Performance Analysis Outdoor

Transcription

1 Assistive Technology & Seating Service Vancouver Coastal Health GF Strong Rehab Centre 4255 Laurel Street Vancouver BC V5Z 2G9 Overview Consumers with a broad spectrum of abilities are using power wheelchairs. At one end of the spectrum we have users with single switch access; at the other end are young paraplegics who want to avoid the long-term consequences of repetitive strain injury to their shoulders. These users demand more from their chairs. Gone are the days when a manufacturer could hide their chairs shortcomings behind the old defence "But quads don?t do that!" Handling characteristics determine to a large extent how well the chair performs outside. Other factors include: - comfort, range, top speed, and tracking. Ability to handle changes in terrain and the occasional lumps along the way are important for people who use the chair both indoors and outdoors. A chair used primarily outdoors should be easy to drive requiring minimal user input to keep going in the desired direction; uneven terrain should not throw the chair for a loop (literally). The chair should offer adequate range such that top up charging is not needed. The chair must be comfortable on rough terrain as well as on smooth surfaces. Users must consider their outdoor performance needs carefully. A chair which performs well indoors may not meet outdoor handling needs. Some chairs offer the driver a chance to programme the chair to respond completely differently in outdoor and indoor modes. The drawback to this solution is the driver must come to a complete stop and cycle a switch through various modes until the appropriate one is selected. None of our testers found this level of control necessary or satisfactory. LR RWD Quite a lot of shock is transmitted through the casters when they encounter a bump. Good tracking properties but reluctant to make sharp turns on slippery surfaces. Range tends to be better than average. HR RWD Less shock transmitted from casters when they encounter a bump. Good tracking properties and a willingness to make sharp turns with minimal skidding. Range tends to be better than average. CWD Less shock transmitted from casters when they encounter a bump. Tendency to lift the drive wheels off the ground when negotiating terrain like curb cuts. Good tracking properties and a willingness to make sharp turns with minimal skidding. Range tends to be better than average. (1 of 16)12/30/2006 3:36:14 PM

2 HR FWD Less shock is transmitted from the pneumatic drive wheels when they encounter a bump. There is a slight tendency for these chairs to over steer making tracking inferior. Tendency for the chair to pitch forwards when descending terrain like a curb cut. Range tends to be average. LR FWD L Less shock is transmitted from the pneumatic drive wheels when they encounter a bump. The weighted casters tend to transmit some impact to the driver. There is a tendency for these chairs to over steer making tracking inferior. Range tends to be inferior. nown exceptions LR FWD Ω megatrac Over-steering is prevented by a unique arrangement of motors. One drives the chair forward. The second adds power to one wheel or the other to create a steering force. Speed User ability plays a significant role in determining the appropriate speed for any given situation. In our experience a top speed of around 6 mph (10 km/h) is fine for open spaces such as shopping malls or sidewalks. However when the route is clear and the journey long, 10 mph (16 km/h) is awfully nice. Speed can reduce the comfort of the ride and must be taken into account. Speed also contributes to user safety and I must stress this is a two edged sword. It is quite easy, not to mention safe, for armchair clinicians and policy makers to limit top speed in the name of safety. In some circumstances this is appropriate. But, anyone who has crossed six lanes of traffic at an uncontrolled intersection in a chair with a top speed of 4.8 mph appreciates just how much safer 6 mph can be. During the course of the testing we had to make a round trip of 5 blocks to test the chairs on an uneven stretch of pavement. See illustration. In the S525 testers did not feel safe crossing at the crosswalk, they felt that the chair wouldn?t respond fast enough to get out of the way of traffic. Instead they chose to drive down to the main intersection to cross. In the S626 the testers felt perfectly safe crossing at the crosswalk. This reduces the round trip time from 14 minutes to 4 minutes. This savings in time goes beyond speed. This is a lifestyles issue. People may nip out to the corner store if it is going to take five minutes. If it?s a fifteen minute slog they may just choose to stay home! (2 of 16)12/30/2006 3:36:14 PM

