I work out of the Loughborough University Centre for Autonomous Systems (LUCAS), and have recently discovered the Ardupilot Mega. We are now constructing a number of vehicles with Ardupilot Mega, using it as a low level control system. Our fleet will include Flying Wings, Wot 4's, and Trex 450's. We will use these to conduct research, which will range from, UAS forced landing, UAS collaboration, autonomous soaring, to collision avoidance.

This post is the first in a line of posts detailing our ArduCopters setup, testing, and eventually non-destructive (i hope) flight tests on a TRex 450. We have had out TRex 450 helicopter for quite some time, and have some rather talented pilots in out lab, but getting it flying outdoor autonomously has been an aim of ours for quite some time now. We have had TRex 250 helicopters flying autonomously using Simulink, and a Vicon camera system for some time now, but the next logical step to to go outdoors.

We have used a TRex 450 Pro so it is a bit nicer, and more powerful than the stock one. we have attached a small training undercarriage on it,in an attempt to protect Ardupilot as much as possible (mostly from my questionable flying skills).

Reading about, many people have made there own undercarriage for their Heli some of them a very nice indeed, but for the prototype it was decided that using foam and lots of zip ties it was attached to the underside of the heli. This is a really quick solution and the foam had the effect of isolating the IMU from many of the multitude of vibrations off the heli. It also protects the pressure sensor from the rotorwash. the DIP switch, USB port CLI switch, and all other outputs are still assessable. Rotary wing craft are extremely sensitive to the mounting of this autopilot system, it needs to be rigidly attached, and perfectly orthogonal to the airframe. This is because Adrupilot Mega for traditional helis does not seem to save the biases on the accelerometers on startup, which is useful but means that if it is mounted at just a few degrees off then in stabilise mode the helicopter will tend to drift off.

The magnetometer was mounted as far from the electronics as possible, so to not be affected by any errant magnetic fields from the high current wires or motor. the mag was placed on the tail boom, and a ribbon cable was ran down the struts to Ardupilot. We found a really nice way of mounting the GPS, using a small bit of aluminum bending it and drilling 2 holes, it can be screwed directly on to the airframe. Remove the screws that fix the tail boom mount to the airframe and put the screws though the aluminum and re screw on to the airframe. This way the GPS mounted on it aiming vertically up. As the GPS will be directly under the rotor disk in flight it might cause signal problems, but i doubt it. Will have to get back to you guys on that one.

 

Found out that there is no manual mode on Arducopter, as you have to send non ECCPM mixed signals from the transmitter. Arducopter does all the mixing for you, this makes me slightly concerned, as if all hell breaks lose I can not simply hit manual, and recover it myself. I guess this is just a matter of building confidence in the system. It did surprise me as my first experience with Ardupilot was with a flying wing which had elevon mixing, but you could send it mixed signals from the transmitter, Ardupilot would demix them to give pitch and roll commands to the fly-by-wire system, while still meaning you had the option to go to manual with the transmitters mixed signal.

Arducopter performs hight hold using the blade pitch only and does not control throttle at all. This is a problem as the pilot will have to control throttle to maintain blade speed, and I don't think anyone would be fast enough to do that especially as we plan to do some aggressive flight profiles in the future. Of course one option is to install a governor, but this is another added expense, and as we plan to get a lot more of these in the future it could soon add up. a solution could be to add the same throttle curve that you use on you transmitter to the code, this will control throttle based on a given pitch angle. This is a form of open loop control and it does work well, but to get it to maintain the optimum blade speed no matter the flight conditions, closed loop throttle control will need to coded. Using a blade speed sensor attached to one of Ardupilots ADCs would do it. In the simple hover condition this should not be a problem and as we don't plan to do anything that crazy for a while the current system will be more than addiquite.

Plan to do some simple familiarisation flights in stablise mode next, to gain some confidence in the system and try to get some good roll, and pitch gains. although the stock gains are for the TRex 450, we have to pro so they might be different, we will see.

I must stress that this is a prototype setup, as i am sure you can see our wiring "solution" is not ideal, this is to familiarise ourselves with ArduCopter. Our intial aim is to try and get as much out of Arducopter as we can with APM mission planner, and without having to change any code.

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Developer
Comment by Randy on November 16, 2011 at 5:46pm

It's fantastic to see another trex450!  Very nice looking set-up.

 

You know, Mounting plates are now available in the DIYDrones store for only $15 as of last week.  11 sets have been sold already which surprises me..didn't know we had that many closet heli people out there!

 

I've put my compass and gps in the opposite positions in my set-up (i.e. compass near the body, gps out on the tail boom) after find that the gps signal wasn't good under the main blades..but who knows...maybe your experiences will be different.  By the way, there are 3 printed parts for the gps and sonar at shapeways.

 

The ESC that comes with the trex450 pro should have a throttle govenor built in.  My super sport did at least.

