Final Design

The final design consisted of four major components: two-dimensional motion stage along with a gantry, a product holding platform with a scissor lift, a spring-cam coupled presser system, and a motion control board. The following figure is a flow chart that shows the composition and control of this design.

The motion stage gantry was composed of three timing-belt-driven motion guides, four 6061 aluminum extrusions and a 6061 aluminum base plate. The presser was mounted on Y axis motion stage. The timing belt was driven by 24V stepper motors. By sending square wave pulses at different frequencies from the motion control board and setting current and microsteps at different levels from TB6600 stepper motor drivers, the rotation speed and output torque of stepper motors could be varied. Photogate sensors were installed at the end of both axes serving as homing position.

The product holding platform locked products of different sizes.  The platform included two fixed walls supported by a corner bracket and two moving walls driven by T8 leadscrews. The linear motion of moving walls came from the rotation of the leadscrew. The scissor lift under the platform changed the height to accommodate test samples in different thickness. The scissor lift was driven by a leadscrew that was rotated manually with a hand wheel.

The cam-spring coupled presser applied variable forces when the cam rotated. The different contour radius compressed the spring connected to the finger to different lengths, which applied forces from 1N to 10N on the tested button. The contour and output force profiles of cam are showed below. From 95° to 260°, the radius increases 1.4 mm as the rotation angle increases 15°.

The SNAP motion control board was designed and provided by the sponsor. The board was capable of controlling the stepper motors, the brushed DC motor, and homing sensors. Meanwhile, the internal proportional-integral-derivative (PID) system helped rotate the cam accurately for outputting variable pressing forces. The board had open-drain GPIO pins that required external pull-up resistors to output 5V control signals to the stepper motor drivers. 

Result

Performance of stepper motors on the motion guides

1. With 1000 pulses sent to the stepper motor driver, the motion stage would move 22.5 mm. 

2. The precision of the motion of motion stages were 0.3 mm.

3. The minimum required period of pulses sent to the motor drivers were 2 milliseconds.

4. Under different microstep settings, the pulses needed for a full displacement (300 mm) on a motion guide is showed below.

5. The speed of motion stages under different microstep settings was measured

Performance of scissor lift

The height change after the handle rotated different revolutions were measured. Initial height was 73 mm.

Performance of product holding platform 

The displacement of moving product holding walls towards the handwheel were measured after the handwheel rotated different revolutions.

The initial distance of moving wall 1 was 70 mm away from fixed wall. 

The initial distance of moving wall 2 was 65 mm away from fixed wall. 

Performance of Presser

The output forces were measured by a force gauge as showed below.

The results were plotted into an error bar plot. The result showed a relatively linear relationship between the number of encoder units sent to the encoder and the output forces. The error bars showed the variance at each measurement point.