE OF GRAVITY The mass (and geometry) of the object to be moved and the position of its centre of gravity as it moves relative to a coordinate or datum point on each axis must be calculated. As a mass is accelerated or decelerated along multiple axes of travel, the position of its centre of gravity relative to each axis will change. This needs careful consideration so that the moment loads at multiple points in the system can be established. Often, calculating the best- and worst-case scenarios and then averaging these is sufficient for most applications.


2. SYSTEM CONFIGURATION & MOUNTING System configuration, including the number of axes of motion, needs careful thought. The most common are two-axis (X-Y) configurations, but less complex single-axis applications and the more complex three-axis configurations are also possible. System orientation and mounting are


important and, in multiple axis systems, this becomes more complex. Factors to consider here include the direction of travel of each axis and the distance between the rails. Does the load need to be moved simultaneously in multiple axes or does each axis move individually? Does the system require a moving carriage or a moving rail? Are the axes vertical, horizontal or inclined?


3. STROKE LENGTHS The ‘effective’ and ‘total’ stroke length for each axis is also critical. With ballscrew driven linear actuators, for example, the


stroke length is limited to the length of the ballscrew itself, so maximum stroke lengths tend to be around 3m. But with belt-driven systems, there are no such restrictions and so stroke lengths can be up to 20m if required. If linear motors are specified, in theory stroke length is unlimited – but in reality lengths above 10m metres are rare.


4. ACCURACY & REPEATABILITY Depending on the application, accuracy and repeatability will differ greatly between applications. For example, if the actuator is for an automated pick-and- place machine, then it is likely that high repeatability and accuracy are required. Typically, the accuracy of a ballscrew- driven linear actuator is 0.16mm/m with repeatability of ±0.01mm. For belt-driven actuators, typical accuracy is around 0.5mm/m with repeatability of ±0.10mm.


5. ACCELERATION, DECELERATION & LINEAR SPEED Acceleration itself is not normally the defining issue in multi-axis positioning systems. It is the loads due to these accelerations that are critical. The highest acceleration of any linear actuator to date is around 50m/s2


are often between 0.5m/s2


, although typically they and 5m/s2


.


Deceleration is also important, particularly if emergency stops are required. Apart from acceleration and deceleration forces, required speed can also dictate the type of linear system chosen, generally to a maximum of 10m/s.


6. EXTERNAL LOADS & FORCES What are the positions and magnitudes of the loads in the system, including external impact forces such as stops or human interventions? Is something pushing or pulling on the load to be moved or does the load need to be brought quickly to a stop at the end of its travel? For example, a drive may bring the linear actuator to a stop or a ‘home’ datum position. How this


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In this article, ‘linear modules’ refers to ‘linear rails’, ‘linear actuators’, ‘driven linear systems’ or ‘linear X-Y tables’. These types of systems typically incorporate a number of different linear drives and actuators, including ballscrew driven systems, belt-driven linear actuators,


ballscrew-driven linear actuators, linear tables and linear motors.


Bob Love, business development manager at Schaeffler UK, outlines


the top ten factors engineers should consider when specifying linear


modules for single and multi-axis handling and positioning systems


Environmental factors such as temperature, humidity and contamination (i.e. dust, oil, water, washdowns, chemicals and coolants, etc.), will affect the choice of linear system


is achieved and how this affects the loads on the mass to be moved are key considerations. It can be useful if the customer provides a 2D line sketch of the application with loads and forces.


7. CYCLE TIMES Cycle times dictate the life of a linear system. If the customer needs the system to last for a minimum of 10 years, changing the tool on a machine five times every hour may not be an issue. But if the tool is changed 10 times per hour, a different type of linear system may be required.


8. ENVIRONMENTAL FACTORS Environmental factors such as temperature, humidity and contamination (i.e. dust, oil, water, washdowns, chemicals and coolants, etc.), will also affect the choice of linear system. A dusty working environment may require the customer to implement external bellows or dust extraction devices for the linear system. The two biggest causes of failures of linear modules are lack of lubrication or re-lubrication, and contamination from the operating environment. Linear actuators can be protected from the environment by incorporating special seals, corrosion-resistant materials and coatings, special greases or by using plastic parts where necessary.


9. ELECTRICAL CONSIDERATIONS For multi-axis positioning systems, drives and other electrical systems are often complex and therefore require careful consideration. A multi-axis linear module is likely to incorporate electric motors, controllers, geared drives, cables, grippers, limit switches, encoders, brakes and other control devices. All of these features can add Mass to the axes.


10. TOTAL COST OF OWNERSHIP Although the initial purchase price of a fully protected and sealed linear system is relatively high compared to a standard linear system, the potential savings that can be achieved in the form of increased productivity, higher operating life and reduced maintenance costs, often more than outweigh the initial higher purchase price of the system.


Schaeffler www.schaeffler.co.uk DESIGN SOLUTIONS | NOVEMBER 2016 13


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