Monday, January 07, 2008

If I stay with the four engine system the Swagelock plug valves are almost perfect for pressurization and fuel. (Won't work for Lox they freeze) One is shown below next to two dynamixel actuators.

These dynamixel actuators are robot servos with a an RS-485 interface will full feedback. The normal model servo interface does not offer any feedback about the torque, actual position temperature etc... One of the test stand reliability issues was that we were never 100% sure that the valve went where commanded, these actuators fix that problem. I've ordered some zero backlash couplers and when they come in I will mount the dynamixel opposite the plug valve and test the combination.

When we get to the point of flying to significant altitudes the brushed motors in these actuators and all standard model/robot servos are not acceptable. brushed motors have arcing problems at high altitude. So we have been testing a sensored brush less motor with a planetary gearbox and a ball valve. This may become the Lox valve for our system. I'm also looking at a small brush less motor as a replacement for the Tonegowa main motors. Below is a "teamnovak" brush less crawler motor attached to a bane bot gearbox and a 3 pc ball valve. This combination has an insane amount of torque and very precise control.

I finally have the necessary sensors on the helicopter working 100% reliably in flight. The last item was to do a 3D hard iron calibration of the microstrain IMU so it would sense magnetic heading correctly. I've evaluate three sources of magnetic heading on the helicopter: (All three are still mounted)

1)The Crescent Vector two antenna GPS. With the 2nd antenna near the helicopter blades its very unreliable with the blades turning. The primary GPS info is fine, the 2nd RTK solution for heading is not very reliable, its almost useless.

2)The Ocean systems magnetic compass module out on the tail boom. Works fine when things are static, but seems very sensitive to vibration and acceleration.

3)The Microstrain IMU. After recalibrating the unit with all the electronics turned on and the blades spinning it works great. I just need to recalibrate after any configuration changes.
This will be the heading solution long term, its the best and seems to be the most reliable.

After getting the sensors working we started working on controlling things automatically. The first control to go under automatic control is heading. To test just heading I built rotary table I can run the helicopter on. Its is a lazy susan bearing between two pieces of plywood with the helicopter tie wrapped to some screwed down brackets (shown bellow)

This allowed me to quickly test my heading control code and get the system gains right in less than an hour. The weather in San Diego was such that I could not really fly this weekend, so I have yet to try the heading code in flight, but it works really well on the lazy susan. It yaws to true north when commanded at a fast but steady rate and holds that position when perturbed.
The data collection shows a tiny bit of overshoot when comming from more than 45 degrees away, but all in all I'm happy with it. the next control step will be to close the loop around commanded attitude using the RC system to command attitude and the IMU to measure attitude. That is my goal for this coming weekend.


Anonymous said...

The standard Dynamixel isnt really RS485, is it ? Its sold with Bioloid humanoid kit, and theres lots of info on hacking and controlling them, plus some good open source software on forums.

Oh, i checked Robotis site, they actually have rs485 versions as well.

Paul Breed said...

They have a number of actuators
I'm evaluating the
RX-28 and RX-64