Case study: Influence of the Mechanical and Electrical Anti-rollback System for Wheelchair When Climbing a Hill

The article deals with the issues of operating wheelchairs with manual drive in areas with a significant inclination angle. The kinematics of a wheelchair while climbing a hill was analyzed. On the basis of the conclusions drawn from the research, two solutions for systems that block the reversal of a wheelchair on a hill have been proposed. Among the solutions mentioned, a mechanical anti-rollback system with three operating modes and an electric anti-rollback system is described. The described electric anti-rollback system is part of a manual-electric hybrid drive unit to the manual wheelchairs.


Introduction
Currently, we can observe a significant increase in the population of people with physical disabilities who must use wheelchairs for everyday functioning. About 75% of wheelchair users can use wheelchairs with manual drives. The use of this type of drive requires from the user increased muscular effort [1] and exposes him to upper limb injuries [2]. In addition to the injuries that can be caused by manual wheelchair drive, attention should be paid to environmental factors that reduce the mobility of manual wheelchairs. Terrain conditions such as surface hardness and slope have a great influence on the user's effort. The use of a wheelchair in the area of which the road profile is characterized by many hills translates into increased muscular effort [3], requiring the human body to use large amounts of energy.The design of the manual wheelchair with pushrims is outdated and requires modernization. As is the case with other off-road machines, which are characterized by impact on the environment [4][5][6] and low energy consumption [7][8][9]. The archaic nature of the current design solutions of the classic manual wheelchair introduces barriers to its place of use. The biggest problem is when the incline angle is greater than 4 degrees. They often constitute an insurmountable barrier for a disabled person. The problem with climbing a hill with a manual wheelchair is due to the way how it is propelled. By analyzing the driving motion of a wheelchair with pushrims, we can divide it into the driving and returning phases [10,11]. In the driving phase, the upper limb is in contact with the strings and transmits the driving force. However, in the return phase, the upper limb lets go of the pushrims to return to the starting position. In this stage the wheelchair begins to reverse when disabled people going up a slope. To avoid this, the operator of the wheelchair in the drive phase exposes itself to excessive effort to build up a sufficient reserve of kinetic energy to allow time to withdraw his hand in the return phase. In order to avoid the wheelchair backing up while climbing a hill, modifications to the manual drive system are used. The innovative solutions include hybrid drives and mechanical systems that block the reverse of the wheelchair. All solutions assisting in climbing gradients should only support the propelling of the wheelchair with the upper limb, and should not exclude their participation in this process. Therefore, the applied technical solutions must match the kinematics of driving the wheelchair through pushrims.

Wheelchair Kinematics During Climbing the Hill
The changes in the wheelchair velocity during climbing the hill are shown in Fig. 1. The graphs show the velocity courses for three people (a, b, c). Between the start of the observation and the SC point (start climbing point), the wheelchair was driven on a flat surface. There the speed increase with each drive cycle was observe. In the section between points SC and END, the wheelchair was traveling on the hill. During the analysis of this stage of the road, an increase in the number of drive cycles(frequency increasing) was found in comparison to driving on a horizontal road. Additionally, analyzing the acceleration graph while driving the wheelchair up the hill (Fig. 2), a significant increase in the acceleration amplitude was observe. The measured amplitude was about 7 m/s2. Sudden changes in the generated acceleration are the result of rapid movements of the upper limbs characterized by intense MVC muscular effort [12]. In order to reduce the frequency of driving cycles, the amplitude of acceleration and muscle effort, assist modules are used. They can secure an uphill ride in two ways. With the help of electric motors that support the muscular system. This support allows, without excessive effort, to generate a sufficiently large acceleration that will enable the generation of kinetic energy allowing for the return movement phase. The second way is a mechanical reverse lock which allows the wheel to turn in only one direction. This solution gives the user the certainty that the wheelchair will not reverse. As a result, it can reduce the frequency of drive cycles. In addition, at any time, he can stop driving the pushrims and rest.

