4 Drive System

The motion of the rover is a critical issue. Four very important factors to be included in choosing a drive and steering motor are power, mass, complexity and overall dimensions. The motor with the least power consumption and adequate torque is, of course, an ideal choice. The mass of the motors is also an important factor. Minimizing the overall mass of the rover is a goal in every component choice. The complexity of the design should also be minimized. Minimizing the number of components in a system generally decreases the cost, mass and complexity of the design. The dimensions of the components must also be considered. Generally, the larger the component, the larger the mass. Also, dimensional constraints must be met.

4.1 Drive Motors

To make the best choice for a drive motor, a compromise must be reached between the deciding factors. All relevant properties for motors are included in Table 4.1. An overall rating of the motors will help to determine the best choice for this application. It must be understood that although the overall rating of an option may be highest, that particular motor may not always be the optimum choice. Factors not considered in the selection of a component may force the designer to reevaluate his/her decision. Only by proving the performance and reliability of the component by testing it for a certain application can the choice be considered final.

Table 4.1 - Considered Motors For Drive System
Motor Company Part # Peak Torque (oz in)* CST (oz in)* Mass (kg)* Power (W)* **
BEI Sensors*** DXP15-07 14.3 4.0 0.0425 71.0

*per unit specification (X 6 for system total)
**maximum power at peak torque
***mass does not include support bearings

4.1.1 Motor Assembly

Two configurations for the motor assembly were considered. They included a direct drive and a geared configuration. The direct drive configuration requires fewer mechanical components, decreasing its complexity. It also provides the basis for a modular wheel design. This self contained wheel design uses a reverse motor assembly as the base of the wheel. Eight spokes connect the motor assembly to the outer surface of the wheel. The design and its dimensions are included in Figure 4.1. The outer cup component and the wheel itself also double as a heat sink for the motor. Only the outer cup assembly, armature and PC board are included in the motor package. This allows for a shaft and motor case to be designed specifically for this application. A modular wheel design is the initial configuration considered. It includes the outer rim of the wheel, the wheel spokes and the outer cup assembly. This design could easily be cast as one component, decreasing the amount of machining required. No modification of the initial wheel design is required for the direct drive configuration.


Figure 4.1 - Modular Wheel Assembly

The geared configuration consists of a standard motor, shaft and gearbox coupled to the wheel as shown in Figure 4.2. This design can possibly maintain a modular design if the entire configuration is placed in a container. This container would provide the base for the wheel assembly as in the modular design discussed earlier. The motor assembly would remain stationary while the container, and therefore the wheel, rotated relative to the assembly. Some extra modifications would be required to support the motor assembly within the wheel. Therefore, this design requires the use of more components, increasing the possibility of failure. A separate heat sink would also need to be implemented into the design for heat dissipation.


Figure 4.2 - Geared Wheel Assembly

4.1.2 Motor Requirements

The main consideration in choosing the motor assembly is the power required for operation. Will the design provide the necessary torque without extensive damage to the battery array? This is the only concern with the modular design. Due to the direct drive configuration, the motor will be operating at a very low speed. At a maximum velocity of 0.1m/s, the motor speed is limited to approximately 21.45 revolutions per minute (rpm). It is possible that at such a low motor speed, the power requirements may outweigh the modular design advantages when the final design is assessed. Testing of this motor configuration should be implemented before the final decision is made. The power requirements for the standard geared configuration are minimal due to the high motor speed. At maximum velocity, the operating speed of the motor would be approximately 17,000 rpm. This configuration would not pose a power problem. The motor would be controlled by a frequency transmitter, which is discussed in the controls section.

An initial torque equal to or higher than 0.0584 N×m (8.27 oz×in) must be available for the system as a whole. A peak torque of 0.262 N×m (37.13 oz×in) must be available to the system as a whole. These calculations provide the basis for proper motor selection (see Appendix A). In accordance with the stated requirements, the DXP15-07 DC motor from BEI Motion Systems Company was selected. The selected motor weighs 36.85g (1.3 oz). Implementing a six wheel drive system provides a degree of redundancy to the system. Choosing a motor with characteristics approximately equal to one half of the stated requirements, allows the rover to remain operational with only two drive motors. The selected motor meets these requirements for continuous torque but does not meet the requirement for peak torque. The peak torque is thus limited to 0.202 N×m (28.6 oz×in).

4.2 Steering Motors

The steering mechanism will be housed in the hollow leg of the rover as was stated earlier in the discussion of the frame. This will provide protection for the system allowing for complete isolation from the surroundings. Figure 4.3 depicts a simple gearing mechanism proposed to allow for a fifteen degree rotation from center. This mechanism consists of a small standard DC motor, a gearbox, one threaded shaft and a threaded sleeve directly attached to the shaft of the wheel. The wheel shaft is allowed to rotate by the ball connections between the sleeve and leg. This is the only configuration being considered for the steering assembly at this time.

An eight millimeter diameter motor from Micro Mo Electronics is the initial choice for the steering motor. The small size, weight and relatively large continuous torque make this motor an ideal selection. The motor has an overall length of 16 mm and weighs 8.79g (0.13 oz). It is rated to operate in a vacuum down to 10-7 Torr. The motors will be used to steer the front and back set of wheels. This will provide a degree of redundancy to the rover allowing it to continue its objective with only two steering motors operational.


Figure 4.3 - Steering Mechanism


Last Modified: Fri Dec 18, 1998