Older engineers will remember the days when machine design had to be based on catalogued electromechanical actuators. Variations from standard were rare, and expensive. Fast forward 40 years and today’s machine designers expect highly configurable and adaptable products and motion technology suppliers have developed their product portfolio specifically for them. Here are some key factors to help in the selection of electromechanical motion products for the machines of today.
Among the fundamental questions to ask is: Does the actuator offer various stroke lengths as standard? A product using a ball or leadscrew drive is commonly restricted to stroke lengths up to around two metres maximum for practicality. Some actuators offer strokes to four metres, however speed is often limited due to screw whip. Very long stroke lengths can, however, be achieved by belt drives, which perform to a similar level regardless of stroke length, but lack the precision of a screw-driven product. A further option is linear motor-driven products, which perform extremely well with increases in stroke. In addition, linear motors do not demonstrate speed restrictions at longer strokes and offer the same repeatability over the full stroke.
Machine builders should determine if the actuator is available in a number of different frame sizes or widths. Having a family of products to select from allows the project to be cost-optimised. Moreover, many multi-axis applications demand different loading for each axis. Having multiple drive-train choices in the same product can be extremely useful to a machine designer.
Modularity and performance
Electromechanical actuators often need to be connected to other actuators or mechanical devices. The ability to combine linear actuators into XY, XZ or XYZ assemblies quickly and effortlessly is vital, and most modern electromechanical products can be bolted together like building blocks, without the use of transition plates for XY systems. Performance-to-size ratios should also be considered carefully. Using a product that is highly condensed leaves more space for machine designers to include end effectors and tooling. For this reason, metrics such as thrust or rated load per height-by-width become important.
Selectable resolutions and encoder types
To retain good servo control, an actuator should have up to ten times more resolution available than the repeatability of motion required. Being able to adjust the resolution is also important. In using analog feedback signatures, two analog signals are passed from the encoder to the drive, 90° out of phase with one another. Equivalent resolution is established within the drive, and is dependent upon the pitch of the linear scale and resolution of the drive’s analog input.
Flexible resolution is another major benefit. In applications that do not require especially high levels of precision, magnetic encoder technology is a cost-effective option, while applications that do require high precision but not long stroke lengths benefit from the very high resolutions of capacitive encoders. Inductive encoders are popular for applications directly exposed to heavy contamination, such as coolant from a machine tool. Applications which require constant positional information will require an absolute feedback source. With regard to communications, until recently most encoder protocols were based on embedded proprietary signals, which meant that designers had to use a limited list of manufacturers. Today, open standard protocols allow design engineers to use a variety of products and even reduce installation and cabling efforts.
In cutting-edge design software, finite element analysis can be used to understand not only the deflection characteristics, but also the thermal or magnetic variations within the product. Naturally, these simulations cannot give results with 100% certainty as they are only as good as the algorithms and assumptions that are used, but modern machine design is starting to leverage these digital design methodologies more heavily to expedite development.
Metrology test data
Test data from metrology can be used to take ballscrew backlash into account and improve overall system accuracy. Selecting products from a design partner that understands the mission-critical aspects of the application and tests will prove particularly advantageous.
The demand for faster turnaround time on machines has become critical. Key to the machine- building race is selecting the right product from a manufacturer that understands the daily design hurdles and has systems in place for rapid machine development. Factors such as breadth of product, range of options, modularity and product test data should be taken into account when designing the machine. This strategy will enhance the machine builder’s ability to respond to customer needs, and provide faster ROI.
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