Development of a Robust Microcontroller Based Intelligent Prosthetic Limb

M. Parashar et al. (Eds.): IC3 2012, CCIS 306, pp. 445–455, 2012. © Springer-Verlag Berlin Heidelberg 2012

Development of a Robust Microcontroller Based Intelligent Prosthetic Limb

Anup Nandy, Soumik Mondal, Pavan Chakraborty, and G.C. Nandi

Indian Institute of Information Technology Allahabad, Robotics & AI Laboratory, India {nandy.anup,mondal.soumik}@gmail.com,

{pavan,gcnandi}@iiita.ac.in

Abstract. Adaptive Modular Active Leg (AMAL), a robotic Intelligent Prosthetic Limb has been developed at the Indian Institute of Information Technology Allahabad. The aim of the project was to provide the comfort of an intelligent prosthetic knee joint for differently abeled person with one leg amputated above the knee. AMAL provides him with the necessary shock absorption and a suitable bending of the knee joint oscillation. The bending and the shock absorption are provided by artificial muscles. In our case, it is the MR (Magneto Rheological) damper which controls the knee movement of an amputee. The feedback signal is provided by the heel’s strike sensor. AMAL has been kept simple with minimal feedback sensors and controls so that the product is economically viable for the patients. In this paper we describe the mechanical design, the electronic control with its successful testing on differently abeled persons.

Keywords: Adaptive Modular Active Leg (AMAL), Robotic knee joint, Magneto-Rheological Damper, Prosthetic Leg, Heel strike sensor, Above knee amputee.

1 Introduction

The knee joint movement is an important aspect for a human biped locomotion [1], [2], [3]. An amputee with a prosthetic rigid leg is deprived of this knee movement. This leads to an uncomfortable walking gait and injuries due to lack of shock absorption which the knee joint provides during walking.

Several prosthetic limbs are available for amputee persons with different costs. The cost of below the knee prosthetics is between $6000 to $8000 and the cost of above the knee prosthetics is between $10,000 to $15,000 [4]. The C-Leg, manufactured by Otto Bock Health care is completely controlled by the microprocessor circuit. It provides better comfort to the trans-femoral amputee during locomotion. It facilitates multiple settings for running, walking, bicycling and even incline-skating. The cost of newest C-Leg is extremely high in the range of $30,000 to $40,000 to be fitted and delivered [5].

The Jaipur above knee prosthesis is built to help active knee amputees with different knee joints such as Exoskeleton (Plastic Knee joint), Endoskeleton (Pylon knee joint) and Exoskeleton (Oilone Axle knee joint) [6]. The mechanical design of this leg involves locking system at the knee joint which helps to oscillate the knee flexion and knee extension whenever required. No other microcontroller based

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controller circuit has been put into this leg. It lacks of calculating the suitable damping force required to the knee joint.

Another Limb manufacturing organization named as Artificial Limb Manufacturing Corp. of India (ALIMCO) [7] is involved to fabricate the above knee prosthesis with conventional hip joint and above knee with Silesian suspension for the betterment of amputee’s gait. The price of this leg is economically cheap compared to other prosthesis manufacturing organizations. This leg also deprives to have a damping controlling circuit.

The Endolite India Limited [8] is the leading manufacturing corporation to develop prosthetic limb. The characteristics of this leg are the knee movement is measured by the Orion sensors in real time and the microprocessor is used to process the sensory information. The knee speed is being controlled by Hydraulic/pneumatic dampers which allow the prosthetic patient to walk on slopes, stairs and flat terrain.

Indian Institute of Information Technology Allahabad has been involved in developing an Adaptive Prosthetic Limb to facilitate differently abled people whose one of the legs has been amputated above the knee. This robotic prosthetic limb [9] has been named as AMAL. It is an acronym for Adaptive Modular Active Leg. AMAL provides him with suitable comfort for walking, by providing a suitable bending of the knee joint and knee joint oscillation.