3 LR RWD HR RWD Caster flutter is the main limit on top speed potential; casters with weight on them tend to flutter at higher speeds than unweighted casters. CWD HR FWD The slight tendency for the casters to overtake the drive wheels makes this configuration unsuitable for speeds in excess of 7mph (11 km/h) LR FWD L The strong tendency for the casters to overtake the drive wheels makes this configuration unsuitable for speeds in excess of 5mph (8 km/h) nown exceptions LR FWD Ω megatrac Over-steering is prevented by a unique arrangement of motors. One drives the chair forward. The second adds power to one wheel or the other to create a steering force. (3 of 16)12/30/2006 3:36:14 PM

4 LR FWD Arrow FWD Invacare uses inertia to reduce the casters attempts to overtake the drive wheels. A 44lb (20kg) weight has been placed over the casters. Handling Predictable handling characteristics are particularly important in a chair being used outdoors. Negotiating poorly located curb cuts is often a risky proposition with traffic flying past your elbow and it is occasionally necessary to take immediate evasive action. Spinning wheels, delayed, or overly enthusiastic response to steering input will make a chair less safe and therefore less desirable. We have found more variety in handling characteristics between brands than between various configurations at outdoor speeds. However, some generalizations may be made since the increased speed usually associated with outdoor use tends to emphasize differences. LR RWD Chairs are reluctant to change direction at speed if the traction is in the least bit compromised e.g., wet pavement. The wheels spin and the chair skids until speed is reduced sufficiently to allow the tires to regain traction. HR RWD Generally the best handling configuration. LR CWD CWD chairs with plenty of weight on the drive wheels respond more accurately to joystick input than CWD chairs with lots of weight on the auxiliary wheels. HR CWD HR FWD Require frequent steering adjustments to keep the chair rolling in the desired direction. (see Tracking) LR FWD L Chairs require continuous steering adjustments to the joystick to keep the chair moving in the desired direction. (see Tracking) (4 of 16)12/30/2006 3:36:15 PM

5 nown exceptions LR FWD Ω megatrac Good tracking is produced by a unique arrangement of motors. One drives the chair forwards. The second adds power to one wheel or the other to create a steering force. Curbs Inevitably, power wheelchair users find themselves in situations where access is less than ideal. Some wheelchairs are able to cope with minor obstacles better than others. If a 2" rise must be negotiated, it will be easier for someone to do in a chair tested at 5" rather than one we tested at 3". (We know C5/6 quads who can do 5" curbs independently albeit reluctantly.) A chairs ability to cope with smaller irregularities on the pavement is proportional to the curb climbing potential. LR RWD are limited to a curb slightly smaller than half the caster diameter, unless the acceleration is set to allow a wheelie. HR RWD can be made to wheelie and, therefore, clear the curb with the smaller caster. The maximum curb is slightly less than half the diameter of the drive wheel. CWD L have very limited curb climbing due to the weighted caster. Even if the caster can be lifted on to the curb the drive wheel will be suspended in the air. HR FWD are limited to half the diameter of the anti tipper plus its height off the ground. There is also a possibility of getting the drive wheels suspended and losing traction. LR FWD have the potential to drive up a curb slightly smaller than the diameter of the drive wheels. Actual height depends on the traction between the wheel and the surface of the curb. nown exceptions (5 of 16)12/30/2006 3:36:15 PM

6 HR FWD azzy The Active-Trac suspension significantly improves the chairs curb climbing performance. When power is being applied to the drive wheels the front anti tippers are lifted, which makes climbing even fairly high curbs a smooth proposition. Curb cuts Chairs used outdoors must be able to master curb cuts. The curb cut represents the transition from relative safety (on the side walk) to an area of vulnerability (among traffic). Transition is not always ideal - often the road camber will make the angle steeper than intended. Footrests low to the ground may dig in. Footrests located a long way from the front wheel are more likely to hit the ground. FWD chairs tend to have footrests mounted closer to the wheels. Control on the transition is very important and all things being equal High Ratio chairs offer more traction. Another problem with curb cuts is the drive wheels being lifted off the ground - this occurs in some CWD and HR FWD chairs. (6 of 16)12/30/2006 3:36:15 PM

