Design and Control of Motorized Foot-rest for Electric Powered Wheelchair


In this project, we aim to enhance some of the features of an Electric-Powered Wheelchair(EPW). With the aim of having simple accessories with user-friendly features, we decided to focus on the foot-rest of the EPW. Extensive literature search and

brainstorm creative and new ideas were conducted. We finally decided to design a working prototype of foot-rest. To build the foot-rest to actual size is too difficult for us given the time frame and with our limited knowledge and experience with engineering design; hence we built a scaled-down model using materials from a modeling kit to demonstrate the basic functions of the foot-rest. We tested the performances of the foot-rest model for both the leg lifting function as well as for exercise mode by connecting it to the electrical power source. We also used two batteries of 3V in total as an alternative power source. The foot-rest is driven by Direct Current (DC) motors: two for the rotational motion and one for the linear motion. Using the experimental results, we have developed an idea about where to modify and improve the existing scaled-down model so as to obtain a more desirable full-fledge product to help more people. For future development of our project, we would like use a microcontroller to run the foot-rest in the exercise mode in a smoother and more convenient manner.

Introduction

Wheelchair foot-rest are known by many names including front rigging, foot rest, wheelchair leg supports, or just wheelchair legs to name a few. The existing foot-rests act as base of support to prevent sliding and improve positioning manually [1]. Usually, EPWs wheelchair users often have limited strength in their arms and torso, and thus most of the time they depend on the wheelchair in sedentary position. Hence, the users hardly move their legs or feet due to weakness in their lower part of their body and the foot-rest is fixed. In this case, it leads to some inconvenience, for example, there must be somebody helping to lift the users’ feet from the ground onto the pad of foot-rest. Moreover, if the users’ legs are too short and the pad is too far down from the seat, it would be uncomfortable to dangle his legs and feet in the air since the foot-rest is unmovable. Another problem found by EPW users is that, as a result of long-time staying still on the wheelchair, the users lack chances to exercise; and thus it brings out the problem of poor blood circulation and physical inactivity [2]. All these difficulties need to be resolved and with this query in our mind, we found it necessary to make a change on the foot-rest.

With considerations of the problems sated above, we decided to develop a simple model which can help people to lift up their feet from the ground without anybody’s assistance and adjust the length and angle of the foot-rest to provide a most comfortable position for the user to put their feet. The foot-rest can also help the users to exercise their legs in order to improve blood circulation in the legs.

Material and Methods

Design
Before we sketched the draft of our design, we took pictures of the present foot-rest so as to have a general idea about the foot-rest. The original one is fixed in position, with fixed length and angle. Therefore, the pad is also unmovable, providing a fixed position for the users to put their feet on. With the purpose to create a mobile position, we designed two types of new foot-rest in total as following:

? The 'L' shape one with two linear motor (Fig.1)

? The 'I' shape design (Fig. 2)

After careful considerations and discussions about the pros and cons of each design, we decided to centre on this 'I' shape design. Basically, we have considered many factors when designing the foot-rest like weight of the foot-rest, number of motors used, construction of the design, amount of materials needed and cost, etc.

Specifically, the first design including two motors to drive the whole construction is obviously more expensive. Moreover, it also adds weight to the wheelchair; thus it requires more electrical forces to lift the foot-rest up and down. Lastly, as the design involves construction of two motors and more circuit connections, the overall construction is deemed to be more complicated.

Judging the designs in these aspects, we find out that the current design is more advantageous because of following reasons:

? The motors are placed at the top and it effectively minimizes the weight on the pad of foot-rest. Hence, it does not need much power to lift up the lower part of foot-rest, so we could we can choose small and low-powered motor which can reduce the cost. Benefiting from this, our design saves much power and is basically environmental friendly.

? For the present design, we use one rotational and one linear movement rather than two linear movements in the first design. It is done so to change the angle more directly and effectively. Moreover, this design makes it easier to control the speed and spare our need for complicated calculation work.

Name Explanation
controller Switch plate: the place the main controlling system/connection and the batteries are
Switch arm and control stick: act as joysticks to control the rotational and linear movements by moving the control stick to the left, right, forwards and backwards.
Rotational and linear motions This part includes the motors and gearbox which control the rotational and linear movements.
Movable mast This part includes movable mast, and threaded shaft/lead-screw. When the motors start to work, the lead-screw rotates accordingly to produce the linear motion.

