Moco Rig 4 – Jan 2017

I have been very slow in adding posts to this, as it has taken so long for me to get round to creating something that actually functions at all. But the resolution for 2017 is to do a post at least once a month!

The rig as built has been functioning, but there are definitely improvements to be made. Firstly, the drivers. It seems the Big Easy Drivers are fairly basic and quite old in design. They have no protection against overloading, which is why I have managed to blow 8 of them! Newer designs are readily available as drivers for 3D printers and CNC machines. They have black boxes and green connection blocks on the side, and come in several different types. All apparently have optical isolation which separates the signal circuits from the higher voltage power circuit. Phew! I chose one which offered more than 2A current, and there are microswitches on the side to specify the current for the motors, which should be around 1.7A.

Attached 2 of these, and only one is currently working. Not sure what has happened with the other, but it is more likely a connection fault than anything else. The 4 BEDs that are still working seem fine, so nothing to change there. Also, I have stripped out an old desktop computer to use the box. This means I can put the mains transformer and all the Arduino stuff in one box.

Power supply at bottom, original box with arduino and BEDs at top

Incidentally, I found a fault with one of the Aviation sockets when testing them with a multimeter. The resistance seemed suspiciously low across a couple of pins, so I replaced it. Might have been the cause of several BED deaths!

The mechanical aspects of the rig have also got teething troubles, but that is hardly surprising.
Track, operated with a ballscrew, is slow in operation but has absolutely no backlash. Very pleased with this as a choice.
Rotation is geared. I could not find the huge diameter gears seen on some rigs, so have a MOD 0.8 128 tooth gear from Servocity on the pillar engaging with a 30 tooth pinion on the 27:1 geared motor. There is a sliding mount for the motor, to disengage the gears, and it is held in place with nuts and threaded rods. Crude but efficient. I think the only problem with this axis is that I have set the motor speed a bit fast.
Crane is giving the biggest headaches. The arm is quite big and counterweighted, so there is a lot of inertia. I went for a belt drive, thinking this would be fairly simple. BUT… at first I connected the pinion directly to the motor shaft – not clever as tensioning the belt puts lots of strain on the gearbox. So I added a couple of bearings and attached the motor with a flexible coupling. The result seemed to be even worse, until I realised that the flexible coupling acted like a spring and needed to coil before powering the belt. So I have ordered a solid coupling…..I am sure this is all very basic to any engineer, but I’m just a filmmaker!
However, I am not sure the coupling (yet to arrive) will solve all the issues. There is still a little stretch and bounce in the belt, and of course this is magnified by the length of the arm, so there may be about a couple of mm wobble at the camera end – unacceptable!
I have seen several, including professional rigs, that use a vertically mounted ballscrew. My experience with the track mechanism suggests this is the answer. It will also help to support the weight of the arm. Now I just have to figure out a way to attach it and the motor.
Pan seems to work just fine, although with the new driver there is a buzzing from the motor. I think I need to reduce the current at the driver to see if it stops.
Tilt works perfectly!
Focus has never worked yet! I had a very small NEMA 11 motor, then swapped it for a NEMA 17, but the driver let me down. Back to testing.
The bearings and the general construction has been very successful. I took trouble to make sure everything was as solid as possible with no slop, and this is critical if the rig is going to function as it should.

I have done one shot using the track function, and it has been a delight to do. Not having to think about the camera once it was set up made it so much easier, and we were able to concentrate on moving the puppets.