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Miniature motors bring new capabilities to prosthetic limbs

Miniature motors bring new capabilities to prosthetic limbs

Miniature DC motors as small as 4mm in diameter are playing an increasingly important role within medical device technology, for example in prosthetic limbs, as Patrick Vega of Maxon Motor explains.

Prosthetic limbs are used to replace what is lost to an amputee. These contain DC motors that play a major part in the capabilities of an artificial limb. Prosthetic limbs allow people to gain a sense of feeling of movement again. Imagine something as simple as picking up a glass of water. We require our fingers to wrap around the glass and apply just the right amount of force around the glass to keep it held in your hand. Small DC motors are necessary for these applications.

When selecting a DC motor for a prosthetic, it will need to fill requirements for compactness, high power density, high acceleration and light weight. And some require high controllability. Next it is a matter of selecting the most suitable drive system to provide the required torque and speed.

Maxon’s brushed motor range start from a diameter of just 6mm, while the brushless range starts as small as 4mm diameter with a length of below 21mm and a mass of just 1.2g. This year Maxon’s EC Flat brushless motors have been joined by the EC Flat frameless. Forget the motor’s housing, bearings and motor shaft – all you have is the rotor and stator so that you can fully integrate the motor into your design – saving even more space with reduced weight.

The brushless frameless motor has a maximum continuous torque of 1.03Nm, and a gearhead can be added to increase torque and reduce speed to meet the application requirements. Combining the size and the pure power of the motor gives an amazing volume-to-power ratio.

Running continuously might be a limitation, however if the duty cycle is intermittent then the motors can be overloaded for a period. So you can benefit from, say, three times the nominal torque for a few seconds while still keeping the motor size small, as highlighted in the graph.

Consider the DCX14L with graphite brushes. If this motor was overloaded to three times the nominal torque it will be able to operate for roughly 10.3 seconds, which is the thermal time-constant of the winding. Now if you overload the motor only two times the motor will able to run for 51.5 second, which is five times longer.

Remember that the motor has been overloaded so the winding is very hot and requires time to cool down, or there is a very real risk of motor failure.

Maxon’s ECX Speed is a popular option in the medical industry for applications that require high speed and high acceleration. Applications include dental drills and centrifuges. This family of products was developed to help the medical industry fulfil its technical and product quality needs.

When it comes to motor control, Maxon can control three main parameters; speed, current and position. The company has 1-Q controllers, known as DEC controllers, that allow you to set a speed and it will run at that set speed. However, there is no active braking, meaning there is no deceleration apart from the moment of inertia of the rotor and the load.

On the other hand, Maxon also has 4-Q controllers which have active braking, so regardless of direction the motor can be decelerated at a controlled rate.

A new controller from Maxon is the EPOS4 EtherCAT card with combination for EPOS4 encased and module controllers which give real-time feedback. EtherCAT cards support CAN Application Layer over EtherCAT standard according to IEC-61800-7 profile type 1 (CiA 402) and fulfils the relevant conformance requirements.

Of particular of interest for precision equipment in the medical device sector, this EtherCAT feature can provide a response as quick as 1 millisecond given the appropriate Process Data Object (PDO) mapping.

Maxon Medical based in Switzerland has been designing a new complete unit that has the motor, encoder and controller completely enclosed. This unit uses the new EC Flat Frameless and has been designed to be applicable for exoskeletons. For product developers in this market, it means there is no need to worry and waste time thinking about how to enclose a unit or the best way to mount a complete system in to a limited space.

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