This is my latest dolly the UM11-Dolly2 for timelapse tracking and videoing. The unit moves by using a cam belt stretched the length of the tracks and a stepper motor with a cambelt pulley mounted to it. My previous dolly just used a wheel and friction for movement but I wanted something that would allow repeat moves and this setup certainly does that. The stepper motor has a microstepper controller allowing upto 25000+ steps per revolution. I'm using it at 3600 which negates the need for a gearbox and the stepper motor is quite able to move the weight of my Canon5D with a large lens. Note that I currently have a Manfrotto head mounted on that top screw.
There are three bearings either side of the dolly which run along the rails. These are off the shelf skate bearings which do the job. The tracks are 20mm alumiunium with a 5mm thick wall for strength. I mount these onto three Manfrotto 190XB tripods which only just cope with this weight. It's fine for timelapse but it can get a little wobbly doing video due to the initiate jolt of the dolly movement. I am currently in the stages of getting a ServoCity Pan/Tilt head which I will modify for use with stepper motors. This will be mounted onto of this dolly which I fear will cause an upgrade in the tripod department. In fact I've already thought about this and I plan to look into getting the rails welded to a point that they do not need three tripods but only two larger ones. We'll see how that goes.
The grey box at the front is where the UM12 plugs in, the camera gets it's power, a shutter trigger connection for direct control and the 24vdc input from my UM6-CTP portable power supply.