7 LR RWD Limited foot clearance on transitions. A small curb rather than a smooth transition causes the weighted casters to transmit quite a lot of shock to the user. HR RWD Limited foot clearance on transitions, good traction on slippery inclines. LR CWD L Limited foot clearance on steep transitions and poor traction on slippery inclines. Tendency for drive wheels to become suspended. HR CWD Limited foot clearance on transitions. Limited traction during transition. HR FWD Good foot clearance. Limited traction during transition, but good traction on slippery inclines. LR FWD Good foot clearance, smooth ride over small curbs and transitions but limited traction when ascending. The poor traction combined with the tendency to over steer can make the chair difficult to control on steep, slippery curb cuts. nown exceptions HR FWD azzy Power applied to the drive wheels causes the Active-Trac suspension to lift the front anti tipper eliminating the tendency for the drive wheels to be lifted. Hills A chair that climbs and descends hills without slowing excessively or building up a head of steam is easier and more satisfying to drive for most people. An ability to climb steep hills without fear of getting stuck or sliding if the sidewalk is wet may be an issue dependant upon the area in which the chair will be used. Most wheelchair users will be reluctant to attempt ascending anything greater than a 15-degree slope. However if steep hills are a fact of life for a person, a chair that can climb and descend a 25-degree slope has more in reserve than one that only manages 20 degrees. (7 of 16)12/30/2006 3:36:15 PM

8 LR RWD L Can climb the steepest hills but tends to skid on moderate descents. HR RWD Best balance of ascending, descending. LR CWD HR CWD Traction is proportional to the weight on the drive wheels. HR FWD Best balance of ascending and descending, although care has to be taken when slowing down during a descent because the chair may tip forwards onto its anti-tippers. LR FWD L Can descend the steepest hills but loses traction on moderate ascents. nown exceptions (8 of 16)12/30/2006 3:36:15 PM

9 CWD Quickie G424 The G 424 has a dynamic motor mount which increases traction when powering up hill. The dynamic element does not work when going down the hill. Acceleration / Deceleration A programmer allows the user to set the chair to respond rapidly or gently to control input on all the chairs tested. While acceleration and deceleration can be precisely programmed in individual drive modes, it has been our experience that people prefer to stay in their outdoor mode when entering a building and control speed by limiting the joystick throw. This can have a dramatic effect on a chair?s braking distance which is usually discovered when the chair collides with something or someone. Our experience has been that deceleration between 0.15 and 0.20 g?s is a suitable rate for most people. Emergency deceleration, where the joystick is momentarily put in reverse should be between 0.3 and 0.4g?s. This provides people with good balance and / or appropriate restraints an opportunity to stop quicker if necessary. Rapid acceleration in a RWD chair is a necessity for people who like to ascend curbs or descend a couple of steps. Gentle acceleration is important for people who have difficulty with fine control of the joystick, poor balance or decision making issues. Braking response is particularly critical in HR FWD chairs. If the deceleration is too rapid the driver will feel as though he or she is being pitched out. If too slow, the chair rolls a long way before finally coming to a halt. The Active? Trac suspension on the azzy reduces the pitch out sensation significantly. As the chair is being slowed the front anti tippers are pushed down into the floor. Tracking A chairs tendency to keep going in the desired direction is a factor most consumers tend to ignore. For people who use the chair in latched mode e.g., sip and puff or switch array, this factor cannot be emphasized enough. To understand how a chair will track three facts about casters must be understood: 1. Casters are passive. Their position is a direct result of the effect of all external forces acting on them. (Forces include: gravity, centrifugal force, and the force from the drive wheels.) 2. When the chair is moving casters inherently want to lead the driving wheels. The manufacturers of FWD chairs have dampened this tendency through the electronic controller. 3. The effect of caster performance on a chair is proportional to the weight passing through the caster. The Ω megatrac LR FWD is unusual in that the drive to the wheels is provided by one motor. A second motor adjusts the power to each wheel according to control displacement produces the turn. The design significantly improves the chair tracking on uneven terrain. (9 of 16)12/30/2006 3:36:15 PM