2 The Human Locomotion and the Use of the Knee

To design a prosthetic leg one should understand the human locomotion [10], [11] and the use of the knee joint. Human locomotion can be separated into two phases. The stance phase and the swing phase (Fig. 1a). Stance phase begins with the right heel strike. By shifting the center of gravity of the body, the body weight is transferred onto the right leg from the left leg. The momentum causes the human to rise on its standing right leg freeing the left leg which then swings into position. The left knee bends, stretched, bends and stretches, going through double oscillation with short and long amplitude. The knee finally straitens to take the left heel strike. The human falls forward naturally and catches itself on the left leg during the heel strike. As the swinging left leg touches the ground, the ground contact triggers the knee to tighten and get ready for the body weight transfer. The legs then swap roles. The Fig. 1b also shows the major muscles involved in human locomotion. The acting muscles are shaded in black.

The knee Joint has a complex structure. The skeleton view in Fig. 2a illustrates this joint. The knee joint connects a single (thigh bone) with the duel bone tibia and fibula which forms the lower part of the leg. The dual bones help to provide the necessary torque for the knee joint movement. The small circular bone patella in front of the knee prevents it from bending forward. The end surfaces of the femur and tibia are convex as well as concave. This surfaces move with respect to one another by simultaneously (1) rolling, (2) gliding, and (3) spinning. When the concave surface is fixed and the convex surface moves on it (Fig. 2b), the convex surface rolls and glides in opposite directions. When the convex surface is fixed and the concave surface moves on it (Fig. 2c), the concave surface rolls and glides in the same direction.

Development of a Robust Microcontroller Based Intelligent Prosthetic Limb 447

(a) Different phases of Human Locomotion

(b) Different muscles involved in human locomotion

Fig. 1.

(a) Skeleton view of knee (b) Convex Surface (c) Concave Surface

Fig. 2.

3 Mechanical Design of AMAL

AMAL, has been designed to be very rugged, easy to use and maintain. The mechanical parts of AMAL are made with strengthened steel and stainless steel. Some of these steel parts can be replaced with strengthened aluminum or advance carbon

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fiber structure to reduce the weight. The design is modular and it may be altered for people with different height. As per our design, the mechanical structure of AMAL was manufactured at the mechanical workshop of the Artificial Limb Manufacturing Corporation of India (ALIMCO), Kanpur, India. An MOU was signed between IIIT-Allahabad and ALIMCO for manufacturing and marketing of AMAL. After satisfactory testing, the technology has been transferred to ALIMCO for mass producing the prosthetic limb and doing the required field study. Preliminary results by this field study have been extremely positive.

While designing AMAL, the human knee has been kept in mind. Fig. 1b illustrates the major muscles involved in human locomotion. We see that there are multiple muscles involved in the process which makes it extremely complicated. It is assumed that the patient will have a thighs stub with quadriceps femoris muscle sufficiently intact and having the capacity to move his hip joint. Fig. 3a shows an MRI picture of a healthy leg. The MRI picture shows the major bones, Femur, Tibia and Fibula and the inner muscles that connects the Femur with the Tibia. Our MR damper will simulate and work, as this muscle. The working of the MR damper is explained in the next section. We also notice from the MRI picture that the central axis of Femur and Tibia have an offset. This allow and additional torque which helps in bending of the knee. The frontward bending of the knee is prevented by the Patella. In our design we have maintained an offset of 11.5 mm (Fig. 3b). The frontward bending is automatically prevented by the maximum limit of the piston of the MR Damper. This piston is also offset from the rotation axis by 35mm. This provides the additional necessary torque and a full bending capability up to crouching/squatting. Fig. 3c shows the front view of the AMAL’s knee. The knee should be able to bear the weight of the person and a higher backward force during heel strike. Earlier design had a thin axel of 5 mm which gave way during trials. Therefore the joint was redesigned with a thicker axel and with 2 heavy-duty bearings. The separation between the bearings had to be less than what is seen in Fig. 3c. This separation was

(a) Knee joint muscles (b) Knee joint socket (c) Bearing hinge joints

Fig. 3.


increased during manufacturing at ALIMCO. A readily available axel of the dimension was used. This has prevented the twisting motion that a human knee has for negotiating uneven terrain.

