CONTROLS
Conceptual Design and Design Selection
Design of the motor controls was probably the most underestimated aspect of the project. It was originally thought that the controls would be easy to design and cheap to produce. Further research and refinement has proven both of these assumptions incorrect.
User Interface: The proposed design for the user interface used four buttons or a simple joystick to control forward, reverse, left, and right. A simple joystick is nothing more than four switches printed onto a circuit board and each switch is activated by moving the stick in the direction of the switch to be closed (see figure 1) (Harris). Both options are inexpensive to produce and have good durability and reliability.
Figure 1: Simple joystick internal view Figure 2: Four pushbutton concept
Using a joystick would be closer to the original goal of using this vehicle as a wheelchair-training device, but the decision of which to use was based upon ease of use for the disabled child. Expert advice from occupational therapist Rocky Landers assisted in concluding that the four-pushbutton design was the best choice for the prototype vehicle.
Further design iteration and consultation with the sponsor showed that an eight push-button interface would provide the best possible control system. This design provides the four basic independent directions, plus the ability to turn in either direction while moving simultaneously in forward or reverse.
Circuitry Design:
Two design options presented themselves after the selection of the four pushbuttons as the user interface. Each of the options that were considered dealt with what response was expected from the vehicle for each button depressed. Both scenarios use the forward and reverse buttons for their expected uses, forward and reverse. The difference in the options is the use of the left and right buttons. Option one would use them simply to turn the wheels left or right, respectively. Option two would use the left button to turn the wheels left and simultaneously drive the vehicle forward; the right button would turn the wheels to the right and drive the vehicle forward.
Figure 3: Motion of Controls (1) Figure 4: Motion of Controls (2)
Option two was chosen and a preliminary circuit design, using four dual pole single throw switches was created. A few problems became apparent. The circuit would work, however, if any two buttons were pushed at the same time, the battery would short. This would cause a severe safety hazard and be expensive to repair. The other problem is that most switches are rated for only a few milliamps. This system would send an excess of 15 Amps through each switch. This again, is dangerous and unacceptable. These problems prompted further research and paved the way for the final design.
Figure 5: Original circuit design. Each yellow box represents a dual pole single throw switch.
The final circuit design utilizes a box with eight pushbuttons as shown in figure 6. This provided a compromise between the two four-button designs. Eight buttons allows all four cardinal directions alone, as well as all possible combinations of those directions. Each button that provides combination motion is actually controlled by two switches. The oversized buttons allow the combination to be activated by a single push.
The problem of excessive current through the switches is solved using relays. Relays allow the large current delivered to the motors to be controlled by a series of low current switches. This final design can be seen in figure 6. It utilizes four single pole single throw switches, two 10 Amp dual pole dual throw relays, and two 30 Amp dual pole dual throw relays. The diagram in figure 6 only shows half of the circuit. Two identical circuits were built. One controls forward-reverse; the controls left-right. Specifications for the linear actuator show a maximum current draw of 6 Amps, therefore, the left-right circuit uses the 10 Amp relays. The forward-reverse circuit utilizes 30 Amp relays to accommodate the larger draw from the DC motors.
The relay design also eliminates the problem of short-circuiting when two switches are thrown simultaneously. This circuit allows the use of the combination motion buttons. If both forward and reverse, or left and right are used at the same time, no circuit is completed.
All of the circuitry is protected from an overload by fuses. One 30 Amp fuse protects the battery from an accidental short circuit and one 8 Amp fuse protects the linear actuator.
A charging port was installed on the rear of the vehicle, providing a quick, convenient method of recharging the battery.
Figure 6: Final Circuit design.
The drawing below is the final design for the control box.
Figure 7: Final Design