10 The Permobil Trax, which was not tested for this report, is unusual in that the casters are not passive and the driver is able to negate some of the downward turning tendency of the chair. Straight line Straight-line tracking is something that can be programmed into most chairs. It is essentially a case of balancing the motor output so the left and right wheels turn at the same speed. All configurations can be made to follow a straight line on flat on smooth level terrain. Cambered sidewalks Sidewalks and roadways are cambered to facilitate water run off. Unfortunately the camber tends to make wheelchairs turn. In all cases the chairs turn so that the casters end up below the driving wheels. The tendency to turn is proportional to the horizontal distance between the system centre of gravity and the mid point between the drive wheels. Therefore: LR RWD L Strong tendency to turn down the slope. HR RWD Slight tendency to turn down the slope. CWD No tendency to turn. HR FWD Slight tendency to turn up the slope. LR FWD L Strong tendency to turn up the slope. Uneven terrain Uneven terrain such as might be found on gravel roads or alleys compromises tracking because it changes the traction available to each driving wheel and changes the load on each motor. If a wheel slips the chair will turn towards the slipping wheel. The likelihood of wheel spin is inversely proportional to the amount of weight on the wheel. If a wheel encounters increased resistance, the chair will turn towards that wheel. (10 of 16)12/30/2006 3:36:15 PM

11 LR RWD Lack of weight on drive wheels causes loss of traction, but the casters want to return to the leading position. HR RWD Weight on the drive wheels minimizes traction differential. CWD Weight on the drive wheels minimizes traction differential. HR FWD Weight on the drive wheels minimizes traction differential but casters want to overtake drive wheels, magnifying any deviation. LR FWD L Lack of weight on drive wheels causes loss of traction. The resulting turn is amplified by the casters desire to be in front of the drive wheels. Changing direction A chair?s response to steering input is important for people who drive in both latched and unlatched modes. RWD chairs have an inherent tendency to under steer in that they want to keep going straight and require an external force to change this tendency. As soon as that force is removed the chair wants to return to rolling in a straight line. FWD chairs have an inherent tendency to over steer. (Pushing an unmanned manual wheelchair backwards in an open space demonstrates this quite clearly. If not pushed perfectly straight the chair will increase its turn rate quite dramatically until the casters lead the fixed wheels.) LR RWD Chair is reluctant to initiate turn due to inherent under steer. HR RWD Slight amount of under steer translates into a feeling of stability. CWD Steering is neutral. (11 of 16)12/30/2006 3:36:15 PM

12 HR FWD Slight tendency to over steer is evident when changing direction at speed. LR FWD L Strong tendency to over steer requires constant steering input to keep the chair on track. To make things even more complicated, consider it is not uncommon to change direction on a cambered sidewalk comprised of uneven terrain. The compounding effect has led to a number of inadvertent donuts in both HR and LR FWD chairs. nown exceptions LR RWD Permobil Trax No tendency to turn on gentle cambers. No tendency to turn on uneven terrain. Turns are created by steering the casters, there is no reluctance to turn but the turning radius is larger than a conventional LR RWD. LR FWD Ω megatrac No tendency to turn up the slope on gentle cambers. No tendency to turn on uneven terrain. The Ω megatrac features oil dampened casters which are slow to react if the displacing force is only applied for a short time. LR FWD The Arrow sports a 44lb (20kg) weight directly over the casters. The inherent inertia in such a large mass tends to keep the casters trailing. Action Arrow Range The distance a chair can go between charges is dependant on the capacity of the battery and the current drawn by the chair. Our chairs were all supplied with M gel batteries. The capacities of the various types are shown in the table. Battery Capacity (12 of 16)12/30/2006 3:36:15 PM

