Author Topic: Scratch built giant rc car  (Read 14572 times)

Offline KB

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Re: Scratch built giant rc car
« Reply #30 on: November 23, 2018, 07:09:12 PM »

Got a little more time in...





Offline KB

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Re: Scratch built giant rc car
« Reply #31 on: November 23, 2018, 07:10:14 PM »

I had a little time to play with the steering. Not sure what is going on here. The servo, by itself, is smooth acting but when it is connected to the steering linkage, it gets super glitchy, some kind of resonance. There is very little play. It quiets down a bit under load.

 I'll try a different motor....


 


Online Vixen

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Re: Scratch built giant rc car
« Reply #32 on: November 23, 2018, 08:46:13 PM »
Looked smoother and less twitchy in your earlier 05 May posting, so it can be done.

Mike.
It is the journey that matters, not the destination

Offline KB

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Re: Scratch built giant rc car
« Reply #33 on: November 23, 2018, 09:19:17 PM »
Thanks for the encouragement.

By itself it is smooth even with a load applied and yanking on it. Only when it is connected to the rest of the linkage, it is twitchy?
Im going to try a longer servo horn on the feedback servo. Also have a wiper motor to try.

Online Vixen

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Re: Scratch built giant rc car
« Reply #34 on: November 23, 2018, 09:25:56 PM »
If it is smooth by itself and only becomes twitchy when you add the weight and inertia of the road wheels, then it sounds like a steering damper cylinder may help.

As you say, changing the servo horn length/ratio may also help.

Mike
It is the journey that matters, not the destination

Offline crueby

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Re: Scratch built giant rc car
« Reply #35 on: November 23, 2018, 09:44:07 PM »
Do you have capacitors on the motor, like you would on a drive motor?

Offline Noitoen

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Re: Scratch built giant rc car
« Reply #36 on: November 23, 2018, 10:05:12 PM »
You could move the feedback potenciometer to the stearing arm itself. This could minimise the "hunting " effect.

Offline KB

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Re: Scratch built giant rc car
« Reply #37 on: November 24, 2018, 12:55:44 AM »
Mike, here is the damper/servo saver set-up. There is no resistance and little weight so I don't know, weird. I'll see if the ratio smooths it out but by itself I can wiggle the servo horn with no effect.





Crueby, this is the h-bridge; I read somewhere that the capacitors were not the best quality. I no nothing of electronics.



Noitoen, good call, I'll try.

Cheers,

KB

Offline Jasonb

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Re: Scratch built giant rc car
« Reply #38 on: November 24, 2018, 08:44:20 AM »
Can you bypass the radio and try moving the servo directly just in case all those metal ball joints are interfering. Also try it with just the link to the servo saver, then add one steering rod and then the other to trace down where it starts to happen.

Also what is the red lead clipped to the toolmakers clamp doing?

Offline MJM460

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Re: Scratch built giant rc car
« Reply #39 on: November 24, 2018, 10:30:35 AM »
Hi KB, you mentioned earlier that you thought the issue might be a resonance issue.  Hard to tell from the video, but if so, it is is about the natural frequency of the steering motion.  That is determined by the moment of inertia of the wheels (about the vertical steering axis) and the springs in that servo saver.  Applying the load by hand does not replicate the natural frequency, so tends to support your view that it is a resonance.

It is easily checked by making a temporary solid replacement for the servo saver unit which will drastically change the natural frequency, and probably eliminate the problem, so confirming whether a resonance is the issue.  Alternatively it will make no difference in which case you need to look elsewhere, possibly in the servo characteristic.  But I imagine you need the servo saver on a real world track.

In that case, to change the natural frequency, you need to change either that moment of inertia of the wheels, which is not very practical, or to experiment with the stiffness of those springs.  You may need to test the system with each of the tyres you intend to use if they are significantly different in weight.

MJM460
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Offline steamer

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Re: Scratch built giant rc car
« Reply #40 on: November 24, 2018, 12:59:55 PM »
Is there any control over the control loop?    Is it a PID control?     Perhaps backing off on the Proportional gain will help?

AS the Servo is a wiper motor, and pretty cheap, I might just take the servo saver out all together for the moment and only put it in if needed.

Most servo savers are of this design by the way.  The design is very damped, and quite stiff, and is a very common design amoungst most manufacturers.

Dave
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Offline steamer

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Re: Scratch built giant rc car
« Reply #41 on: November 24, 2018, 01:18:06 PM »
A_dk, indeed!

Thanks Dave,

The way I made the top caps of the shocks didn't allow me to have room for additional mounting points.
I should have used rod-ends on both ends
The front lowers have a few options.

I just have high pressure grease in the diff for now.

The shocks are 11/2" bore by 3" stroke, what do you think I should try as a starting point for piston holes? Do you have any recommendations for cheap shock oil?

I don't have a lot of time these days but I am starting to look at the drive train;






You should check out my friend Matt Olson's web page   MOD speed shop
 
https://modspeedshop.com/

His 1/8 and 1/5 scale bypass shocks are the technology you should be looking at.

His 1/5 scale shocks are 32mm bore


Dave




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Damned ijjit!

Offline steamer

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Re: Scratch built giant rc car
« Reply #42 on: November 24, 2018, 01:22:40 PM »

William and Matt throwing down at the King of the Dirt after race grudge match....  1/10 vs 1/5 scale, both with bypass pistons


"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

Offline steamer

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Re: Scratch built giant rc car
« Reply #43 on: November 24, 2018, 01:27:47 PM »
As far as cheap shock oil, I would look at most hobby shops or online shops.    TLR and Team Associated will have the least expensive oils.

You really want a silicone based oil, and I would shoot for piston port sizes that get you in the TLR  "35 weight" range, as you will then be in the middle of their product range for oils

That will give you the most adjustability for a wide range of surface conditions.

I would start at 10 hole x 1.0 mm    You can then go up from there easily.

Dave
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

Offline KB

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Re: Scratch built giant rc car
« Reply #44 on: November 24, 2018, 06:14:07 PM »
Thanks for the advice.

Jason, everything is quite free to move, but I'll try to power the motor directly, good idea.
The red lead was an attempt at grounding everything.

MJM460, thanks, the springs are quite stiff and shouldn't be a factor in a free state but it is certainly worth a try. I'll just swap out the spring cylinder for a threaded rod.

Dave, lots of good info there, thanks. The gutted servo provides the feedback. I got this radio a while ago, so that I would be able to set the endpoints. I thought that the proportional gain might make a difference, but I only just started playing with it and don't really know what I am doing.. It also has adjustable frame rate speed. The old servo is analog, so I think I am supposed to set it to the slowest?

https://www.rccaraction.com/radiopost-ts401-2-4ghz-color-lcd-radio/

I have that cam style servo saver on my T-Maxx, I considered it but got lazy.
I'll start with 10x1mm thanks, looking at those pistons the is a lot less through surface are than I would have thought.

Cheers!