Besides assembling all the parts together, we also add in a couple of resistors to reduce the voltage applied to the motor. It is necessary as if the voltage applied to the motor is too high, it would cause the speed of rotational motion to be very fast. If that is so, it is inconvenient and too fast for the user to adjust the position of the foot-rest. Therefore, in order to slow down the speed, we construct the motor equivalent circuit according to the theories we learnt before (i.e. the four basic equations pertaining to Direct Current motor)

In the equations, Va is the overall voltage of the circuit and Ea is the electromagnetic motive force/ the voltage over the motor. The speed of the motor is proportional to the back e.m.f. induced by the motor. When the DC power source (Va) is reduced, the value of Ea would then slow down the motor’s speed. As a result of this, the speed of the rotational motion of the foot-rest would also be slowed down.

Results

After the whole construction is completed, we use two batteries of 3V in total as the power source and supply the foot-rest model with a current of 1.5A. Under these conditions, the foot-rest moves up and down with the linear motion at an average speed of 6 mm/s. It can extend to a maximum length of 30cm from the original/minimum length of 18cm. For the rotational motion, if only one foot-rest is working, it can reach a maximum angle of 90 degrees. If two of them are working together, the maximum angle they can reach is 60 degrees. This is because when two of them are working together, they will share the voltage and the voltage applied to each of them is smaller. Therefore, there will not be enough electrical forces to rotate one to 90 degrees.

As abovementioned, our new design of foot-rest is capable of lifting the user’s feet from the ground. Firstly, we can apply the linear motion – the length is increased – until the pads touch the ground. Then when the user’s feet are dragged onto the pad and the user is sitting in the wheelchair, apply the linear motion again. Consequently, the user’s feet on the pad can be lifted up in the air, sparing the need of others helping to lift up the feet. Next, the user could adjust his sitting position in the seat to a most comfortable one and then adjust the position of the foot-rest accordingly. In order to achieve this, the user could simply control the joy sticks/control sticks in the switch plate to apply both the linear and the rotational motion. Both the vertical length and angle can be changed. For example, if the user had longer legs, he can increase both the length and the angle to stretch his legs and to seek a suitable position. If he is obstructed by objects in front, he merely needs to decrease the angle and increase more in length (as shown in Fig.5 to Fig.7).

Furthermore, our foot-rest can provide an exercise mode to help users exercise their legs. This can be done by applying the linear or rotational motions continuously; as in making the foot-rest moving up and down (Fig.8) or rotating the angle continuously (Fig.9). It is achieved by pushing the control sticks forward and backward alternatively to apply linear motion; it is similar with the rotational motion. In this sense, the users’ legs are moving all the time, just like walking. This encourages blood circulation and physical reaction, which is beneficial to the users’ health and recovery.

In conclusion, our new design is very convenient and requires little strength from the user. Furthermore, as we have adjusted the speed to moderate, the user could adjust the length and angle bit by bit in a more accurate way.

Discussions

In overall, we feel that we have achieved our main objectives under our continuous and arduous efforts. Our model is completed in time and successfully demonstrates all of our suggested functions. As we connect our model to the power source, we manage to control the movement of the foot-rest using the controller. Any movement is seen immediately.

However, there are still some areas that we can work on. Firstly, when there is any movement on the foot-rest (rotary or vertical motions), much noise is produced. Partially, it is due to the friction between gears and we need to apply oil on the gear constantly, which is very inconvenient. In addition, according to our estimation, the speed of linear motion is a bit slower and it may not lift up users’ legs. The major reason lies in the small power of the motor. Therefore, the model is merely for demonstrating the fundamental working principles of the new foot-rest. For the real one, we still need motor with larger power to load efficiently.

Further development of project

We plan to extend our project as we find out that it is very inconvenient for the users who are weak in strength to manually control the foot-rest for exercise mode. Therefore, we foresee the need to improve our model with more effectiveness to make it more realistic and feasible. We plan to add microcontroller into our design and thus it can automatically manipulate the foot-rest. When the user is stepping on the pad, the microcontroller can help to exercise his legs like riding the bicycle in a circular motion. In other words, it can adjust the length and angel continuously and simultaneously and thus a more convenient exercise mode is created to help the users to automatically exercise their legs and even lower body for physical recreation.

Acknowledgement
We would like to thank our mentors and teacher in charge for patient and continuous help and support.

Reference:
[1] Colin A McLaurin and Peter Axelson, 1990. "Wheelchair standards: an overview" Journal of Rehabilitation Research & Development, vol. Suppl, pp. 100-109.
[2] R.A. Cooper, K. L. Stewart, D. P. VanSickle, S. J. Albright, T. Heil, 1994. "Manual wheelchair ISO-ANSI/RESNA fatigue testing experience" in Proc. RESNA '94, Nashville, TN, pp. 324-326.

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