13 Model Capacity Amp hr Weight - kg (lb) U-1 Gel (24) 22NF Gel (40) 24 Gel (55) 27 Gel (64) It is reasonable to assume a chair fitted with Group 24 batteries will go 25% farther than the same chair with Group 22?s. Current draw When manufacturers test their chair?s range in accordance with ANSI / RESNA standards the chairs are driven along a flat hard test track for a distance of between 1 and 2 km (0.6 to 1.2 miles). The chairs are driven at full speed and the current draw is recorded. A calculation of the theoretical range is than extrapolated. We tested our chairs around a 500 metre (540 yd) loop in both directions for a total distance of 1 km (0.6 miles). The test track was made of asphalt and concrete and had a number of irregularities and inclines up to 7 degrees (1 in 8) or 12.5% depending upon your language. Our findings were dramatically different from those claimed by the manufacturers. Our results are more in keeping with what a user might typically expect. ANSI / RESNA tests have to be repeatable at various sites around the world. We believe our tests give people a more accurate estimate of the range they can reasonably expect. Whenever the power to the motors is increased there is a significant draw from the batteries. This occurs whenever the chair is started, accelerated, turned, or encounters a hill. The following table shows the range of current draw observed during testing. Status Draw in Amps Chair on, stationary 0.1 to 0.3 Starting out or correction 20 to 40 (13 of 16)12/30/2006 3:36:15 PM

14 Full speed flat smooth terrain 8 to 12 Down 7 degree incline 0 to?5 (the batteries actually receive a charge) Up 7 degree incline 40 to 80 Every time a correction is made the power drawn by the motors changes. Ascending an incline draws a large current while descending is almost the same as standing still. These draws are not factored into the ANSI / RESNA tests, a fact that is particularly significant when the tracking characteristics of power wheelchairs are considered. A FWD chair tends to over steer and requires constant steering input to keep on track. So instead of the chair drawing 8 to 12 amps going full belt down a fairly straight path it repeatedly draws 20 to 40 amps every time a steering correction is made. LR RWD Range is compromised by parasitic drag from weighted casters. HR RWD Drag is minimal, steering corrections minimal. LR CWD Drag depends on weight on casters, steering correction minimal. (14 of 16)12/30/2006 3:36:15 PM

15 HR CWD HR FWD Slight tendency to over steer requires frequent corrections, minimal drag. LR FWD L Strong tendency to over steer requires constant joystick input to keep the chair on track. Also significant parasitic drag from weighted casters. Comfort To some extent comfort is subjective. What one person favours in the way of seat angle and back height, for example, another may find intolerable. We focused our attention on dynamic comfort, assessing the smoothness of the ride over a variety of terrain. Suspension and weight distribution, wheelbase, tire diameter, and a smooth response to steering input all combine to determine how comfortable the user perceives the chair to be. Comfort should be considered both statically and dynamically. Static comfort Seat sling or base, cushion, back upholstery or insert, and the frame geometry (which determines hip, knee, and ankle angles) all factor in the user?s comfort when stationary. When selecting a chair ensure it fits the user. Dynamic comfort Dynamic comfort is the ability of the chair to minimize the vibration and shock transmitted from the driving surface to the user. Reducing shock and/or vibration can help to reduce spasticity, minimize low back pain, decrease the likelihood of skin breakdown, as well as increase comfort and sitting tolerance, to say nothing of a better frame of mind. Shock can be absorbed in four ways: (in order of preference) 1. Suspension between the frame and wheels. Some can be adjusted to user weight and the terrain to be experienced. Suspension can be found on all wheels. The wheels with most weight on them have most impact on comfort. The wheel that first contacts the obstacle also has a more profound impact on comfort. 2. Pneumatic tires on the drive wheels and casters. Large fat pneumatic tires are best at absorbing shock. Since the drive wheels are invariably bigger than casters, a chair with a larger percentage of its weight on the drive wheels produces a softer ride. Also, a chair with drive wheels at the front produces a softer ride. (15 of 16)12/30/2006 3:36:15 PM

16 3. Stretchy upholstery Upholstery can absorb shock by stretching, however it tends to sag a little, resulting in a less than perfect platform from which to support the users position. 4. Shock absorbing cushions. Cushions can be added to any of the chairs tested. If extra shock absorption is required users are advised to try out the various types of cushion (air, gel, or foam). However the user must remember the primary purpose of the cushion is not to provide shock absorption. Go to Performance Analysis Off Road (16 of 16)12/30/2006 3:36:15 PM