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Motor controllers - build or buy?

Motor controllers - build or buy?

Experienced engineers often talk of developing their own motor controller for an application, as it's perceived as being easy and, therefore, a cheaper option. But is it? We put this question to Mark Gibbons, technical engineer at Maxon Motor.

There can be a number of attractions to building your own motor controller. Certainly if your specification requests that the intellectual property of the device, firmware and inner workings is known by the customer then you may want to consider going down that route. But if you haven't developed a controller before and are considering whether to build or buy, there are some factors to be aware of.

The initially cheap overheads of buying the separate components and making a customised drive can be attractive, but an important point is that often the entire cost of developing a motor controller is not realised. The solution and the effort for optimising the control algorithms, the power stage design and current measurement are very often underestimated.

This task is made more difficult when trying to control high dynamic, low inductance, coreless/slotless permanent magnet motors. Sensorless control of brushless (electrically commutated) motors is even more demanding and the success strongly depends on the application's requirements. The effort of developing a controller and power stage from scratch (based on a general application note) will 'cost' at least 6-12 months of development time and the 'risks' involved will require management and liability insurance. The developed controller will also have to be inspected, and maybe certified to the standards required by the application.

Engineering a controller to run a specialist motor is not just a simple matter of selecting a microcontroller and copying the schematics of an application note, it is considerably more complex and there are often unforeseen issues that can jeopardise the success of the entire project. These complexities increase with the demands of the particular application and its environmental conditions; for example, developing highly sophisticated sensorless control for low speed or changing load operation would be extensively more demanding and and could take 12-24 months of development time and costs.

Why, though, reinvent the wheel. Companies such as Maxon can offer not just off-the-shelf controllers but also customised controllers with fair development costs and a reduced development time, thanks to a wealth of experience with different DSPs, control algorithms and power stage design. Further, the testing and certification can be facilitated and because of experience the units are more likely to pass these certifications, so eliminating the wasting of precious development time on multiple revisions and repeating attempts to achieve those specified certification levels.

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