POWERED STEERING
Electromechanical Linear Actuator
Our design utilized an electromechanical linear actuator (see Figure 2). When coupled with the current linkages, this motor provides steering control similar to a rack and pinion steering system. Advantages of this method included a simplified linkage between the actuator and the wheels, a rugged well-protected steering system, and abundance of power to steer the wheels. The sole disadvantage was the high cost of the actuator.
Figure 1: Electromechanical Linear Actuator
Among the primary advantages of the Linear Actuator were the minimal additional design necessary, and the relative ease of integration into the chassis steering system. The actuator chosen was capable of applying a maximum 100 lbs of linear force at a rate of half and inch per second. This provided a smooth turn through the steering radius. The actuator was also equipped with adjustable limit switches. These switches cut power to the actuator once it reached a pre-set travel distance. These switches were set to match the travel of the steering drive-rod – approximately +/- 1” from center (straight forward) position.
The actuator was mounted in the vacant space under the hood of the chassis. While the confined space increased the difficulty of this task, this location provided the actuator the most possible protection. As a result each mounting piece was custom designed and manufactured.
The plastic under the hood was reinforced with a steel plate due to the high force the actuator was capable of applying (see figure ). Five 3/8” bolts were used to distribute the loads from the actuator to the steel plate and plastic walls. This design prevents the actuator from tearing out of the weak plastic walls of the chassis.
A two-piece flange was designed and built to attach the stationary rear mount of the actuator to the chassis. This mount provides the linear force the actuator works against. It also helps stabilize its other degrees of freedom. These parts were machined from ¾” aluminum and were redundantly constrained to provide an additional factor of safety without adding too much weight.
The brass-linear bearing provides the main axial and vertical support to the actuator through its linear motion. The bearing and its mount were custom-manufactured to precisely fit and align the actuator. Brass was chosen for the bearing surface because it is soft and will not cause wear on the actuator, and because friction between steel and brass is relatively low.
The C-bar transmits the force from the actuator to the steering linkage drive-rod. This component was also produced from ¾” aluminum due to potentially high stresses in the structure. The pin that connects the C-bar to the drive-rod was designed as a fail-point for the assembly. The bolt was much closer to reaching its fail-point than was the C-bar at a load of 100 lbs.
Figure 2: Steering System