The mechanical design of AMAL has been kept simple and made it economically viable. The price at which AMAL is being marketed is approximate. $300. The knee joint movement of AMAL using the MR damper should be controlled, synchronized and coordinated with the knee joint movement of the other healthy leg. A control circuit which has been developed at IIIT-Allahabad provides the required current in the MR damper which tightens or relaxes as a normal human muscle involved in knee joint movement.

Fig. 4. The total layout of AMAL

The AMAL mechanical design can be divided into two parts; the upper and the lower. The upper part of the AMAL which is the limb socket (Fig. 4) has to be tailor made for the amputee. It has to be designed to the specification of the amputated limb and should be comfortable and tightly fitted to the amputee and giving him a good grip. The lower part of AMAL contains the MR damper; the electronic controller and battery; which will be common to all patients except the foot size may be changed to match the other feet.

The upper and lower parts of the AMAL structure can move freely back and forth in the same way as that of a natural limb of a human can move along the knee joint. The MR damper is placed such that the free movement of the AMAL structure can be controlled by the changing the damping profile of the MR liquid inside the damper. Fig. 5a shows electronic circuit is integrated with AMAL before field trials. AMAL has been tested on actual patient (Fig. 5b) whose one of the legs has been amputated from above the knee. The impressive result has been obtained from testing field.

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(a) Electronic circuit integrated with AMAL (b) Field testing on actual amputee

Fig. 5.

4 MR Damper Specifications

A Magneto-Rheological (MR) Damper [12] simulates the natural knee joint for a human being. The MR damper is a sophisticated shock absorber whose damping profile can be accurately adjusted as required at a very fast rate (few milliseconds). The MR damper (Fig. 6) is filled with the Magneto-Rheological fluid which is a smart fluid. It is a colloidal mixture of microscopic magnetic particle and carrier liquid (oil) (Fig. 7a). During un-biased conditions the smart fluid behaves like any other viscous oil, free to flow in any direction.

Fig. 6. MR Damper designed by LORD’s corporation


(a) MR Damper with liquid (b) Without Magnetic Field (c) With Magnetic Field

Fig. 7.

Fig. 7b shows the MR damper without magnetic field. Fig. 7c shows the MR damper whose fluid’s viscosity changes with the applied magnetic field. Iron/ magnetic particles are suspension in glycol and get aligned with the magnetic field and thus increase its viscosity [13].

When subjected to a magnetic field the micrometer sized magnetic particles align themselves according to the magnetic line of force and transform into a viscoelastic solid. Therefore, the viscosity of the smart fluid can be varied over a wide range, from free flowing to fully rigid, by changing the strength of the applied magnetic field. An electro-magnet is used to apply the magnetic field. The electro-magnet coil can be excited using a battery. The damping profile of the MR damper is controlled using an advanced electronic circuit.

A typical Magneto-Rheological (MR) fluid consists of 20-40% by volume of relatively pure, 3-10 micron diameter iron particles, suspended in a carrier liquid such as mineral oil, synthetic oil, water or glycol.

Iron particles in suspension align and develop yield strength in the presence of a magnetic field. The change from a free-flowing liquid to a semi-solid when a magnetic field is applied is rapid and reversible.

5 Controller Circuit Description

We have designed and built a new control circuit (Fig. 8) using commonly available components in India. The circuit has been successfully tested and deployed on the AMAL.

The AMAL electronic controller works on 12V D.C and drains 2 Amps of current at full load. The power going to the MR damper is controlled using an advanced power management IC and it can extent the battery life three folds. Essentially it works in a pulse mode. Peak power is only applied for about 50µs over an average gait cycle of one second. The approximate consumption is about 1miliwatt/second. This drastically reduces the battery consumption. With a high capacity modern NiMH rechargeable cells which has a rating of 60 – 120 watt-hr the leg can easily perform for more than 8hrs of continuous working. However the power efficiency always been

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Fig. 8. AMAL controller circuit and PCB design

the major concern in such endeavor. The control circuit employs a microcontroller – AVR Atmel Mega32 for computational needs. The microcontroller is used to process the data coming from the hell strike sensor. The heel strike sensor provides a pulse as the heel strikes the ground. It is essentially a rugged switch. This pulse is used as start and stop for computations of gait period. The output of microcontroller is directed to the input pin of the MR damper controller and the damping profile is controlled.

