Putting Prosthetic Hands into Motion

Introduction

A prosthesis or prosthetic implant is an artificial device that replaces a missing part of the body, which might have been lost due to accident, trauma, disease, or congenital disorder (birth defect). An orthopedic prosthetic is an artificial limb designed to substitute the missing body part of a patient. It is intended to restore a degree of normal function, thus providing precise movements and efficient operation.

In the past, a hand amputee would have been asked to use a hook prosthesis that had limited functions and carried a significant amount of social stigma. However, in today’s society, a hand amputee can expect a replacement hand that replicates a whole host of normal hand functions. (see image 1 on right)

Upper limb prostheses can be categorized in three main categories: passive devices, body powered devices, externally powered (myoelectric) devices. 

A myoelectric prosthesis uses the electrical tension that is being generated every time a muscle contracts. This tension can be captured from the voluntarily contracted muscles through electrodes applied on the skin to control the movements of the prosthesis, such as elbow flexion/extension, wrist supination/pronation (rotation) or opening/closing of the fingers. A prosthesis of this type utilizes the residual neuromuscular system of the human body to control the functions of an electric-powered prosthetic hand, wrist, elbow, or foot.

Most prosthesis manufacturers use EMG (electromyography) signals for actuation. In 1919, G. Schlesinger defined six basic ways of grasping as shown in the figure on the right (image 2). 

Based on these basic prehensions, price and functionality, manufacturers can choose the appropriate type of mechanism and determine the necessary number of motors to be used in these actuation mechanisms. It can be a fully articulated arm or a semi-articulated arm. In a fully articulated arm, manufacturers use five motors for the actuation of all five fingers to achieve multiple degrees of movements. This provides better flexibility to the end user for achieving almost all major configurations of the arm. In a semi-articulated arm, a single motor is used for actuation of two or three fingers (the middle finger, ring finger, and little finger). Individual motors are provided for actuation of the index finger and thumb. This helps to reduce cost and simplify design. Naturally, there are limitations to the degrees of movement. It is the major movements used frequently in day-to-day life that are the focus in these designs.

Customer Requirements for a Good Prosthesis

  • Functionally efficient
  • Durable
  • Comfortable
  • Lightweight
  • Low maintenance
  • Long battery life
  • Aesthetically pleasing

Standard Motion Solutions for Prosthetic Hands

Coreless brush DC motors, being highly efficient, reliable and cost effective, are an ideal solution for prosthetic arms. A coreless brush DC motor with its incorporated gearbox operates at low noise – an understandable requirement for myoelectric prosthetic arm users. For prosthetic hands, the typical requirement for motors is 50 mNm @ 150 rpm to 100 mNm @ 50 rpm. A lighter motor reduces the overall weight of prosthetic hands and allows users to move each finger independently with minimum effort in single finger actuation designs. The design of the coreless brush DC motor also offers high power density and ensures optimum space utilization. It suits the compactness and portability requirement of prosthesis application.

Some of the lower cost myoelectric semi-articulated hand prosthesis manufacturers use one motor for four single-finger actuations. Here, the degree of freedom is two; one for movement of the four fingers, and one for manual movement of the thumb. The typical torque requirement in this design can range between 300 mNm @ 1500 rpm and 400 mNm @ 1000 rpm.

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 is required so the user can grip objects like an egg with the delicate precision necessary to prevent breakage. 

Motor and gearbox selection is a direct contributor to the success of the application. The solution chosen should offer sufficient torque and speed (power), to ensure proper holding force and linear speed necessary for grasping. The type of mechanism used for finger actuation should also be a deciding factor in motor selection.

Motor regulation (R/K2) 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 - 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. (see formula on right)

If motor regulation is a critical parameter of the motor, then efficiency is certainly the critical parameter for new generation prosthetic hands, where smaller and lighter weight batteries are considered now. High efficiency results in less power losses and less current consumption, which increases battery life.

The selection of a motor and gearbox depend heavily on torque/speed requirements and size constraints of the application. In the above generalized chart, we'll use a few Portescap DC motors to compare. As you can see in the chart, the slope of the 10N motor + R10 gearbox with ratio 64 is nearly the same as the 12G motor + R13 gearbox with ratio 30.2, although the 12G + R13 provides a wider range of torque and speed. This means by increasing size, we can increase the power of the motor-gearbox composite. The slope of the 12G motor + R10 gearbox with ratio 64 is lower, hence it can operate at a higher working torque, though at a lower speed.