Mechanical Anti-rollback System
Mechanical reversing locks are characterized by a simple, reliable design and a low price. Their principle of operation is based on frictional engagement with the drive wheel of an element equipped with a single-directional clutch. An important advantage of this solution is the permanent blockade of reversing, as a result of which the wheelchair does not roll down even after the disabled person stops driving it. The disadvantage of this solution is the lack of support for the muscular system in the drive phase and a slight increase in the eternal frictional resistance of the entire drive system. An example of such a solution is described in the patent application to the UPRP entitled Module for the universal lever brake of a wheelchair wheel (P.431924) By B. Wieczorek, Ł. Warguła. The essence of this invention is the modification of the classic parking brake, which consists in replacing the bar pressing the wheel with a special roller (Fig. 3). The brake module consists of a central axis 3 bolted to the arm of a classic parking brake 12 by means of a screw 1 and a spring washer 2. A one-way clutch 5 was slid onto the central axle 3, which was previously pressed into the brake roller 4. One-way clutch 5 was secured on the central axis 3 with a disc the locking device 6 is pressed against it by the screw 7. The latch protection 9 is screwed into the blocking disc 6. When the latch is in the hole in the brake roller 4, the device functions as a parking brake. After unscrewing the snap lock 9, the device acts as a blockade for reversing the trolley. The use of the snap lock 9 facilitates the operation, because it is enough to screw it into the blocking disc 6 and when the brake roller 4 rotates freely, it will automatically lock. Optimally, the brake roller 4 is covered with rubber. The brake prototype and its mode of operation are shown in Fig. 4. The impact of using the mechanical anti-rollback system is shown by exemplary studies conducted on wheelchair users. Each tested patient climbed the ramp six times. Three times with the activated anti-rollback system and three times with the deactivated anti-rollback system. Three complete propelling cycles were separated from each climbing made by a patient, and they were subject to a further kinematic and biomechanical analysis. The following was verified as part of the kinematic analysis: propelling cycle maximum velocity vmax, propelling cycle minimum velocity vmin, propelling cycle medium velocity M v, road covered during a single propelling cycle s, propelling cycle duration tcycle, a push phase share in the propelling cycle, a return phase share in the propelling cycle, the ratio of generated average power and average loss power for a single propelling cycle λ, and the total mechanical energy after a single propelling cycle E. The average values of these kinematic parameters for 72 propulsion cycles performed in total by 8 users are presented in Table 1.
Based on the mean from the performed 72 measurement tests it was determined that the average maximum velocity vmax in a single propelling cycle with the activated anti-rollback system was 0.47 m/s and with the deactivated anti-rollback system it was 0.54 m/s. This results in 13% decrease in velocity during ramp climbing with the activated anti-rollback system in relation to the ramp climbing with the deactivated anti-rollback system. In the event of the minimum velocity of the propelling cycle, the following values were recorded: 0.13 m/s for the activated anti-rollback system and 0.27 m/s for the deactivated antirollback system. This results in 52% decrease in velocity during ramp climbing with the activated anti-rollback system in relation to the ramp climbing with the deactivated antirollback system. When averaging the velocity of a single propelling cycle and then determining a mean for all the performed measurement tests, the results were as follows: 0.31 m/s for the activated anti-rollback system and 0.42 m/s for the deactivated antirollback system. For this parameter, the decrease amounted to 26% during ramp climbing with the activated anti-rollback system. Despite this decrease of the average propelling cycle velocity with the activated anti-rollback system, a minor difference in the road covered in a single propelling cycle was observed. The decrease in the road covered in a single propelling cycle s equalling 7% with the activated anti-rollback system was determined. Moreover, during ramp climbing with the activated anti-rollback system, it was determined that the average increase in the propelling cycle duration time tcycle was 1%.

