The View Up Here

Random scribblings about kites, photography, machining, and anything else

Offroad Camera Project

Posted by Tom Benedict on 11/07/2011

I’m building an offroad camera for a fellow photographer. The goal is to have a remotely controllable camera platform that can drive over rough terrain, position a camera, orient it, and release the shutter. It’s almost done, but not quite.

All in all it’s been a really fun project to work on, but I’d be lying if I said it was all clear sailing. It’s unfortunately been a case of one step up and two steps back, or on good days two steps up and one step back. And just when I thought the end was in sight, I managed to set myself back by a week.

The vehicle is a 6WD Dagu Wild Thumper from Pololu Robotics.  Since the goal is controllability rather than speed, we went with the 75:1 gear train for the drive motors rather than the speedier 30:1. The motors are driven by a pair of Pololu Simple High-Power Motor Controller 18v25 electronic speed controllers. These are really nice units that can be configured using a USB cable and software from Pololu. They can be driven by an R/C signal, or via an analog input, or over a TTL serial connection. Even better, you can use a pair of Y-splitter cables to feed them a steering/velocity signal, and the two ESCs will do the channel mixing necessary to drive a tank-style steering chassis like the Dagu.

The robot is powered by a pair of 7.4v 3000mAh NiMH batteries. They’re wired in parallel to provide 6000mAh of capacity, so it can run with one or both batteries installed.

The Pololu ESCs offer a battery elimination circuit (BEC) for powering the radios and other servos, but they can only source a small amount of current. The other R/C components of the robot draw several amps, so the on-board BECs couldn’t be used. Instead we used a 5V 3A BEC that is installed in the same enclosure as the ESCs. Pololu now offers a BEC of similar capacity, but at about a quarter the size of the BEC we used. In the future, I’d probably go with the Pololu unit.

The pan/tilt unit is a combination of two gearboxes from ServoCity. The pan unit is an SPG400A set up for 360 degree rotation, with a Hitec HS-7985MG servo and 5:1 metal gears.  The tilt unit is an SPT400 with a Hitec HS-7985MG servo and 5:1 metal gears. This provides ample torque for holding the camera and lens combination in position, regardless of the orientation of the chassis. The pan/tilt unit is attached to the Dagu chassis via six standoffs and a custom machined aluminum plate. The plate also serves to distribute the load of the pan/tilt unit to the lower chassis through four of the eight standoffs that attach the two halves of the chassis.

One problem I ran into with the ServoCity units is the exposed gear trains. In most situations exposed gears are no-nos. This is why most equipment that contains gear trains also includes safety covers. The ServoCity units didn’t have a provision for a safety cover, so initially I ran without one. Sure enough, the pan gear box ate the tilt unit’s servo cable during initial integration and testing. I added a simple ABS cover to keep the cables from getting chewed in the future.

Crawler 2

I’m still waiting on a couple of parts, one of which is a 50′ length of heavy duty servo cable to replace the one the pan gearbox ate. But this is the current state of the thing:

Crawler 1

It uses a 75MHz surface radio to drive the chassis and control the pan/tilt unit, and a Hahnel Inspire 2.4GHz remote unit to control the camera. I’m waiting until the owner has had a chance to drive it around before deciding how to mount the Hahnel unit, if at all.

One surprise I had was that the center of gravity of the thing is below the level of the top deck. At first glance it appears to be horrifically top heavy, but that isn’t the case. Even more surprising was how hard it is to push over. The wheels all move independently, which gives the chassis an incredible amount of compliance. It’s quite hard to tip it over.

How this will play out in use I don’t yet know. It’s one thing to test a system when static and declare it stable. It’s quite another to test it in motion and see what the real dynamics are. I have a couple of good test spots where I plan to run it with a dummy weight in place of the camera, just to be sure. Once I know it’s safe and won’t tip over in normal use, it’s on to testing in the field.

– Tom

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One Response to “Offroad Camera Project”

  1. […] did some more work on the offroad camera platform I wrote about back in 2011. As delivered back then, the platform had a couple of serious weaknesses. First, having a highly […]

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