RESNA 27th International Annual Confence
The Measurement of Whole-Body Vibration Levels Produced by Electrically Powered Wheelchairs without Suspension
Wheelchair users are particularly at risk from whole-body vibration (WBV) related complaints due to their reduced muscle activity, relaxed posture and musculoskeletal weakness caused through long periods of sitting.
Legislation is proposed to reduce levels of WBV exposure in the workplace by incorporating vibration minimising systems into the design of workstations. If vibration levels experienced by powered wheelchair users exceed stated limits it could be argued that similar legislation would require vibration damping (suspension) systems to be incorporated into wheelchair designs.
Vibration levels were assessed on four powered wheelchairs without suspension systems by recording vertical and horizontal accelerations on the operators seat base over two test surfaces. All the wheelchairs on the rough track above 0.5ms-1 forward speed showed RMS acceleration values in both axes in excess of the proposed European Union PAVD (Physical Agents (Vibration) Directive (2000)) 'daily exposure action value' of 0.6ms-2.
Wheelchair, Seating, Vibration, Suspension
Today there is tremendous variety in the specifications and performance of powered wheelchairs but it is often the case that innovations such as automotive type suspension systems come at a considerable cost. Vibration exposure and ride comfort may be less important than retail cost in powered wheelchair development.
In the EU new legislation is set to limit the time individuals can be exposed to various magnitudes of vibration. The European Union PAVD (Physical Agents (Vibration) Directive (2000)), will potentially limit the WBV daily exposure levels of vehicle operators (1). Previous legislation stated that measures to minimise WBV could be added between the operator and machine. In practical terms this used to mean simply the addition of a more comfortable cushion and or effective seating system. The new directive, however, requires the whole machine to be designed specifically with measures to reduce WBV to a minimum (2). With such legislation in force in the workplace then all equipment that subjects the user to WBV should conform to similar restrictions.
A number of studies infer that wheelchair users are particularly at risk from whole-body vibration (WBV) related complaints due to their reduced muscle activity, relaxed posture and musculoskeletal weakness caused through long periods of sitting (3)(4)(5). Although considerable literature exists relating to whole-body vibration levels experienced during manual wheelchair propulsion the author has found no research related to powered wheelchair use.
The wheelchairs studied are the; Sunrise Medical F45, Sunrise Medical F16, Scandinavian Mobility Harrier and Days Medical Group DMA. None of the wheelchairs studied incorporate suspension systems.
The first factor considered when selecting test surfaces was what type of input would be most relevant. The Simulated Road Course (SRC) used by VanSickle et al. (6), Liu et al (7) and DiGiovaine and Cooper (3), was developed to mimic a number of obstacles encountered when operating a wheelchair on a daily basis. These obstacles are very short term when considered over a day of wheelchair use. The majority of time will be spent crossing surfaces such as Tarmac, concrete or gravel rather than purely negotiating curbs or door thresholds. It was therefore decided to make the track from long lengths of test surfaces.
A further consideration was to make the runs over the surfaces repeatable. There are many different types of paving available which could be used for testing. However, an uncorrelated surface or wheel inputs would mean that any analysis of vibration would be extremely difficult as the wheelchair would not only be pitching and bouncing but also rolling and twisting. It was therefore desirable to have identical inputs on both the right and left sides of the chair.
Two tracks were decided on; a random rough track and a regular smooth track. The rough track was constructed by nailing 300mm x 8mm x 10mm batons at an ad hoc spacing. The right and left wheelways have the same arrangement of batons.
The smooth track was constructed using 300mm x 25mm x 0.8 mm metal strips spaced at 25 mm intervals.
Two strain gauge type accelerometers were secured to a seat plate so that their active axes lay vertically and horizontally in the direction of travel.
BS 6841,3.3 (8) states that where vibration enters the body from a non rigid or resilient surface (eg. seat cushion) it is necessary to place the transducers between the body and the seat surface. Unfortunately, the pad developed to carry the transducers in the correct planes proved too uncomfortable to sit on directly. It was therefore decided to place the plate on top of the wheelchair seat, beneath the users Jay2 cushion as with VanSickle et al. (6).
Acceleration data was recorded at 1000 Hz for ten seconds using a laptop PC, LabView™ software and a DAQCard-700 data acquisition PCMCIA card via a two channel amplifier. The final data was saved as a two column file each containing 10,000 data points, in ASCII .txt format.
The testing for each wheelchair consisted of five runs at five speeds on both the rough and smooth tracks with a maximum duration of 10 seconds.
Acceleration data was recorded and manipulated to give the unweighted RMS (Root Mean Square) accelerations for each run. The majority of the runs produced acceleration levels in excess of the proposed European Union PAVD (Physical Agents (Vibration) Directive (2000)). A example of such data is presented in Figure 1.
Each set of selected acceleration data was analysed using MathSoft MathCAD™ software to produce a graphical representation of its frequency components between 1Hz and 80Hz.
A number of traces of z axis (vertical) vibration show two defined peaks at 0 Hz to 2Hz and again between 9 and 13Hz when operating on the smooth track. On the rough track the bulk of vibration frequencies appear between 0 and 11Hz. At all speeds z axis vibrations show a peak between 3 & 6Hz This is illustrated in Figure 2. . Horizontal (x axis) vibrations are predominantly located between 0 and 6Hz at slow speeds but then increase to between 0 and 11Hz as speed increases.
The PA(V)D directive quantifies 'limit' and 'action' exposure in terms of Vibration Dose Values (VDV's) of 21ms-1.75 and 11ms-1.75 respectively.
All of the wheelchairs exceeded the daily exposure action value (11ms-1.75) once forward speed on the rough track increased above 0.6ms-1 when calculated over a theoretical 45-minute journey time. The Harrier, F45 and DMA wheelchairs exceeded the higher vibration dose action value once forward speed increased above 0.8ms-1. The highest z axis VDV values when crossing the smooth track were 6.6ms-1.75 produced by the F45.
From the PSD analysis it was seen that the bulk of z axis vibrations occur in the range from 3Hz to 6Hz for all test wheelchairs and the majority of test speeds. The most sensitive frequency band for vertical motions of humans is 4-6Hz (9). The study by Lundström et al. (5) reviewed states that absorbed energy was related to the frequency of vibration and peaked within the same range (4-6Hz). Individuals are generally more sensitive to these random vibrations than to sinusoidal vibrations (10).
The test wheelchairs when crossing the rough test track would appear to produce vibrations which exceed current legislation governing exposure levels to WBV in the workplace. They also appear to produce the majority of vibrations in the frequency range most strongly linked to health risk.
This study was carried out as an undergraduate final year thesis at Harper Adams University College, Newport, Telford, Shropshire UK.
Christopher Swift MRes,
Cranfield University at Silsoe,
Silsoe, Bedfordshire, UK MK45 4DT
Phone +44 (0)1525 861669