Page 65 - Mechatronics for Safety, Security and Dependability in a New Era

Ch11-I044963.fm Page 49 Tuesday, August 1, 2006 8:51 PM
1, 2006
8:51PM
Page 49
Tuesday, August
Ch11-I044963.fm
49
49
Rack
Pinion gaer
Ascend —a ckPinion gaer
Ascend
Sprockets
Chain
Lift Chain
Rollers Stage
Stage
Gas springs
Descend Gas springs
Base
Base
v//////////////////////////
Figure 3: Motion of the lift when Figure 4: Mechanism to
ascending / descending decrease driving torque
work under outdoor, power failure and some disaster. A problem is that the lift can't ascend / descend
without a wheelchairs. One wheelchair user can't use this lift after another. It must be used personally.
3. MECHANISM TO DECREASE DRIVING TORQUE

Driving torque to ascend the lift is larger than that to descend it. This isn't efficient because
mechanical parameters of driving parts should be determined under the condition of the maximum
torque. In order to decrease the difference of driving torque between ascent and descent, assist
mechanism with gas springs are attached. In comparison with coil spring, gas spring has a
characteristic that the reaction force doesn't change so much while extending. Though it is good to
cancel out the constant load, its length is over twice as long as its stroke. By applying the principle of
a moving pulley, the assist mechanism with long stroke can be realized as shown in Figure 4. It
consists of short gas springs, chains and sprockets. Tt can double the stroke of gas springs, however,
the reaction force of gas springs should be two times as large as that without this mechanism. Then, it
uses double the number of the gas springs. When there is no wheelchair on the stage, the worm gear
holds the stage against the gas spring force.

4. MECHANICAL ANALYSIS
The ascending (/ descending) speed and the driving torque are analyzed. The ascending height of the
stagey and ascending speedy become

D P-D-i Dp-D- Dp-
y = co y = 0)
2-d 2-d d
where 0 is the rotation angle of rear wheels of a wheelchair, D is the diameter of rear wheels, <iis the
diameter of rollers, / is the total ratio of the worm gear and sprockets, D p is the diameter of the pinion
gear, co is the angular velocity of the rear wheels, and v is the running velocity of the wheelchair as
shown in Figure 5. When the rear wheels are rotated at a constant velocity, the lift stage ascends
uniformly. If rear wheels are rotated at the velocity of 0.3 revolution per second (1.88 rad/s), which
assumes a manual wheelchair for example, the ascending speed becomes 10 mm/s in the case of D =
570 mm (22 inches), D p = 28 mm, d = 30 mm, / = 1/50. Assume that the velocity of a powered
wheelchair is v = 6 km/h = 1.67 m/s, the ascending speed becomes 31 mm/s.
The driving torque of the rear wheels T is expressed by