Mechanical Stair-Climbing Transporter
The main purpose of this mechanical stair-climbing transporter is to carry or transport loads up or down a stair case. The main features of the transporter include the supporting frame, tilt switch to adjust the angle of the platform, and triangular belt attached to wheels.
The supporting frame is designed to support all the essential parts of the transporter. The motor drive, driven shafts and chains, and platform are all attached to the supporting frame. The tilt switch will help to determine the angle that the platform is currently at. A motor will rotate a lead screw rod that will raise or lower the angle of the platform until the desired tilt angle of 0° is obtained. This will keep that platform parallel to the ground. The screw rod is connected to a clamp, which is attached to the frame, that will allow it to rotate clockwise or counterclockwise. The platform will be attached to the frame using hinges. These hinges will allow the platform to rotate clockwise or counterclockwise.
The use of a triangular belt with wheels is to allow the transporter to go up the stairs from the friction of the belts. The design has four sets of these triangular belt with wheels. Pulleys will be used as the wheels. The front wheels will be driven by small pulleys and the back wheels will be driven by sprockets that are driven by shafts that, in turn, are to be driven by a chain.
An AC motor will be used to drive the transporter. The motor’s rpm will be reduced to the desired speed. This will be done by using a worm gear, bevel gear and sprockets inside a gear box.
The design of the Mechanical Stair-Climbing Transporter had been done in several steps. The initial steps involved planning out the motions of the transporter. This includes the design of the wheels and the platform. Research has been done to see what some designs were already done. The design that was chosen was not one of the design that was found from the past research. The design chosen has wheels on a triangular plate.
Some of the other steps involved calculations, obtaining the materials and assembling the transporter. The final steps involved testing and fixing any problems that has occurred.
Most of the problems were cost considerations. They were solved with the used of cheapest available materials in the market. Another problem was that the rear wheels belt was slipping. This was solved with the replaced of the rear wheels’ small pulleys with small sprockets.
Mechanical System Design
Drive Mechanism Design
The initial drive mechanism of the stair-climbing transporter was shaft driven. The shaft was to be turning two bevel gears, one on each shaft on the wheels. A motor was to turn the drive shaft using a bevel gear. The current design is chain driven. The chain drives the two shafts on the wheels (See Figure 6).
Figure 6: Bottom view showing chain drive
The difference between the two designs are that the chain drive design requires two sprockets and one chain and the shaft driven design required three bevel gears and a shaft. The chain drive design costs less than the shaft drive because the cost of bevel gears are greater than the cost of the sprockets. Because of this reason, the chain drive was chosen and the shaft drive was rejected.
The design of the wheels is set to be triangular in shape. This shape is chosen to allow the wheels easily rotating up the stairs and keeping more surface contact for the rear wheels and the floor to attain stability, as opposed to circular shaped. The triangular wheels rotate about its center axis (centroid in this case). The triangular shape wheels are made up of three V-shape pulleys and each pulley is attached to a triangular plate. A #25 V-belt is wrapped around the pulleys. A timing belt is wrapped around the V-belt with the grooves facing the outside. The grooves provide some grip on the wheels to the stairs. A small pulley is attached to one of the big pulleys on the two front wheels. This small pulley will drive the other big pulleys. A small pulley is attached to a shaft that will drive the small pulley on the wheel with the help of a small #25 V-belt. (See Figure 7A). The rear wheels are made the same as the front wheels but have small sprockets instead of small pulleys to drive the other big pulleys (See Figure 7B).
Figure 7A: Front Wheel
Figure 7B: Rear Wheel
In order to have the transporter to go up and down stable, a low rpm from the AC motor is required. The reduced speed also gives a higher torque that is needed. The AC motor has a high speed that need to be reduced to meet the desired speed. The reduction of the speed is done with a worm gear and bevel gear (See Appendix C). The gears are placed inside a gearbox with the shaft of the motor sticking into the box. The bevel gear is attached to the shaft that turns the rear wheels (See Figure 8).
Figure 8: Gear box
Transporter Motion Design
An AC motor is used to drive the transporter. The motion of the Mechanical Stair-Climbing Transporter is controlled by a toggle switch. This switch controls the forward and backward motion of the transporter. The transporter is primary not designed to turn left or right (See Figure 21).
Platform Motion Design
The platform is designed to flip so it will remain horizontal. This design is done with the use of attaching a screw rod from the transporter to the platform. The platform can be able to move up and down along the screw rod. A DC motor will turn the rod so the platform will move along the rod. Gears will be placed between the motor and the rod to increase the revolutions per minute (See Figure 9). A simple control system is used to control the DC motor (See Control System Design Section, p.15)
Figure 9: Platform with DC Motor and Screw Rod attached
Control System Design
Figure 10 How the Tilt Switch works
Tilt switches control the raising and lowering of the platform. They are switches that turn on and off at certain angles (See Figure 10). There is liquid mercury (metal conductor) inside the tilt switch that is free to move around inside. When the switch is on, voltage flows through switch. When it is off, no voltage flows through the switch. The operating angle is the between the angle in the off position and the on position. Two switches are used. One turns on when the platform’s angle is below the horizontal and the other turns off. The other one turns on when the platform’s angle is above the horizontal.
Gates, and Relays
The problem with the tilt switch was that there is a chance that both will turn on. To prevent the motor from shorting out, an AND gate and a NAND gate was used. They are setup to allow voltage to go to the motor only if one of the switches was on. If both are on or off, no voltage will be sent to the motor. Since the gates cannot output 24VDC to the motor directly, relays were used. Four relays were used to provide the necessary voltage to the DC motor.
The motion of the entire transporter as it is going up the stairs is shown in Figure 12. The platform is rotating as the transporter is going stairs. The wheels rotate as it hits each step. Each of the kinematical linkage and control system are decoupling together and running functional and properly together.
Figure 12 Motion of the Transporter as it is going up