Putting prosthetic hands in motion
There was a time when a hand amputee would have been asked to use a hook prosthesis that had limited functions and carried a significant amount of social stigma. Today, designers are looking to build prosthetics that are not only functionally efficient, but also durable, comfortable, lightweight and aesthetically pleasing.
Here, Chetan Kale, R&D Engineer at Portescap has dicussed how standard motion solutions can meet all of these requirements in packages that also offer long battery life and low maintenance.
Prosthetic technology has developed enormously in recent years, with a hand amputee for example today being able to receive a prosthetic that will replicate a whole host of normal hand functions. Underpinning this functionality is a mechatronic system that combines high power density with optimum performance, addressing considerations for efficiency, reliability, accuracy, size, noise and overall weight.
Based on the essential prehension requirements of the given prosthetic, a 180-degree linear actuator is the start point for each of the motion elements. Such an actuator will typically comprise a motor that transfers the rotary motion to a lead screw by using a spur gear pair. The lead screw is attached to one spur gear and transfers motion to the piston which has internal threads. The piston moves forward and backward due to the screw and nut mechanism between the lead screw and the piston. This linear motion of the piston is what creates finger actuation.
There are a number of key parameters to consider within this integrated solution as they relate to the operation of the prosthetic hand. The opening and closing of the palm translate into a requirement for a number of cycles per day, and 100 to 500 cycles might be typical. Then there is the time taken to actuate the hand to grip an object – the cycle time. For high speed gripping, this cycle time might be 1-2 seconds. This in turn will imply an actuation speed (the movement of the fingers) of perhaps 12-15mm/sec. The holding force is another consideration, perhaps in the range 30-80N. And if the prosthetic is to be battery operated, the supply voltage and current must also be considered.
The linear speed of the actuator depends on the weight of the object the user is handling. Higher linear speeds allow the user to grasp objects quickly. The force and speed depend on the designer’s choice of motor, gearbox, and the size of the leadscrew used for the linear actuator.
Selection the motor/gearbox combination is no trivial task, with the designer needing to find the right balance of performance characteristics to ensure the proper holding force and linear speed necessary for grasping, while also optimising the package for the available space and the required battery life.
Electric motor selection
Motor regulation is a critical parameter of the motor which defines speed-torque characteristics. Lower motor regulation results in a more powerful motor, but it is important to remember that as torque (load) increases, speed decreases. The speed drop rate is less in the case of better motor regulation. Good motor regulation provides high power density, which leads to less power losses and better efficiency.
Coreless brush DC motors provide an ideal solution in prosthetic hand applications. Affording the required good motor regulation, they are also highly efficient, reliable and cost effective. A coreless brush DC motor with its incorporated gearbox operates at low noise, and offers high power density. This ensures optimum space utilisation and low weight, with the motor able to meet not only the mechanical performance criteria of the application, but also the size and portability requirements of the prosthetic. A lighter motor reduces the overall weight of prosthetic hands and which helps the user to use the prosthetic with minimum effort.
For accurate positioning and motion control, a suitable encoder is recommended for use with the motor and gearbox. Motors with integrated gearboxes and encoders enable the user to move fingers more nimbly to grasp objects. Magnetic encoders provide a high degree of accuracy that is ideal for prosthetics applications which require incredibly accurate positioning with closed loop motion feedback. The accurate position information is required so the user can grip objects like an egg with the delicate precision necessary to prevent breakage.
Optimisation of the mechatronic package for prosthetic hands is a delicate balance of different (and sometimes contradictory) performance requirements. For the designer, it can be enormously beneficial to collaborate with a motion technology supplier who can help in tailoring or customising the package to provide the best fit. With expertise in all of the technologies discussed, and the knowhow to bring them all together to provide an enhanced mechatronic package, Portescap can help bring a prosthetic application to life.