Design Proposal

Problem Overview 

While there have been several advances in upper limb prosthetics there is still a lot of room left to bridge the gap between biological and artificial limbs. One of the leading problems with new robotic limbs is force sensory. The patient has no way of registering how much force the robotic arm is exerting on an object, and unless continuous eye contact is made with the artificial limb, no way of registering
that the limb is grasping an object. According to the International Conference on Robotics and Automation, the lack of sensory feedback in artificially motorized limbs is one of the main reasons for prosthetic rejection. Some sort of force registration and feedback is needed to register when the prosthetic limb is grasping an object and how much force is being applied. This information must then be incorporated into a feedback system that alerts the patient to exactly how much force is being applied. This system will be incorporated with modern motorized prosthetics to improve the quality of life in upper limb amputees.

Constraints

• Space: The prosthetic arm is already limited for space due to the mechanisms for moving the hand and housing the power supply required. As such, there is very limited space in which to add touch sensors, their wiring, a processing chip, and the vibrator.

• Weight: Difficulty in lifting the prosthetic arm becomes an issue depending on the remaining length of user’s arm from their elbow; the shorter the length from the elbow to the end of the arm, the more difficult it is to lift the artificial limb. In order for the design to be a feasible addition to the prosthetic arm, the weight added needs to as small as possible; otherwise the user may encounter trouble lifting his or her limb.

• Cost: The budget of this design is dependent upon the money the group members are willing to give to buying the necessary supplies. It is in the group’s best interest to keep the cost as low as possible, as well as keeping the cost to the user to a minimum.

• Power Supply: The prosthetic arm houses its own power supply in order to move the fingers in the hand. The power supply needs to be recharged after a length of time dependent upon the amount of use the arm has during a day. The touch sensors and vibrator need to both be hooked up to the internal power supply as well as use as little power as possible. The design should not draw more power than it needs to from the internal power supply, in order to avoid inconveniencing the user.

• Aesthetics: By adding the necessary components for the proposed design advancement, the visual appeal of the prosthetic arm might deteriorate. It may become important to the user that the added functionality the design proposes to give won’t detract from the look of the artificial limb. 

Pre-Existing Solutions

Most prosthetics currently lack the function to vary force while grasping an object and the user doesn’t know how hard or soft they are gripping. There are several research projects working on fixing this problem. They use force sensors to detect how hard the prosthetic is gripping the object and send feedback to the user.

One study published by the American Academy of Orthotists and Prosthetists discusses a possible design using force detectors. There is one force detector on the thumb of the hand and it sends a vibration back to the user when an object is gripped. The strength of the vibrations is proportional to the strength of the force. All patients in the study improved their ability to grip objects of different weights.

Another study is being done at the University of Maine. Their lab is developing a skin embedded with force sensors that will cover a prosthetic hand. They are trying to get the robotic hand to mimic the movements of a human hand as closely as possible, and are studying possible ways of feedback such as visual and vibrotactile.

Design Goal

Many difficulties exist in making an upper limb prosthetic look, feel, and act as natural as a real arm.  Although most problems associated with artificial limbs cannot be addressed without extraordinary costs and novel technology, the bind between force applied by the prosthetic and user recognition of said force can be resolved with force sensory and feedback.

Most users of prosthetics experience problems with how much force is applied on an external object when in the grip of the phalanges and palm. When holding an object, say, picking up a mug, the contracting of the “hand” will break the mug due to an inordinate amount of force applied, not proportional to how much is necessary for the action. On the opposite side of the spectrum, the hand will not contract enough and objects will fall from a grip. In order to solve this, a design involving force sensory and vibration feedback with the purpose of notifying the user of when the “right” force is being applied will be created.

Through the detection of force exerted on the object, MatLab will interpret the quantifiable data and then output a proportional electric signal which will vibrate a vibrator against the user’s “stump”. The vibration will correlate to how much force is being applied; the more force applied, the higher the frequency of vibration. The goal of this design is to allow prosthetic wearers to know how much force they are applying.

Deliverables

The goal deliverable for this project is to produce a system using force detectors that will simulate what would happen when the prosthetic arm grips an object. The force detectors will be connected to a MatLab program that would interpret the signals and allow pancake vibrator to vibrate and let the user know how firmly they are gripping the object.  

Project Schedule:

Week 4        Order the necessary items to set up the system and contact professors and students that can help connect MatLab to the sensors and the output

Week 5        Look at helpful resources that have been used to create a similar system and begin planning the electrical system and sketching how it would be incorporated into a prosthetic

Week 6        Explore MatLab to begin programming the system and seek help if needed

Week 7        Incorporate input system (force sensors), take data on the force sensor readings, and finish Matlab coding

                               
Week 8        Build full system with vibrator and debug MatLab program

Week 9        Critique system and the layout of how the system would be incorporated into a real artificial arm and begin Powerpoint.

Week 10        Have final working system between force sensors and vibrator. Write lab report and finish Powerpoint.

Projected Budget:

Equipment	Cost/unit	Quantity	Cost
Force Sensitive Resistor	$15	2	$30
Pancake Vibrator	$4	2	$8

                            Total:        $38

The Force Sensitive Resistor is to be used as the input of the feedback system. Ideally, there would be one sensor placed on every finger of the prosthetic. However, in this project that is focusing on small or light objects, it is expected that two sensors will be adequate to “pinch” objects to simulate one small motion of a robotic arm. The sensors are 4mm in diameter and will be able to fit in the fingertip region.

The Pancake Vibrator is the feedback generated from the force received from the sensors. After MatLab translates the force received, the vibrator will go off indicating a grasp on the object between the sensors. This will tell the user to stop adding power to the robotic arm- that is capable of breaking the small object. The vibrator will be placed in the prosthetic in a place where the device has contact with the user’s body.  

References

[1]   Morita S, Kondo T, Ito K. Estimation of forearm movement from EMG signal and application
to prosthetic hand control. IEEE Intl Conf on Robotics and Automation (ICRA), Seoul, May
21–26 2001; 3692–3697.

[2]   Pylatiuk, Christian; Kargov, Artem; Schulz, Stefan. “Design and Evaluation of a Low-Cost Force Feedback System for Myoelectric Prosthetic Hands”. Journal of Orthotics and Prosthetics, 18., pp 57-61, 2006.

[3]   Stepp, Cara; Malhotra, Mark. “Sensory Feedback for Prosthetic and Robotic Hands.” Bridging Human Hand Research and the Development of Robotic Technology for Hands, 2010, pp 1.

[4]   Blum, Jeremy. Intel Science Talent Search, Topic: “Using Force Sensors to Effectively Control A Below-Elbow Intelligent Prosthetic Device.” Washington, D.C., March 13, 2012.

[5]   J. Blum. (2005, September). FSR-controlled Prosthetic Hand [Online]. Available: http://www.jeremyblum.com/portfolio/fsr-controlled-prosthetic-hand/

[6]   C. Cipriani, M. D’Alonzo, and M.C. Carrozza. “A Miniature Vibrotactile Sensory
Substitution Device for Multifingered Hand Prosthetics.” IEEE Transactions on
Biomedical Engineering, vol. 59, pp. 400-408, Feb. 2012.