3axis KAP Gimbal part 3

Voltage regulation. Oh boy, this is fun. Having got the balance happy, the PID settings clean and video relay done I find myself stymied by failure of the RC control aspect. I can have the gimbal powered happily on a 1600mAh 3s battery. ..but as soon as I wire in the RC receiver I get direction control failure. Change battery, repeat, fail.

The plan was to leech power off the VTx as its supplied with a 5v outlet. The meter reads a steady 5.12v but I can’t get the receiver to do much except blink low power warning LEDs at me. On checking the documentation from Immersion RC I see the VTx  5v outlet is  a miserable 300mA max output. Enough juice to pair the receiver but not enough to drive a servo, let alone control a gimbal motor. Pah! I revert to the tried and tested Y cable and UBEC method which gets 5v at 3A to the receiver. The shutter is tripped by radio with a satisfyingly quiet ztt..sktchuck at last.

The wiring schematic ended up like this:

A ferrite bead is required on the output side of the UBEC as its switching method interferes with the 12c connections.

Direction control. This proves to be the last hurdle. 2 channels are required. Pan and Tilt or in gimbal speak yaw and pitch. Pan is awkward as the gimbal is designed around a drone which can turn to point a camera on a fixed heading.

PIDs pan

The pitch settings seemed obvious, the yaw less so:

  • Pan mode control=0ff
  • Pan mode default setting=pan hold pan*
  • Yaw Pan hold= 2.7
  • RC Yaw mode= relative
  • RC Yaw Trim=18 us (the RC transmitter trimmer has no effect)

Using the rudder stick for pan requires the yaw control to be set to ‘relative’ to take account of the self centring stick. A small yaw trim was needed to stop the pan creeping:

PIDs rc

*I swapped for ‘hold hold hold’ on advice from Al, the  yaw PID had to be re-set for it to work but this is the control required to pan independently from the gimbal heading.

Ground station. The other half of the system is the radio control transmitter and video reciever.

DX5e-mods-05042016_01Looking at a minimum effort approach a video reciever and screen are slapped onto a ‘standard’ 2.4GHz RC transmitter. I have been here before and learned the video reciever can run for a good stint on a 2sLiPo, this time round I discovered the DX5e can also be powered by the same 7.4v source without damage even though it’s quoted voltage is 6v. This means 1 battery can power 2 devices, fitting it in the transmitter case involves hacking a fair bit of plastic out of the battery compartment.

With the battery compartment cut away the power supply and video leads are fed through the transmitter case. The ‘system’ is powered without loose leads or external batteries: win. Velcro holds the screen and video receiver in place.


To protect the battery against damage from rubbing about in the case a card shield is fitted. With the battery cover plate re-fitted the disquieting mess is neatly hidden.

It’s a screwdeiver job to get the battery in and out so chrging is via the DC port on the tranmitter. For reasons known only to themseleves Spektrum use a centre pin NEGATIVE connection to charge the battery. This is the only device I own which uses this so a one off lead is required for charging and I dread the day I grab the wrong cable.

The screen is independently powered by its own 3.1v cell. A 10mm hole was required to feed the video cable to the screen, this turned out to be the only serious damage to the transmitter case: I taped over the hole once the cable was threaded through. I don’t work in the rain so the tape should keep the muck out.


Runtime for the gimbal and video transmitter is just short of an hour on a 1600mAh 3s battery pack, the RC transmitter and video reciever are out running the air side easily on a beefy 4000mAh 2s pack.

Combined 5.8GHz video and 2.4GHz RC units are available but pairing with DSM2/DSMX recievers is not well documented for them, they seem to be set up for proprietary DJI or Hubsan frequencies which are not known to be compatable with the Immersion RC vTx so I have cobbled  together generic parts: the key components of tansmitter and video reciever cost me much less than a ‘ready to fly’ UAV set up. The 2.5″ screen is something I have had for a while, it’s becoming scarce but reasonably priced alternatives are out there including intresting addons for the Rasperry Pi.

Suspension. The short pendulum used for testing is extended to 55cm giving slower cycles (but for slightly longer duration). The length is arbitray based on what will fit in a convienient case.




About billboyheritagesurvey

Heritage worker
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