Miniature motors for medical applications

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 enable people to regain a sense of movement. 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 the hand. Small DC motors are necessary for these applications. When selecting a DC motor for a prosthetic, it will need to meet the following requirements:

• compactness
• high power density
• high acceleration
• lightweight
• some also require high controllability

maxon products succeed in every aspect.

Next it is a matter of selecting the most suitable drive system to provide the required torque and speed.

Compact and lightweight

In maxon's brushed motor range, starting from the smallest there is the DCX 6 that has a diameter of just 6mm. Remarkably, maxon can get smaller than that. maxon's specialism is creating small drives below 70V. The company's brushless range starts as small as 4mm diameter with a length of less than 21mm and a mass of just 1.2g.

EC Flat motors are popular for many reasons: brushless technology, lightweight, high torque, compactness and low cost. These motors are used in many applications throughout all areas. This year maxon has developed 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 any design, saving even more space with reduced weight.

High power density

maxon's brushless frameless motor has a maximum continuous torque of 1.03Nm; add a gearhead to increase torque and reduce speed to what you want. Combining the size and the pure power of the motor gives an amazing volume-to-power ratio. Running continuously can limit performance, but if the duty cycle is intermittent then the motors can be overloaded for short periods. This is very useful if, for example, you want to keep the motor size small and cannot go bigger, but require three times the nominal torque of the motor for only a few seconds.

Take a look at the graph below:

Let us take, for example, 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 to only two times, the motor will be 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 at 125degC. The motor requires time to cool down. Be aware if time ON is exceeded and not enough OFF time is provided, this may cause failure. Time required OFF can be calculated using this formula:

maxon's ECX SPEED is very popular 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 their technical and product quality needs.

High speed and acceleration

In a race, the ECX SPEED will take first place every time straight from the start line. These motors top out at 120,000rpm. To reach such speeds you will need the available voltage, as speed is proportional to the voltage.

Take the ECX Speed 13L, for example; it has a stunning 1.58ms mechanical time constant. This is the time taken for the rotor to accelerate from standstill to 63 per cent of the no-load speed of 66,200rpm. So the next issue is how to control such speeds?

High controllability

maxon can control three main parameters: speed, current and position. The company has 1-Q controllers, known as DEC controllers, that enable users 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 has 4-Q controllers that have active braking, so regardless of direction, it can be decelerated at a controlled rate. These controller products are the maxon ESCON, EPOS2 and EPOS4. ESCON controls speed and current, and the EPOS controls all three parameters.

New control cards

maxon is introducing the new EPOS4 EtherCAT cards with combination for EPOS4 encased and module controllers that give real-time feedback. The EtherCAT cards support the CAN Application Layer over EtherCAT standard according to IEC-61800-7 profile type 1 (CiA 402) and fulfil the relevant conformance requirements. Particularly of interest for precise medical equipment, this EtherCAT feature can provide a response as quick as 1ms given the appropriate Process Data Object (PDO) mapping.

maxon medical, based in Switzerland, has been developing a new complete unit that incorporates the motor, encoder and controller, all completely enclosed. This unit uses the new EC Flat Frameless motor and has been designed to be applicable for exoskeletons. 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. maxon can work with customers to design and produce a simple plug-and-play unit.

maxon's industry engineers work with medical companies to design and develop complete systems. Contact maxon on [email protected] or go to www.maxonmotor.co.uk. maxon motor is ISO 13485 accredited.

Visit maxon on stand E12 at Med-Tech Expo 2018, 26-27 April at the Ricoh Arena, Coventry.