Construct Projects are things that I have worked on while employed at The Construct, the makerspace at RIT. I officially work here as a lab manager, but among my many roles, I am also a WebDev, Graphic Designer, Systems Engineer, and even a Carpenter! We’re a small space with a small budget, so many of these projects involve developing equipment and infrastructure for low costs.
This project was built in July/August of 2016
We have always had a projector at The Construct, but it was never useful. we would sometimes use it for social events and training sessions, but usually it sat in a cabinet. With a growing number of lab users, and more requests to learn more about how to use equipment in the space, we started to realize that it was necessary to have a method for turning the space into a classroom, for tool training sessions, interactive workshops, lectures, and social events.
To make a long story short, we needed a projector and projector screen, but had no way of purchasing a commercial system. Due to the location of the screen in the room it would need to be remotely controlled, and motorized. Most motorized projector screens available were either too costly ($300 or more), or were smaller than what we wanted. Having RIT’s IT department install the projector would also be a significant expense due to labor costs. So, I was inevitably tasked with designing the cheapest possible automatic projector screen for the space. How hard could it be?
Raising the bar
These were the requirements I gave myself for the project:
- Budget: must cost less than $200 total
- Ease of Use: must be able to automatically raise and retract the correct amount with a single interaction
- Longevity: must be stable and unmoving when in either the up or down position.
- Power: must be able to lift weight of projector fabric.
A typical projector screen consists of a fixed frame, a cylinder which can rotate to take up the screen fabric, the fabric and tension bar, weight an a motor/gearbox to spin the cylinder. A typical screen of the size selected (128″ diagonal) has a weight of about 12lbs, and I assumed the tension bar would weigh 5lb, making the total weight 17lb (or 75.62 Newtons), so I would need a serious motor or gearbox to lift it. Worst case scenario is when the screen is all the way down, as the motor will have to lift the full weight of the fabric+bar. The torque of the motor is then dependent on the gear ratio and radius of the takeup cylinder:
m = (75.62N*R)/n
I made R as small as possible (0.5″ or 0.0127 M) to minimize the torque required . With a 1:1 gear reduction I would need a motor with about 1NM of torque. This is much more than the 0.3 nM that the motor I scavenged could provide. I scaled the gear reduction to 5, in order to make sure the motor wouldn’t have to operate close to the stall torque value.
I sourced as many parts as I could from our collection of scrap parts, to reduce cost as much as possible. In the end, I had everything I would need for the electronics of the display, only needing to purchase some hardware for mounting the projector, the display fabric itself, and raw materials for holding up the fabric. The total cost for the project is shown below.
I leveraged our space’s 3D printers to create custom parts necessary to spin the takeup spool. I created the entire design in Autodesk Inventor (overkill? only if you have to pay for the commercial license 😉 ), which included using their amazing gear generator. To make these objects printable, I used my offset technique to shrink the part to compensate for the tolerances of 3D printing ( I talk about it in my Design for 3D printing course that I’ve been running at The Construct) .
I created the rest of the parts using our space’s shop tools, and assembled everything. To allow the cylinder to rotate smoothly, I created a printed spacer with a captive 3/8″ bolt. this spacer is secured through the cylinder with two smaller bolts, which thread into tapped holes in the plastic.
I originally intended to use a DC motor driver to power the screen, controlled by an unused Arduino Microcontroller. However, I didn’t have one on hand that functioned at the 18V that the motor operated at. So instead, I went with the simple route of using two DPDT Relays configured in an H bridge. I re-purposed a reed switch from a door sensor to detect when the screen has made a complete revolution.
In the end, the hardest portion of the project ended up being installation of the actual screen, which was a heavy piece of coated canvas 128″ in diameter. To make matters worse, I had to attach an extra bit of drop cloth to allow the screen to hang at the intended height. I’ll never get those 2 hours back…
After some delicate maneuvers on a ladder and some minor revisions to the code to make the screen rotate the proper number of revolutions between the down and up states, the screen was finished! Here I am teaching a LaserCutter Training course using it. of course, this doesn’t fix the fact that our projector is old, dim, and low-res, so we will hopefully be able to replace it sometime in the near future.