Kinematics and Motion Analysis

With the geometry of the mechanism defined through the development of a Solidworks model of our synthesized design, we set out to analyze the position, velocity, and acceleration of the two "table links" to determine a suitable motor choice and confirm our preliminary findings regarding the viability of the mechanism in accomplishing the prescribed design features.

To accomplish this goal we constructed a motion analysis simulation in Solidworks to model a motor activation. It was decided that 10 seconds would be a reasonable amount of time for Ceci to wait for the mechanism to open or close, so a simulated servo-motor with a constant acceleration ramp profile at the beginning and end of activation and a 10 second operation time was modeled. The motor was set to rotate 125 degrees over a period of 10 seconds (an average angular velocity of 0.219 rad/s) and record the torque required by the motor to do so. The results of this simulation are presented in Fig. 1, Fig. 2, and Fig 3. An animation of the motion profile of the motor is shown in the video below:

An estimation of the maximum holding torque required to begin activation of the mechanism at minimum mechanical advantage (i.e the initial position of the crank-slider mechanism driven by the motor) is given by the torque value at t < 5 s in Fig. 3 (1.246 Nm) and the maximum torque due to mechanism acceleration and deceleration is given by the peak torque in Fig. 3 (3.496 Nm). With these values, a safety factor of 3 was applied to account for friction effects and other errors in assembly and fabricated part tolerances not considered in the initial simulation. Thus, the maximum required torque of the motor was taken to be 10.488 Nm.

To find a suitable actuator within the budget of the project, we turned towards the automotive industry. Window regulator motors in cars typically actuate windows weighing upwards of 30 pounds, and by sourcing from an aftermarket supplier, can be purchased for ~$20. While exact specifications are not given for each motor, it is widely accepted that they ranged from about 10 - 20 Nm of peak torque. With this in mind, and our calculated 10.488 Nm maximum, we selected the cheapest window regulator motor we could source with the plan that, if it proved to posses insufficient power to reach 10 Nm of torque, we could always increase the mechanism activation time to lower the jerk of the mechanism and thus also the overall peak torque required.