[HELP] Servo Alternative - Return-to-Center Steering with no motor stalling

[Mpafr]

Hello all, 

I am building a medium car model using an infrared remote control and a buggy motor, all generic. To give a bit more juice I am using 3 x 3.7V, 3400 mAH, 18650 li ion, rechargeable batteries.

The two servo mottors I have are giving me all sorts of trouble. From my experience and from the topics I have seen here, it seems they are a big source of headache. 

It seems to me that an alternative for the servo motor is to use a return to center mechanism with a normal motor and shock absorbers or rubber-bands, like to ones seen here: https://brickshelf.com/cgi-bin/gallery.cgi?f=468843

My understanding is that these designs will stall the motor, which might result on high currents and a potential motor failure - I am considering that my generic PF M motor does not have a reliable protection system that will prevent it from frying, especially as my power pack is already over tha standard specs. I found more info on motors stalling here: https://philohome.com/motors/motorcomp.htm.

Therefore I am looking for ideas on how to have a good steering system for a medium sized car without a servo, and without stalling the motor.

Thanks!

I have found this similar question from some 12 years ago, but many of its references are dead, and hopefully some new ideas came up since then.

 

 

[2GodBDGlory]

That's a tricky one.
Do I understand that your parameters are:

1. No PF-style Servo motors

2. No continuous stalling of motors

and

3. Return-to-center steering?

If so, there's not much I can suggest...

One option (that requires spending money!) would be to buy a GeekServo, which is pretty cheap and offers nice, more reliable control, but requires a hobby RC-style controller. Here's a link to a place to buy one:

Spoiler

https://www.aliexpress.us/item/2255801055693447.html?spm=a2g0o.productlist.main.5.682f741bk1TjMi&algo_pvid=7c755618-b73b-46a0-9bf9-0ed6ace00b1a&algo_exp_id=7c755618-b73b-46a0-9bf9-0ed6ace00b1a-2&pdp_ext_f={"sku_id"%3A"10000015429872429"}&pdp_npi=2%40dis!USD!8.89!8.89!!!!!%402122457116701179600072050d0732!10000015429872429!sea&curPageLogUid=OGTS72VxzwW2

If my inferred parameter #3 isn't actually crucial, which seems doubtful given how fast your car sounds, you could always try just putting a good 1:8 or 1:9 ratio between the motor and the steering rack and just forget the return-to-center steering, but I doubt that'll work well.

[msk6003]

Use subtractor to other way, one of builder in korea made this.

CASTER - Brickinside

Very hard to doing backward steering but looks good for steering without servo.

[Mpafr]

On 12/3/2022 at 10:41 PM, 2GodBDGlory said:

One option (that requires spending money!) would be to buy a GeekServo, which is pretty cheap and offers nice, more reliable control, but requires a hobby RC-style controller. Here's a link to a place to buy one:

Very nice, it is the first time I hear about GeekServo. Do you have any suggestions on how to control it? I assume I need a receiver board + controller.

[2GodBDGlory]

Just now, Mpafr said:

Very nice, it is the first time I hear about GeekServo. Do you have any suggestions on how to control it? I assume I need a receiver board + controller.

It does seem like a very cool motor! I don't have any, so I'm just going off what I've seen other people do. Yeah, you will need those things, but I don't think I can give you any more specifics personally!

[Mpafr]

On 12/4/2022 at 3:54 AM, msk6003 said:

Use subtractor to other way, one of builder in korea made this.

CASTER - Brickinside

Very hard to doing backward steering but looks good for steering without servo.

Impressive! It took me a while to digest it all, but I really need backward steering and I am not sure how the system would behave at high speeds. 

[brunojj1]

I remember having run into the same kind of problems when PF servos even didn´t exist some 10y ago. But the solution isn´t that complicated: prevent the M motor from stalling by using 1 white clutch gear between the motor and streering rack. If 1 is too weak, use 2 or 3 of them on the same axle. Try soft or hard springs to make the steering rack return to center, 1 per each side.

[Mpafr]

10 minutes ago, 2GodBDGlory said:

It does seem like a very cool motor! I don't have any, so I'm just going off what I've seen other people do. Yeah, you will need those things, but I don't think I can give you any more specifics personally!

No worries. Thanks for all the help you have given me here and in the other topic. Really appreciated.

I have an arduino with bluetooth controler and a raspberry pi somewhere, I plan to do some lego experimenting with them and lego at some point in the future. The servo will come handy then.

