Getting Telemetry Working, A Short Review of Excelvan Radio Telemetry Kit 915Mhz

I recently purchased a small Telemetry module from Newegg.com for about $25, it runs on the unlicensed 900 MHz band.  It was quite easy to hook it up to my Hobbyking Micro APM and now I can monitor the Quadcopter while in flight, just as if it was connected to my PC via USB.  No need to connect USB on the ground, as this telemetry connection handles all communications.

A new cable must be made using the supplied telemetry cable from the telemetry module and the one supplied with the APM.  Also remember to connect as follows:

Telemetry Module with black antenna

Telemetry Module	APM
5V	5V
Gnd	Gnd
TX	RX
RX	TX

The PC connection was also painless, stuck the module into a USB port and the driver loaded automatically onto my Windows 7 laptop.

The only other trick is to select 57.6Kbps in Mission Planner to get the link to work.

Note: Telemetry module is from Excelvan and is called "Radio Telemetry Kit 915Mhz Module for APM APM2.5 2.6 Pixhawk PX4 RC Multicopter Quadcopter" and is available on Newegg.com

Update-Pixhawk Test

I also purchased a unit for use with my Pixhawk-S500 quadcopter covered elsewhere on this blog.  This time, I could use the included cable to connect the telemetry unit to my Pixhawk, although I had to shave a little plastic on the Pixhawk end, as the Pixhawk uses unusual connectors.  As with the APM, I set the rate to 57600 and was connected to Mavlink on the Pixhawk.  Works like a charm.

More Fixes to the SK450

I am plagued by poor loiter on the SK450 and have made further changes to improve things:

1. Fixed the slop or wobble in the GPS mast.  It turns out that the cheap mounts rely on being able to screw down the knurled flange until the mast mount inside is snug.  However, a lot of them cannot be tightened.  I added a plastic washer from an old prop to the mast as shown and the wobble stopped.
2. The APM mount was maybe moving in flight, so I remounted it with the special vibration reducing foam rubber and I secured the USB cable better so it could not move the APM.
3. Added a little foam rubber inside the micro APM case to ensure the barometers were not getting fooled by the prop wash.
4. Wait 5 minutes after power on to get the GPS and all sensors stable before trying a flight.
5. Added some practice golf balls to the landing gear so they do not catch in the grass when taking off or landing.
   
6. I very carefully recalibrated the accelerometers, making sure that the quad was absolutely level during the first stage of calibration.  This made a HUGE difference versus my previous "it looks level" calibrations - the test flight was much beter with much less yaw and pitch on takeoff and during flight.
7. Made a stand out of a medium size plastic storage container as shown:

Pixhawk Versus APM Comparison Review

This blog entry compares two flight controllers from the Ardupilot 3DR-Open-Source stream: the generic APM 2.X controller and the Pixhawk.  I am an intermediate multirotor builder and flyer, so this comparison will focus on functionality and useability, not on advanced features.  Also, this review is based on my experience with 5 different APM modules (none sourced from 3DR) in three different physical forms: standard, mini, and micro (also called mini by some) sourced from Hobbyking and generic Chinese manufacturers.  The Pixhawk is available from multiple domestic and Chinese sources and you should check the reviews to make sure you are getting one with decent build quality.  Each was used with corresponding GPS/Compass units from the same manufacturers.  I built three different quadcopters and used them with different controllers.

Feature/Characteristic	Generic APM 2.X	3DR Pixhawk	Comment
Size/Weight	Small	Larger	APM is available in 35x35x5 mm board.  Pixhawk is 81x50x16 mm
Cost	Low	Higher, about 4X	APM with GPS ~$65
Quality	Variable	Better	Inspect a generic APM after you get it for poor soldering, loose USB..
Support	Good	Good	The best support is from peers on DIYdrones and APM forums
Flight Stability	Good	Good	This is my experience with a well setup APM and Pixhawk, your experience may vary
Accuracy	Good	Better	It does depend on your GPS and its accuracy.  Pixhawk has a more powerful processor and more memory
Onboard indicators	Poor	Good	Pixhawk has multiple LEDs and tones to tell you status, APM has a few LEDs
Ease of Setup	Moderate	Moderate	More components to interconnect on Pixhawk, but well documented. Cables and connectors often an issue with APM and documentation must be found on the Internet.
Ability to fly autonomous missions	Yes	Yes
Mission Planner Ground Station Compatibility	Yes	Yes
Flight logging Capability	Good	Better	Pixhawk logs more information and has a microSD for storage. APM has limited storage of most important variables.
Upgradeability	None	Yes	APM code is now frozen to my knowledge, but Pixhawk software with improvements still being released
Debugging difficulty	Good	More difficult	APMs seem to always work, Pixhawk is more finicky about ESCs, setup, etc.