Electric Anti-rollback System
The electric reversing lock system was used in the ARMedical AR-405 cross-frame wheelchair equipped with a prototype hybrid drive system module (Fig. 5). Modification consisted in equipping the wheelchair with two 500W BLDCMagicPie 5 motors built in the drive wheel hubs (a), incremental encoders (b), proprietary control system with a gyroscope (c) and a touch screen controller (d). The display was used to displace the assistant mode and the assist gain factor w. The assistance factor made it possible to adjust the operation of the algorithm to the place of use and the user's type of service. The presented wheelchair has several assistive modes, including the Hill-Assist mode, which makes it easier to move on a slope. In hill-assist mode, the system controller used an accelerometer and a gyroscope. Read values of acceleration from the accelerometer and angular velocity were converted into angular position by means of a complementary filter. In addition, the acceleration values were corrected with a low-pass filter. While using the hill climb assist mode, the drive system did not permanently block the rotation of the drive wheels as it was in the case of the mechanical system. The solution used in it supported the upper limb during the driving phase. Thanks to this, without excessive muscular effort, the wheelchair operator while pushing the puhsrims generated enough kinetic energy to perform the return phase of the hand without the wheelchair rolling away. In addition, the delay of the system meant that for 300 ms after the end of driving the strings, the trolley was in dynamic equilibrium. The impact of using the electric anti-rollback system is shown by exemplary studies conducted on wheelchair users. For each patient, the velocity was graphed and measured in each mode with assistance gain coefficient values ranging from 0% to 100%, where 0% means no assistance. The wheelchair velocity of one of the patients in gradability assistance mode (t3) is presented in Fig. 6.  Fig. 6. Graphs of wheelchair velocity on an incline in gradability assistance mode (t3), where wassistance gain coefficient, SC -the point where the incline climb commenced, SA -the starting point of the assistance mode When using the gradability assistance mode (t3), a characteristic property was observed. In each analysed course, a characteristic section was separated starting from the SC point (the start of climbing the hill) and finishing at the SA point (assistance start). Within this section, the wheelchair velocity was the lowest. This is caused by the motor control system delay in mode t3. The velocity drop results from the fact that the wheelchair has entered the elevation but the motors assisting climbing have not yet been activated by the control system. For all the measurement trials, the assistance activation delay did not exceed 3 seconds. When changing the assistance gain coefficient value, it was determined that for w=25% the velocity amplitude Δv was reduced from 1.76 km/h (for w=0%) to 0.83 km/h (for w=25%). For the remaining assistance gain coefficient values (w=25-75%), the velocity amplitude maintained a constant level within the range of 0.73-0.83 km/h.
The gradability assistance mode (t3) was designed for application when moving the wheelchairs on inclinations. Its application in the tests provided the best results with the improvement in the kinematic parameters. The velocity amplitude and the number of propulsion cycles was reduced as the assistance gain coefficient increased.

Summary
Based on the analysis of the speed course and the number of drive cycles performed while climbing the hill, the need to modify the current wheelchair structures was noticed. Using the classic traction wheelchair drive, the uphill climb is often a barrier requiring the assistance of third parties. With inclination angles greater than 4 °, there is a risk of the trolley rolling back while the operator returns his hand to the starting position on the lines. The presented technical solution solves the problem of the driveway up the hill using two different methods with their advantages and disadvantages. The mechanical lock is a simple, cheap and reliable construction. The mechanical reversing lock module fits all manual wheelchairs. The method of operation itself guarantees a complete blockade of the wheelchair reversing, therefore the wheelchair operator may stop driving the wheelchair at any stage of the ramp. The disadvantage of the mechanical solution is the increase in the internal frictional resistance of the system, and the lack of support for the muscular system while driving the pushrims.
In the case of a trolley system with a hybrid electric-manual drive, we are dealing with a structure that has many functions, including hill climbing assistance. This solution supports the human muscular system. As a result, when climbing a hill, the wheelchair user only needs to generate part of the force needed to climb. Additionally, the delay applied causes that after the end of pushing the pushrims and returning the hand to the starting position, the electric motors generate driving torque for 300 ms. They thus induce a reversing delay of the trolley. The biggest disadvantage of this solution is the cost of the module itself and the need to significantly modify the wheelchair. Additionally, this system requires the thrusts to be repelled at a constant frequency. Because when a disabled person stops pushing pushrims, the wheelchair will start to reverse.