The input from the heel strike sensor is attached to the “Sensor IN” of the electronics circuit. There is no polarity of connecting MR Damper. The battery unit is connected to the PCB (Fig. 8) using “Battery”. The toggle switch is the ON/OFF switch for the circuit. The electronics circuit is powered using a battery pack of 12.6V. The 7V line is tapped from the same battery unit. A standard voltage regulator IC (IC7805) is used to regulate supply to 5V. The 5V is used to power the microcontroller circuit. The 500 ohms reference potentiometer is used to supply the analog reference voltage to the microcontroller for A/D conversion. Its screw should be set in the middle for 250 Ohms drop. The microcontroller is programmed using a PC serial port and the 6 pin ISP PORT. The output of the microcontroller can be viewed on a PC computer using a serial port and UART PORT on the PCB. The Red LED shows the ON/OFF status of the circuit. The yellow LED on the PCB shows the over temperature or over current protection enable. The MR damper controller has an inbuilt over-temperature and over-current protection. The IC DRV102 and IC 7805 are expected to heat when used continuously and therefore need heat sinks. The Fig. 8 shows the electronic circuit.


6 Working of AMAL

The control of AMAL is done by a specially designed sophisticated electronics control circuit which is fixed to the support bars of the MR Damper. The electronics should be hermetically sealed and properly cushioned to keep it safe. The control circuit consists of the AMAL electronics circuit and the batteries. The battery is fixed at the ankle joint of the AMAL. It solves two purposes, firstly the battery unit is near to the electronics circuit and secondly the weight of the battery will help the AMAL to retrieve the stable position quickly while walking (like head mass effect in a hammer).

The electronic control circuit is powered by advanced rechargeable NiMH rechargeable cells. The charging of the NiMH battery needs an advanced intelligent charging circuit which is provided with the pack. The control circuit can be switched ON/OFF by a toggle button on the circuit board. There is a status LED on the circuit which indicates the normal working of the circuit. The control circuit uses a latest microcontroller to analyze the heel strike and produce digital output signal. The microcontroller has advance software written into its memory.

The active control of AMAL is initiated by the foot heel strike of the AMAL. The heel strike sensor (Switch, Foot, Small, IP67, and Black) is pressure detector. The sensor detects the pressure applied on the heel of the foot while natural walking; it sends digital signals to the microcontroller. The microcontroller analysis the signal using the software and sends the control signal to the PWM controller. The PWM damper controller changes the stiffness of the MR Damper and thus changing the free movement of the AMAL Knee. If the control circuit detects the heel strike it energizes the MR Damper to its maximum value. The moment pressure is removed from the heel sensor the MR damper returns to its flexible condition. This gives amputee a comfortable gait and a stable standing position. All the above mentioned process happens in split of a second and the amputee will never feel any lag in the response.

7 Conclusion and Future Work

AMAL is now being manufactured by ALIMCO and tested on different patients. The feedback has been extremely positive. The likely impact of this innovation will be to the physically challenged persons whose one of the legs has been amputated above the knee. AMAL provides the suitable comfort for walking, by providing the required bending of the knee joint and knee joint oscillation. The robotic leg coordinates excellently with the other healthy leg in an extremely synchronized fashion. The heel strike follows essentially the hip joint movement of the subject. The hip joint is part of the subject’s natural hip which follows the will of the subject. Thus the will of the subject is captured through the heel strike pulse obtained from the heel strike sensor and sent to the microcontroller which provides the excellent coordination with the healthy leg. The Knee joint movement is controlled by an MR Damper which simulates a human muscle. A control circuit which has been developed at IIIT-Allahabad provides the required current in the MR damper which tightens or relaxes

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as a normal human muscle involved in knee joint movement. The robustness of the design is by minimal use of sensors (only heel strike) and economical use of battery. The mechanical design provides the required gravity aided torque which reduces the power consumption. The mechanical lock bears the weight of the patient and the knee does not buckle during stand still mode. During this period the electronics goes into hibernation which further reduces the power consumption.