Overall, both controllers are excellent and have worked well for me.  The APM is a better fit for small quadcopters and is probably better for the beginner builder due to its simpler setup and low cost.  The Pixhawk is very good but it takes more time to set up and it is likely higher quality and performance, so it works well in larger camera quadcopters and similar expensive vehicles.

APM equipped SK450 "Dead Cat" Quadcopter

S500 Quadcopter with Pixhawk

Fixed the Loiter

Well I fixed the Loiter problem, the quadcopter will now hold position and altitude on command.  The changes were detailed in the previous blog post

1. Put in a new GPS.  
2. Improve power feed to the controller and receiver by using a UBEC (battery eliminator) instead of the power feed from the ESCs.  
3. Get rid of extra wires in the controller wiring harness.
4. Performed an indoor test flight and analyzed logs.  
5. Performed an outdoor test flight which was successful.

I also included a parameter change:

- changed INS_MPU6K_FILTER from 0 to 20 to filter out barometer fluctuations

Further Enhancements

After the exciting flight last week, I had to repair the Quadcopter as one of the arms broke in a landing.  So I decide to make a few enhancements:

1. Put in a new APM controller and GPS to fix the Loiter issue.  This did not turn out well as the new controller was defective but the GPS seemed to improve things.  The new GPS is Hobbyking brand, old one was a "no-name" with some mods to make the cable work.
2. Improve power feed to the controller and receiver by using a UBEC (battery eliminator) instead of the power feed from the ESCs.  This is recommended due to the interference from the ESC and I do think  it improved things.
3. Get rid of extra wires in the controller wiring harness.  Done.
4. Install new arm.  Did this, I learned from the dearly departed Flying Flowerpot quadcopter that it is almost impossible to properly fix a quad's arm and they are reasonably cheap.
5. Performed a test flight and analyzed logs.  Seemed to improve things with few glitches on barometer and GPS and more stable flight.  Testing will continue.

Don't Try Auto if You Cannot Loiter

Learned a new lesson yesterday, should not have needed to learn it, because it is obvious.

If your quadcopter cannot LOITER successfully, don't try an AUTO mission.

I headed over to the local "park" - more like a remote muddy field that the county calls a park - to fly my quad.  The plan was to check out AUTO mode with the program shown in the previous blog entry.  I forgot, or ignored the previous tests that showed that LOITER mode did not work well, the copter bobbed up and down and did not hold position.

So I try taking off in AUTO mode with no luck, then do a manual takeoff and flip the switch for Auto mode.  At first, things seemed OK, then the quad headed for the river, gaining altitude.  Before I could react, it was over the Potomac at about 75 feet, then it dipped below the tree line and I could not see it.  I flipped back to manual (Stabilize) control and thought "that quad is gone".  I hit the throttle to try and bring it back up above the trees and it appeared as a little speck in the distance.  I then was able to bring it back with some skillful (lucky) stick work and land it.  Was I ever lucky!

The saving grace is that it took some great video footage while on its semi-controlled mission.  Also, this is why you should not fly near people, as this could happen to you and someone could get hurt.

Here is the video:

Status of the DeadCat SK450

The Deadcat SK450 is working fine.  A few minor modifications:

• packaged up the APM controller and mounted it on anti-vibration foam
• added a LiPo batter monitor
• tidied up some cables

A photo with some details is shown below.

I am also working on a new mission which includes takeoff and landing in the Mission Planner software, see below.