An Introduction To Plug-And-Play Motion Subsystems

An Introduction To Plug-And-Play Motion Subsystems
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Embedding mechanical motion subsystems into machines improves performance and reduces cost. By Mike Everman, Founder and Chief Technical Officer, Bell Everman.

If you build machines, you probably work with actuators and positioning stages every day. But do you truly get the best performance or lowest cost of ownership from these motion devices? The answer may not be what you expect.

All too often, engineers think of stages or actuators as just another item on the bill of materials. As long as the motion device nominally meets the desired positioning, force, payload, speed and cost requirements, it is good to go.

With simple motion requirements, this approach to stage or actuator selection may yield acceptable results. However, machines with complex mechanical motion requirements will benefit from an embedded motion design strategy. Rather than a collection of electromechanical components, which may or may not work well together, embedded motion systems function as true plug-and-play machine subsystems.

Embedded motion systems are engineered to fit within a predefined physical space on a machine and tie into the machine’s motion control system, ready to accept commands from a top-level computer interface, control card or PLC. At their simplest, embedded motion systems may consist of little more than a stage or actuator that has been connectorized to make the drop-in installation easier. At their most complex, these motion subsystems extend from pinout to payload. They encompass not only the motion device itself but also everything it carries.

Compared to a component-by-component approach to machine movement, embedded motion offers some compelling advantages:

Mechanical Performance. Even when they use the same stage or actuator, embedded motion systems will typically outperform component-built motion systems. The reason why comes down to application and assembly expertise. A good embedded motion vendor will have years of experience solving difficult positioning problems and a collection of proven motion building blocks that can be customised for the task at hand. They will have an intimate understanding of how the stage dynamics, the motion control architecture and the operating environment will affect positioning requirements.

As for assembly, many machine builders lack the skilled technicians, specialised fixtures, laser interferometers and other metrology systems needed to align the most precise multi-axis stages — which often have axis-to-axis alignment tolerances measured in microns.

Controls Expertise. Embedded motion systems may or may not ship with motion controls, depending on the customer requirements. But a control strategy should always be part of the embedded motion equation. A good embedded motion vendor will have an extensive knowledge of how different motion control platforms and their kinematic capabilities will interact with the mechanical motion systems. This knowledge can allow us to push the envelope on what is possible in terms of dynamic capabilities, such as acceptable inertia mismatch ratios.

Reliability. When commissioning a new motion system, some of the most common problems occur because individual, seemingly minor components fail to work properly — or fail to work properly with one another. For instance, a single faulty connector or the wrong wire can leave even the best motion stage motionless. Embedded motion systems avoid this type of failure because they are assembled and tested as a system before integration on the production machine. With motion systems composed of individual components, small failures and incompatibilities can go undetected until the production machine comes together.

Cost Reduction. Embedded motion systems typically cost 25 to 50 percent less than their component-based counterparts. In part, this savings comes from the ability to reduce parts count — for instance, by designing in brackets, connectors and other components. The cost reduction can skyrocket well above 50 percent when you factor in all the hidden cost components associated with building and installing a motion system. These include costs related to design engineering, inventory, time-to-market and more.

Many types of applications can reap the benefits of embedded motion. We have implemented this approach on dozens of semiconductor, wet bench, laser cutting, packaging and lab automation machines. The case studies that follow highlight two applications whose performance and cost requirements would have been impossible to meet without an embedded motion solution. One is a linear embedded motion system for a precision semiconductor singulation operation. The other is a rotary motion subsystem we created for a unique type of CNC machine.

Linear Stage For Semiconductor Operation

Stacked stages of any kind can suffer from alignment and control issues when you design and build them as a collection of individual axes. If that stacked stage has to meet challenging positioning accuracy or speed requirements, it is crucial that stage function as an integrated system. We recently delivered just such a stage for a semiconductor singulation operation.

This multi-axis stage required a linear drive system versatile enough to make two very different types of moves. One was a long travel move at 400 mm/sec. The other was a short, high-speed move of 13 mm that must settle to 10 microns in 150 milliseconds. The moving mass seen by the lowest axis in the system was 38 Kg with a bi-directional accuracy goal of ± 5 microns based on the positioning reference from a 1-micron Renishaw optical linear encoder.

The customer first tried to use an existing XY ballscrew stage design. By a wide margin, it simply failed to make the desired moves and meet the customer’s throughput requirements. It was possible, in theory, to come up with a ballscrew-based design that would meet the motion requirements. Such a design, however, would require expensive zero-backlash ballscrews and encoders that would exceed the project’s cost targets. The customer next turned to linear motors. While capable of making the desired moves, linear motors for this application would have been large and expensive due to the long motor coil needed to meet the application’s 300 N continuous force requirements. The length of the coil would have required sweeping changes to the overall machine design. And the cost of the linear motor would have been more than 50 percent higher than the customer’s cost targets.

Ultimately, the customer went with an embedded motion system based on our ServoBelt Linear drives and a counterintuitive control strategy that rejected dual-loop control in favor of single-loop control using only the linear encoder.

Getting Started With Embedded Motion

The jump from component-by-component motion systems to embedded motion systems may seem like a leap of faith. You will, after all, be outsourcing the motion control to a vendor.

If you pick the right vendor, though, the outsourcing will pay off with improved performance and reliability. Costs will fall too as motion subsystems arrive at your plant fully-tested, warrantied and ready to drop into your machine.

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