TLA – Prototype V2 – Pan Stage Complete
I guess I didn’t mention that I was completely re-building the robot, did I? Well, now I just have!
Here’re the new specs:
Gearing: ~ 20.8:1 (vs. ~ 3.4:1 of the previous prototype)
Minimum movement degrees: ~ 0.011 (vs. ~ 0.067 of the v1 proto)
Housing: 1/4″ Translucent Bronze Cast Acrylic (vs. 5/32″ white opaque cast acrylic)
For trucking motion, the previous model used a hacked-together worm drive from low-quality parts, and suffered dramatic issues due to it, after much tuning and hassle, I decided I was either going to have to replace the low-quality parts with high-quality parts ($$$!) or choose a different means of achieving lateral motion. Not of the mind to spend at least $200 more on quality Acme components, I decided I’d throw my money in another direction, I would go with a linear rail system. So, finding VXB having a great deal on a 55″ long 20mm Linear Rail Guideway System, I bought one and with fast shipping it showed up a few days later. I still haven’t gotten to work on the new trucking setup, but essentially it will have an “outrigger” attached to the guideway that will serve as a friction plane, for a friction wheel that will be driven by a stepper motor. As the slide units have some rotational slop (pitch), this will be compensated for by hanging the truck motor off the side of the housing that contains the pan unit and motor control unit (rather large, see photos below), such that the motor its self is a few inches out past the outrigger plane. This will increase lever forces to both apply extra pressure to the friction wheel, preventing slippage, and also increase the amount of force necessary to pitch the pan unit in the opposite direction.
As my trip to Big Bend was delayed a week by hurricane Ike (I was supposed to leave this past Friday, and return this Friday, now I leave this Sunday and return next Saturday), I decided I wanted to get something completed before I went out there, thus the pan/control unit took first priority.
So, the fabrication was easy once the parts arrived, due to the fact that I sketched everything up in 2D CAD while I waited for my orders to come in, this let me have a very good idea of hole placement, etc. (Well, with some caveats I’ll go into below =) well before I put a single piece of acrylic on the drill press. The net result was the only thing that was needed in the end was some fine-tuning, and the correction of a bad assumption.
The bad assumption was that I could attach the acrylic pieces directly by drilling and tapping the sides dead-center on their thinnest plane. This might’ve worked, had I a milling vise for the drill press, but hand-holding the pieces made it nearly impossible to drill the holes on the bottom panel in a straight line, so some holes were too near to the sides to effectively tap them, and my choice of a #6/32 screw as the primary fastener meant that the threads were too fine to hold being screwed in and out more than a few times, so I went back to the old aluminum bracket method of holding the panels together. The acrylic panels were ordered pre-cut from Professional Plastics. They offer good pricing on pre-cut panels (make sure to adjust your count, the minimum line-item price makes them seem expensive, but you can keep upping the count until you start to exceed the minimum line item price) and a quick turn-around.
An increased resolution of movement in the pan capabilities is achieved by adding a second stage of gearing. The first stage, from the stepper, is 3.33:1 using 32pitch gears (as I had some handy already), and the second stage is 6.24:1 using 42 pitch gears (best ratio I could get using the 150 tooth gear I had, and a very small 1/4″ shaft press-mount gear), giving me roughly 20.8:1 gearing, or approximately 0.01 degrees/step. That is, 360 / 1600 / 20.8 – 1600 is the steps of the stepper using the micro-stepping driver. The first stage uses a 1/4″ shaft, and is run through a ball-bearing, and the second stage uses a 3/8″ shaft and two bronze bushings, one inside and one placed through the top of the housing. My choice to use the bushings instead of bearings is based on sizing issues and cost – namely, I didn’t want to spend any more money on the pan stage than I already had =) I’ll probably add a thrust bearing later to reduce friction, but with the massive amount of torque being elicited through the gear train, I’m not losing a lot here. Gears, hubs, etc. were ordered from ServoCity, who has been nothing but excellent to me as a customer.
Each gear stage is held up by an acrylic platform. The platforms are raised through the use of 3″ #6/32 bolts that are run through 2.25″ long, 3/8″ brass tubes (cut from a longer tube). The brass tubes add a lot more stability to the platform and prevent twist (the screws alone will not) of the platform under torque load.
There is some mis-alignment to the top panel, this is due to me measuring the placement for the final shaft while test-assembling (before I removed the paper from the acrylic). The final assembly found the gears slightly out of alignment, and therefor had to be re-adjusted, resulting in the top being ever slightly off. If you’re wondering how the top is held on, as you’ll see in the pics below, thumb screws are screwed into speed-nuts attached to angle brackets.
One tip for those reading this thinking about building their own system of this sort and who, like me, are anything but “master craftsmen” — drill the holes for your critical attachments just slightly larger than you need, this will allow you to make fine adjustments by moving screws to one side or another. Washers will cover up the slop and help to prevent excess stress on a cut area of acrylic. Another note about acrylic: you should only be working with cast acrylic. Don’t buy the cheap extruded variety, it doesn’t handle drilling, cutting, or stress nearly as well! Almost all cast acrylic I’ve seen comes with a paper coating, vs. plastic coating. This is by design, as it’s easer to draw on, drill, machine, etc – the plastic coating will peel up on you often and occasionally melt as well.
So, without further ado, here are some photos… Scroll down to the bottom for the first test video and some talk about the issues it presented and what I have done / am doing about them.
And, here’s the first test video. It was shot at a rate of 1 shot every two seconds, with a single step between shots. Below the video I’ll discuss the issues encountered and what has been done already, and what will be done.
So, there are three primary problems with this video:
Horizon drop due to the camera platform not being perfectly level throughout its entire rotation
This was caused by a number of factors, originally the 3/8″ final shaft rested on the bottom plate, this meant that the bottom of the shaft had to be perfectly flat to prevent vertical wobble. It wasn’t. Next, the hole through which the top bushing went through wasn’t _perfectly_ straight, resulting in a barely noticeable, but impactful rise on one edge of the bushing (about 1/64″ !) causing further wobble. Correcting these issues (shortening the shaft so that it did not rest on the bottom, and fine-tuning the upper bushing mount) reduced the level issues by 90%. (this can be tested by leveling the device and then slowly rotating the camera mount, noting the position of the spirit bubbles every few degrees of rotation.) Not exactly perfect, but good enough.
Camera wobble due to high winds
This was a two factor cause, the first being that my original shaft design had the final shaft rising up and out a couple of inches – causing the camera to move as if at the top of a flag-pole. My previous fix brought the camera as close to the top plate as is possible, reducing this effect. The second factor being that it was attached to a single tripod via a single point (a plate was made out part of the v1 prototype body bottom plate, and a hole screwed and tapped to allow a quick-release plate for my geared head to be attached. This causes this bulky item’s weight be secured by a single point. The final version, attached to the linear motion slides will result in it being attached to a longer system at 8 points, reducing the likelihood of this occurring.
Slow take up before movent
No, that’s not a sloppy ramping implementation, its the combined effect of the backlash in the gear train – that is, backlash is amplified as you add more gears. The backlash must be taken up before actual movement can occur. This is easily dealt with by “preloading” the gears into a particular direction with the manual control mode.
Overall, I’m much happier with this prototype, and very pleased with the results. Some short testing has brought back to life my belief that I can slow it down even further by skipping shots, as it turns out the jerkiness was caused by my incorrect use of Vegas, and not so much a failure of the assumption. Will post some videos later that show the varying speeds of movement available.