Integrated Solutions for Prosthetic Hands

Average operational requirements for prosthetic arms can look something like this:

Cycles Per Day

Cycle Time

Voltage Supply

Holding Force

Actuation Speed

100 to 500 cycles per day (e.g. closing and opening of palm)

1-2 seconds

(for high-speed gripping)

6-8 V

(for a battery-operated arm)

30 N to 80 N

12 mm/s to 15 mm/s

 

For 180-degree transmission with a lead screw mechanism, the torque and speed requirement of motor-gearbox composite depends on the type of lead screw (no. of start, lead, thread type).

Often, a customized solution is needed to meet specific application requirements. An integrated design can best suit the size and weight constraints as well as the performance, reliability, and accuracy requirements of each application. An optimized design can withstand the specific axial forces necessary for the application. An example of an integrated solution prototype (created by Portescap) is shown on the right (image 5).

In this example prototype, the mechanism consists of 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.

Portescap’s prototype with the 12GS+R10 motor-gearbox composite was tested at 1V, and the composite provided linear speed of 7 mm/s at no load with an electric current consumption of 15 mA. Linear speed can be increased by increasing the voltage. The linear speed of the actuator depends on the weight of the object the user is handling. No load linear speed of an actuator is associated with the motion of finger without an object. Higher linear speeds allow the user to grasp objects quickly. Based on the calculations, the designed load for this actuator is 60 N. The force and speed depend on one’s choice of motor, gearbox, and the size of the leadscrew used for the linear actuator. Naturally, the size of the linear actuator varies from application to application. The low current consumption of the linear actuator provides a longer battery life.

You'll also want to consider if your application requires motion-feedback solutions, including linear actuators, that can be customized precisely for the output needed and is able to fit inside the actuator, so that the operation can be controlled accurately. The linear motion of the piston can be controlled by using a linear potentiometer or a linear magnetic encoder.

A linear potentiometer works on principles based on calculating the variation of the resistance produced by the displacement of the object inside the electric circuit. It consists of a spring-loaded head and it produces a sufficient amount of analog output without using the amplifier. It is easy to install and has a superior ability for measuring narrow angles. The benefit of using a potentiometer feedback system in a linear actuator is that the controller does not have to track the position of the piston.  It offers the absolute position of the piston at any given moment. It is suitable for low speed requirement and is less costly. As compared with magnetic encoder it offers 50% of life.

A linear magnetic encoder is a hall-sensor-based encoder which detects the magnetic field. There is no friction between any parts in the sensor, because the parts are not in contact with one another. In linear actuators, the magnet attaches to the piston, and the two hall sensors are placed near the magnet.  As the magnet (piston) moves, the hall sensor detects the magnetic field.  This produces a pulse which is used by the controller for keeping track of the position. The two hall sensors which are offset from each other produce two overlapping pulses. The overlapping signals determine the direction of travel.

For encoders, even a slight misalignment of the shaft reduces accuracy and increases hysteresis losses. Linear magnetic encoders have more accurate positional measurement, creating more reliability for high speed applications, as well as longer life. Magnetic encoders offer a consistent signal throughout the life of the linear actuator. They provide very precise and fine increments of positional data.  The controller keeps the track of the position, offering a ‘home detection’ functionality to reset position to a known state.

A 180-degree linear actuator is an ideal choice for prosthetic hand applications. It provides a single motion solution to prosthetic hand manufacturers. It is efficient, reliable, accurate and lightweight. It has high power density and provides optimum performance for prosthetic hand. It is available in compact sizes and it meets the low noise requirement of the prosthetic hand applications. Portescap offers solutions in all of the above-mentioned technologies. Contact an engineer today to learn how Portescap can help bring your prosthetic application to life.

About Portescap

Portescap is a manufacturer of miniature Brushless DC (both slotted and slotless), Brush DC, Stepper, and linear actuator motors, as well as related components such as gearheads, encoders, and controllers. Portescap is a leading supplier of sterilizable motors for powered surgical hand tools and robotically assisted surgical devices. Sterilizable slotted BLDC motors by Portescap have been used in tens of millions of surgeries worldwide, in every conceivable surgical application. Our engineering team has spent over 30 years continuously improving our sterilizable motor designs, which have been shown to survive in excess of 3,000+ autoclave cycles, far exceeding the useful life of a surgical device. Portescap offers complete motor customizations tailored around surgical device needs: shaft cannulation, ground-up electromagnetic design, mounting features, custom gear ratios, pin connections vs flying leads, and more. Portescap's industry expert design engineers will collaborate with your team to customize any and all features for your unique surgical hand tool or surgical robotic application.