Hello.

I was reading through the group and I was trying to figure out the best chassis for my application.

I was choosing between Wild Thumper 6WD Chassis and one of Traxxas RC cars (Slash or Stampede). After valuable feedback from TCIII (Thanks!) I figured out I should go for Traxxas cars. And here I have a few questions for you.

I want to be able to mount something on top of my car (e.g. average size WiFi access point or a camera). I think of going for 1/10 scale for that. Would that be enough? And what is the practical difference between 2WD and 4WD models in terms of how they drive? Is setting up 2WD with Pixhawk is as easy as 4WD one?

2WD: https://traxxas.com/products/models/electric/58024slash
4WD: https://traxxas.com/products/models/electric/6804Rslash4x4platinum
4WD: https://traxxas.com/products/models/electric/67054-1stampede4x4

Do you have any experiences with those?


My second question is about all the other parts I need (t will be my first rover). As far as I understand, if I buy one of those RC cars, I have ESC there already, don't need to buy separate one. Therefore, I would only need:
- Pixhawk (with regular setup, so 3DR telemetry radio, 3DR Power module, GPS module, and maybe a second one for redundancy, buzzer and safety switch)
- battery
- a bunch of female to female servo cables to connect to the rover

Is that correct or am I missing some crucial elements?

What is the advantage of second GPS module? How much does it improve performance of a car? (I want to use GPS way-point mode a lot, and then test Follow Me mode).

Thanks in advance for your feedback,
Michal

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Replies to This Discussion

@Michal G,

Here is a Pixhawk mounted on a Traxxas 4WD Slash Link

Regards,

TCIII AVD

At the moment there isn't any real advantage of a second GPS on a Rover.  Its primary use is if your doing long distance remote auto piloting and have a problem with the first GPS you can disable it and use the second.  Its effectively a backup at the moment.

2WD and 4WD makes no difference to the pixhawk as it is still only connected to one ESC.  As you can imagine a 4WD version corners a bit better then 2WD version of the same car.  If your comparing a racing 4WD to a RC off road 4WD then they are different vehicles and will perform very differently.

You are correct if you buy an RC car it will come with an ESC.

The 3DR Pixhawk will come with the 3DR power module.

You will also need a battery charger and Radio Transmitter and receiver.

Make sure you post pictures/videos of your Rover :-)

Thanks, Grant.

Hi Michal,

I will be setting up the wild thumper with pixhawk near the end of this calendar year as part of a high school program I run. Its called applied tech high school and we will be using the 6WD thumper for the entire year. First setting it up as a straight rc vehicle, and then adapting to pixhawk.

Since you've opted for the traxxas vehicle you needn't be concerned about escs or skid steering setup.

I would suggest the 4wd traxxas stampede as it has the largest wheelbase - we used the 1/16 scale last summer for a robotics camp and even that was plenty big for the APM - we hadn't purchased the pixhawk yet. 4WD will handle most terrain and still only uses 1 esc and a differential to drive all 4 wheels.

The one criticism I have of the stampede was that I was constantly repairing it due to students operating it aggressively. Replaced many a servo gear, control arm, and differential over the course of two weeks that I suggested that we keep replacement parts in stock.

TCIII is clearly more knowledgable here - he's been advising me in the forums on skid steer setup with the pixhawk and helping immensely.

It sounds like you have all you need aside from batteries - use 3S 3000-4000mah and convert your connectors to XT60's. I prefer their tighter fit.

Not sure you need a 2nd Gps nor how you would connect it - honestly that has never been an issue with the rovers. 

I found the biggest issue was how to get the traxxas esc calibrated. Once that was accomplished, the rest was settings in mission planner pertaining to cruising speed, throttle min and so forth.

Post your progress here and good luck!

Steve

@Steven,

I have reviewed my dual GPS equipped Rover datatflash logs and you would be surprised at the number of dropouts that are experienced by the GPSs during a competition run.

The Pixhawk will use which ever GPS has the most sats at any given moment.

Regards,

TCIII AVD

Thanks for the clarification. Please refer to my statement in my prior reply: "TCIII is clearly more knowledgable here"!

Looking up - 2 gps setup with pixhawk. i2c connection, no problem, what about bringing in other connection from 2nd GPS?

Much appreciated - again.

@Steven,

I only use one compass in the primary GPS/Compass module.

The Secondary GPS is connected to the Pixhawk through Serial Port 4 on the Serial 4/5 connector.

Regards,

TCIII AVD

Hi Thomas,
I'm having issues with the compass variance error. I've done several live calibrations with tremendous offsets (900 or more) and have tried entering the declination info manually to no avail.
Next step would be to elevate the compass/gps unit away from dual esc and flight controller. Any thing else I may be over looking?
Behavior is rover seems to circle slowly and heading drifts in Mission Planner.
Tried to connect via CLI but I get garbage on screen. I think I need to remove the wireless telemetry for it to communicate properly. 
Thanks!

@Steve,

I mounted the Thumper GPS/Compass module at the front of the chassis between the two front motors on a four inch high wooden pedestal to get it away from the front motors' magnetic fields.

Doing so produced reasonable compass offsets and reduced the compass deviation while underway from around 30 deg to around 3 deg which is acceptable.

Regards,

TCIII AVD

Cool. I deduced (albeit late) that when it was powered via 2S LiPo, it drifted. Connected to usb directly and no issue with compass. 

Unplugged the wireless telemetry to see if that made a difference too. 

I may have time to get to it tomorrow afternoon.

I've got extra 3DR GPS units. I will try a 2nd.

Steve

Hi Tom.  Can you send me that log with the dropouts please?  I would find that very interesting.

Thanks, Grant.

@Grant,

I have over 50 bin logs from the pre Sparkfun AVC testing that I performed in 2014.

I suspect that all 50 bin logs will show GPS dropouts which is to be expected when the rover makes quick 90 deg turns and the orientation of the GPS antenna is changed by 90 deg to its original view of the sky.

Usually only one GPS would lose lock for a brief instant, while the other GPS would maintain lock. That is why I subscribe to the use of dual GPSs. 

All my testing bin logs are on my home server and I am on travel at the moment so I will identify and send you the appropriate logs when I get back home. 

Regards,

Tom C AVD

Hey Michal,

I've used the 1/10 2WD Brushless Stampede and the 1/10 4WS Summit with the APM 2.5.  

The Stampede was really fast but could be geared down.  I wouldn't weigh it down with much, although you can get stiffer springs.

The Summit is my test vehicle of choice.  It will carry a pretty good load (upgradeable with springs too), is wide enough for a large platform, lots of ground clearance, huge tires, and a two speed gearbox.  Of course the APM won't control the gearbox.  It's been very reliable as well.

Falcon

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