After satisfactory testing, the technology has been transferred to ALIMCO for mass producing the prosthetic limb and doing the required field study. Preliminary results by this field study have been extremely positive. AMAL was also tried on a female patient and was found successful. The only complain was the weight of the prosthetic limb. Though the prosthetic limb weight much less than an actual leg would, it feels as an additional weight. This weight could easily be reduced by replacing some of the heavy steel with the much lighter carbon fiber material. This will however escalate the price of AMAL. The weight of the prosthetic limb is a factor that one needs to gets accustomed and acclimatized with.

From field studies we have realized that AMAL requires further modifications and improvement. In our present design we have only considered the rotation aspect of the knee joint. The gliding and rolling aspect of a normal human knee joint is not provided in the prosthetic knee. This aspect of rolling and gliding will be implemented in our next design. The knee pivot and the linking with the human stub will be modified to provide the required rolling and gliding.

Acknowledgments. This work was fully supported by Indian Institute of Information Technology, Allahabad. We would like to thank to Dr. M.D. Tiwari, Director of IIIT-Allahabad to bring this project from Department of Science of Technology (DST), Govt. of India. We would also like to thank to ALIMCO, Kanpur, India for supporting field testing of AMAL on actual patients. Mr. Mahendra Singh Khuswa had lost his right leg in a railway mishap. Almost all of our testing and field trials were made on him. We are extremely indebted to him for all his support and feedback from the patient’s point of view. We also thank Mr. Advitiya Saxena for the help of electronics part related to the project.

References

1. QiDi, W., ChengJu, L., JiaQi, Z., QiJun, C.: Survey of locomotion control of legged robot inspired by biological concept. In: Information Science. Springer, Science in China Series F, pp. 1715–1729 (2009)

2. Marzani, F., Calais, E., Legrand, L.: A 3-D marker-free system for the analysis of movement disabilities - an application to the legs. IEEE Transactions on Information Technology in Biomedicine 5(1), 18–26 (2001)

3. Dejnabadi, H., Jolles, B.M., Aminian, K.: A New Approach for Quantitative Analysis of Inter-Joint Coordination During Gait. IEEE Transactions on Biomedical Engineering 55(2), 755–764 (2008)

4. Prosthetic leg costs, http://www.scipolicy.net/prosthetic-legs/ 5. ottobock C-Leg, http://c-leg.ottobock.com/en/


6. Jaipur Foot, http://www.jaipurfoot.org/ 04_Services_Above_Knee_Prosthesis.asp

7. Artificial Limb Manufacturing Corporation of India, http://www.artlimbs.com/lower_limb_prosthetics.htm

8. Endolite India Limited, http://endoliteindia.com/home/main.aspx 9. Nandi, G.C., Ijspeert, A.J., Chakraborty, P., Nandi, A.: Development of Adaptive Modular

Active Leg (AMAL) using bipedal robotics technology. In: Robotics and Autonomous Systems, vol. 57, pp. 603–616 (2009)

10. Mondal, S., Nandy, A., Chakrabarti, A., Chakraborty, P., Nandi, G.C.: A Framework for Synthesis of Human Gait Oscillation Using Intelligent Gait Oscillation Detector (IGOD). In: Ranka, S., Banerjee, A., Biswas, K.K., Dua, S., Mishra, P., Moona, R., Poon, S.-H., Wang, C.-L. (eds.) IC3 2010, Part I. CCIS, vol. 94, pp. 340–349. Springer, Heidelberg (2010)

11. Mondal, S., Nandy, A., Chandrapal, Chakraborty, P., Nandi, G.C.: A Central Pattern Generator based Nonlinear Controller to Simulate Biped Locomotion with a Stable Human Gait Oscillation. International Journal of Robotics and Automation (IJRA) 2(2), 93–106 (2011)

12. LORD MR Damper, http://www.lord.com/ products-and-solutions/magneto-rheological-%28mr%29.xml

13. Magnetorheological Fluids, http://www.hs-owl.de/fb5/labor/rt/en/mrf_aktorik_en.html

Development of a Robust Microcontroller Based Intelligent Prosthetic Limb

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