“Get that Door Closed!”
By Allied Motion | Apr 08, 2015
In a military personnel land vehicle, the occupants are exposed, perhaps lethally so, when the vehicle’s rear personnel loading door is open. So closure of that draw-bridge style door must occur as rapidly as possible. Opening quickly is no less important.
Allied Motion was recently consulted to develop an electric door actuator system for such a vehicle to replace the potentially leaky hydraulic system used in earlier vehicle designs. Eight seconds was the target for either closing or opening the 440 kg (970 lblb.) door. An additional challenge was the stingy “space claim” (allowed space for the actuator) that had to be respected.
Allied Motion’s customer had chosen a simple circular pulley and wire rope system to operate the door. The pulley would attach to Allied’s gearmotor actuator. The only problem was, the size of the gearmotor necessary to operate the door in the required eight seconds was too large and violated the allowed space claim. The project ground to a halt, until Allied developed an ingenious solution.
Jeff Shearer, Allied’s application engineer assigned to the door project, knew he could use a smaller motor for the task – the total required power was well within the capability of a smaller gearmotor – but the necessary torque needed at the start of the door closing cycle was much more than the smaller motor could provide.
The vehicle’s rear personnel door was to be raised or lowered by two, geared BLDC motors equipped with integrated electronic drives. A single 24 VDC, 50 A battery was to supply power to the gearmotors but it also constituted a limit on the available power. Finally, only a certain allowed space (“space claim”) was provided on each side of the door for the gearmotor and actuator assembly.
To close the door with a simple circular pulley, which was Allied’s customer’s original intent, the motor needed to be large enough to provide sufficient initial torque at the fully open position of the door. Because of the changing moment arm between the door’s hinge and its center of gravity, the tension in the actuator cables is at maximum at the start of the door closing cycle. Hence, the gearmotors needed to be sized for the initial maximum torque load, but would be oversized for the remainder of the closing cycle.
Jeff realized that the total work required to raise (or lower) the door is constant no matter what shape the pulley is. If the power required to close the door in 8 seconds could be distributed more evenly across the entire angle of pulley rotation, rather than just being concentrated in one part of the rotation, then it would be possible to reduce the motor size.
Applying this concept, Jeff effected an even distribution of power by changing the pulley profile from a constant radius circle to an expanding radius – a spiral – where the radius increases proportionally with the degree of rotation. With the spiral design pulley, Jeff showed that the required peak torque would be reduced by 37%.
Figure 2: The constant radius pulley design (modeled in Solidworks)
Figure 3: Graph of velocity and torque required to close the door using a circular pulley – note that torque is highest at the beginning of the closing cycle and drops off nearly proportionally with time.
Figure 4: Spiral radius pulley design – starting torque reduced by 37% (modeled in Solidworks)
Figure 5: Velocity and torque graph for the Spiral radius pulley design – note that torque is distributed fairly evenly across the entire range of motion.