 

Re roll and pitch PIDs, I wouldn't be too surprised if you could come up with better PID values..I strongly suspect that at least the I values are too low.  You can see a this if you leave the heli on the ground a long time before take off that it sometimes leans over for a couple of seconds before sorting itself out...this indicates to me that the I values are too low (or IMAX is too high).

 

I'd recommend going flybarless!  It's like trading in your 80s pontiac for a porche.

 

My guess is that you'll want to upgrade your stock BEC once you add a sonar.  I think the stock bec is good enough to handle the servos but with a lot of additional electronics you can start getting brown-outs.  I had more than my share of those until I upgraded to an over-the-top castle creations Bec PRO and all my problems went away.

 

Great post!

Comment by Matthew Coombes on November 16, 2011 at 6:31pm

Cheers for your comment very helpful

 

Did not consider brownout, will have to monitor that. Although if that is a problem i will just stick another power source on it, still got plenty of payload.

have read about and it look like other have had GPS signal probs under the disk, will give it a go but looks like i will have to move that back to the tail boom as well.

ahh those plates are finally out, sweet. but the problem is, that i am in england and postage is rather expensive for such a small item, also had trouble ordering of this site before, think there was some customs thing. hope it comes soon to the UK sister site of DIY Drones Build Your Own Drone.

Have been looking about and have been looking at flybarless, and it looks like the way forwards. extra maneuverability is always good. but got about a million other things to do before doing that.

What kind of stuff have you done with your TRex 450 setup?

Comment by Mike on November 16, 2011 at 6:44pm

You really need to use the governor. Having the throttle through the rx and by-passing the autopilot is a good safety feature because if the autopilot loses the plot you can dump the heli fast with the normal RC control by cutting the throttle. Handing everything over to the autopilot, especially one that is still far from mature, might be rather dangerous!

Comment by Matthew Coombes on November 16, 2011 at 7:21pm

Worry not i still have direct control over the throttle. i was talking in the far future. Can anyone recommend a good governor for a 450, will have a look keep all my options open.


Developer
Comment by Randy on November 16, 2011 at 7:39pm

I've had no issues with the govenor that came with the trex450 super sport...but i've heard the castle creations ice is a really good one.  I don't know but their BEC is great!


Developer
Comment by Jason Short on November 16, 2011 at 8:11pm

I'm not a Heli guy so I don't know the answer, but why not move your control switch to Channel 8 to leverage the hardware Mux that the plane uses for manual pass through? If your radio channels are in the right spot it should just work. 

Quads on the other hand would crash in a heartbeat if the control switch was place on CH8.

Jason


Developer
Comment by Randy on November 16, 2011 at 11:12pm

It's because of the mixing required for the swash plate servos.  raw Roll, pitch and collective commands fromthe receiver need to be translated to the appropriate movements for each of the three servos supporting the swash plate.

 

The next question asked is, "why not have the radio pass in already mixed signals and decode them on the APM side.  then in case of failure the mux could pass them directly to the servos"..the answer is that we could (in fact we did in the older ArduCopterNG code) but there's not much point because once you go flybarless, a human has no chance of keeping it in the air anyway soit won't increase your heli's survival rates.


Distributor
Comment by UnmannedTechShop.co.uk on November 17, 2011 at 1:32am

I was going to say but Randy mentioned already about the Trex450 mounting plates.  But we sell them here in UK from our store so shipping is much faster and cheaper.

 

Otherwise great work look forward to seeing the updates!

unmannedtechshop.co.uk

Comment by Matthew Coombes on November 17, 2011 at 3:00am

So the governor is a part of the ESC, cool. have no idea if mine does that don't think it does when i flew it a few weeks ago the throttle curve was all wrong and so some serious blade stall. How does the governor control work? does it try to maintain a commanded blade speed, or does it keep your blade speed in an optimal range? also how does it now blade speed, does it measure back EMF over the 3 phase becuase if so that is really cool.


Developer
Comment by Randy on November 17, 2011 at 4:29am

I'm pretty sure your esc has a govenor built in which, as you say, knows the blade speed and corrects it.  You can set-up your esc by attaching a radio channel directly to your esc..then hold the channel high as you power on your esc..probably best to look in the  manual..you want to set the 4th option, "aircraft type", to helicopter 1 (soft start).

 

By the way, i connect my ESC directly to my radio channel 7 which is attached to a 3-position switch on my transmitter.  I don't use a throttle curve at all and in fact, i nearly always just use the middle position of the switch.  This is not ideal for all types of flight so it may become an issue in the future but at the moment, my heli spends 90% of it's time hovering or doing slow maneuvers.

 

by the way, the stock esc/bec outputs 6V i think.  Good for fast servos but you're not really suppose to power the apm at that voltage.  As mentioned, I use the castle creations bec pro and I've cut the red line which connects the radio to the esc (black/gnd and white/signal lines remain).

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