The assignment for this week is to work on the mechanical design for our final project. I would like to make a portable scoreboard for my final project. An important aspect of the scoreboard is a mechanical seven segment display.
My first design uses two actuators for each seven segment digit. I constructed a prototype using cardboard, clear acrylic, and vinyl decals.
The design is very scalable. For this prototype, I made the example digit 12.5 inches tall by 6.5 inches wide. Each segment was 0.5 inches wide by 5.5 inches long.
A vertical linear slide with 5 unique positions is used to control the display of top, middle, and bottom segments.
A horizontal linear slide with six unique positions is used to control the display the four remaining segment.
By moving the vertical slide up/down and the horizontal slide left/right, it is possible to display all the digits from 0 - 9.
Although the vertical and horizontal slides do overlap, the acrylic does not need to be transparent for this design to work.
This design worked well. It was easy to construct and moved easily.
I felt that the segments were not wide enough. I want the digits to be much bolder so that they will be very easy to read.
Since the vinyl "segments" on the vertical and horizontal slides are the exact same size as the aperture holes of the seven segment, I am concerned that it would take too much precision to position the slides so the digits look good.
When you move the slides, it is possible to feel slight resistance when you hit the edges of each vinyl segment.
My second design is the same as the first design except that I made the segments 0.875 inches wide. This is almost twice as wide as the original.
This change made the digit slightly larger overall - 13.7 inches tall by 7.3 inches wide.
I didn't bother to physically make a prototype of this design. While evaluating the design in the CAD software, I realized that the additional side to side movement of the slide would require 10.2 inches between digits. I felt that this spacing was way too large.
Design #2 - Change makes digit easier to read but pairs of digits would need to be spaced too far apart.
My third design is the same as the second design except that I added a third slide to reduce the space needed between digits.
This approach makes the top two left/right segments controlled independently from the bottom two left/right segments.
I didn't physically make this design. It is a feasible design solution. Since it is based upon the first prototype, I am sure it will work.
This approach reduces the space needed between digits by using an additional actuator per digit. However, this also adds the costs, complexity, and control issues related to the additional actuator per digit. Since a full scoreboard would use at least four digits, the cost related to adding four more actuators to the product adds up.
Design #3 - Reduces the space needed between digits but adds the costs of an additional actuator per digit.
My fourth design looks similar to the previous design but reduces the number of actuators needed per digit back down to only two.
In this approach each of the four side segments move independently and have only two states each.
A linear vertical sliding plate with specially configured slots is used to move all four segments simultaneously to achieve the six different combinations needed to display the digits 0-9.
Instead of using vinyl stickers, I cut the aperture and backdrop parts from black acrylic and cut the segment parts from white acrylic. The remaining parts are made from clear acrylic.
Design #4 - Design colored to show how one vertical plate moved up/down moves all four side segments left/right.
The wider segments make the digits much easier to read but it also increases movement and space needed for the segment combinations.
The vertical slide related to the three middle segments works very well.
I had a unique problem when lasering the counter bored hole into the rectangular pieces for the side segments. I had made several test parts to make sure I had the proper laser settings and hole dimensions to compensate for the laser kerf. However when I lasered the final parts the counter bore was too shallow and the hole was too small. I found that if I lasered one hole at a time it worked perfectly, but when I lasered all four at the same time the results were different.
The overall structure bends much more lengthwise than expected. This causes binding if not kept flat. This should not be an issue once the segment is mounted as part of a cabinet.
The four side segments move very easily by hand but bind when driven by the slotted sliding plate. I suspected that this might be a problem since I was attempting to move the segment side-to-side one inch by moving the slotted plate one inch up-and-down.
I believe the binding is caused by the resulting forces rotating the segment sliding piece.
Design #4 - Possible explanation of segment binding problem.
I revised design #4 to see if I could reduce the binding and friction so that it would work as intended.
The sliding segment pieces had very sharp corners which seemed to dig in if the part skewed on an angle or passed another piece. I added fillets to the corners. This greatly reduced the amount of binding.
I made the sliding segment pieces wider. This involved a more complicated design with 3 layered pieces instead of just one simple flat piece.
To reduce friction, bearings were added where the sliding segments ride on the cam slots.
Instead of a narrow cam slot, the cam path pushes the sliding segments outward and a rubber band is used to pull the sliding segment back towards center.
Larger pieces of acrylic are used across the very back to tie everything together. This makes the structure much thicker. Even without being mounted to the cabinet, it no longer bends to the point of causing binding.
Design #5 - Prototype displaying the segments for the number three.
The good news is that this prototype proves that my concept can work. Once I resolved some of the friction and binding issues, it finally functioned the way that I intended.
This means that it should also be possible to control a similar seven segment digit with a single axis, because I have already developed a variation of this design that does just that.
However, the structure has evolved into something more complex than I intended. To save the cost of $5 actuator, I have needed to use four bearings and six layers of 1/8" acrylic. This design utilizes 28 different laser cut pieces of acrylic which are held together with 30 bolts.
Video of the prototype for design #5. The white slide controls the three middle segments. The clear slide moves the cams that control the four side segments. With this prototype, you can see all the way through when the white segments are not in place. Normally black acrylic would be used instead of clear for the background piece of acrylic. The clear acrylic makes it easier to see how the device functions.