For now I will give it a go with the a return to center system using shock absorbers and a PF L Motor (I have only 1 L and 1 XL).

https://rebrickable.com/mocs/MOC-0514/Driva00sCreations/lego-return-to-center-steering/#details

Hopefully the RC receiver + the L Motor protection systems will be able to protect the motor while still letting things work properly. Otherwise I will try feeding the motor with 6.8V with some relays.

Cheers!  

6 minutes ago, brunojj1 said:

I remember having run into the same kind of problems when PF servos even didn´t exist some 10y ago. But the solution isn´t that complicated: prevent the M motor from stalling by using 1 white clutch gear between the motor and streering rack. If 1 is too weak, use 2 or 3 of them on the same axle. Try soft or hard springs to make the steering rack return to center, 1 per each side.

Thats a good idea. I didn´t think of adding more of them together. I actually might need to buy some. I will give it a go, thanks!

[Zerobricks]

There's another way. You can use the 1x4 rack bricks and set the steering in such a way that the gear just about slips at the end of the steering rack when the steering reaches the maximum angle. This way when the end is reached, the gear will simply start clicking. I used this technique in this old model:

 

[Alex Ilea]

I also found that the lego servo motors are unreliable, so I bought some copies from aliexpress and they seem to be rock solid. And for the price you don't really care that much if they break.

[brunojj1]

50 minutes ago, Alex Ilea said:

I also found that the lego servo motors are unreliable, so I bought some copies from aliexpress and they seem to be rock solid. And for the price you don't really care that much if they break.

The question goes to @Zerobricks as well. Did you try them on BuWizz as well as being reliable and most important – responding gradually? E.g. I´m having many CaDA servos of different types and generations, some not yet released and none of them do respond proportionally on BuWizz 2.0 and 3.0. The funny thing (not really) I experienced the original Lego servos are doing that, but cost a fortune on bricklink. Another funny thing: the topseller is sitting in China, seemingly hoarding piles of them...

[Zerobricks]

I only have original Lego motors, so I don't know how others behave...

[2GodBDGlory]

6 minutes ago, brunojj1 said:

The question goes to @Zerobricks as well. Did you try them on BuWizz as well as being reliable and most important – responding gradually? E.g. I´m having many CaDA servos of different types and generations, some not yet released and none of them do respond proportionally on BuWizz 2.0 and 3.0. The funny thing (not really) I experienced the original Lego servos are doing that, but cost a fortune on bricklink. Another funny thing: the topseller is sitting in China, seemingly hoarding piles of them...

All the Chinese servos I know of (except for the aforementioned and discontinued MouldKing ones) have a basic internal track that only allows for the three positions, regardless of the controller.

[Alex Ilea]

12 hours ago, brunojj1 said:

The question goes to @Zerobricks as well. Did you try them on BuWizz as well as being reliable and most important – responding gradually?

Tried, and they don't work like that, only 3 positions.

[brunojj1]

5 hours ago, Alex Ilea said:

Tried, and they don't work like that, only 3 positions.

Thank you! Hopefully all of this will help the OP, considering he is sitting in Brazil, supposedly having opportunities very different from ours. (I just hate the fact that sometimes you´d have to direct people to cheap but more reliable and poweful MK products)

[Mpafr]

Thanks, I appreciate all of your inputs. I will play around with all the inputs Ive got and post the results here at some point!

 

2 hours ago, brunojj1 said:

Thank you! Hopefully all of this will help the OP, considering he is sitting in Brazil, supposedly having opportunities very different from ours. (I just hate the fact that sometimes you´d have to direct people to cheap but more reliable and poweful MK products)

I can source generic parts from China with reasonable (not cheap) prices, but 60 days delivery time. My local currency is very weak, buying original parts here is prohibitive, all my original sets are from when I lived overseas. 

 

21 hours ago, Zerobricks said:

There's another way. You can use the 1x4 rack bricks and set the steering in such a way that the gear just about slips at the end of the steering rack when the steering reaches the maximum angle. 

Nice! Awesome model and  a creative workaround!

[2GodBDGlory]

I wonder if there's a way to make some kind of smart adapter between the servo outputs from a Lego IR receiver and the standard servo control of the GeekServo. I'm imagining something based on some kind of microcontroller running a program that can recognize when the voltage changes and then control the standard-style servo based on that. I might give that a shot whenever I get around to figuring out how the Arduino I bought at a garage sale works.

[Zerobricks]

One more idea. You can also use a Mini Linear Actuator to steer and have it geared up some 3x for faster response.

[brunojj1]

1 hour ago, Zerobricks said:

One more idea. You can also use a Mini Linear Actuator to steer and have it geared up some 3x for faster response.

But it doesn´t solve the OP´s problem with the return-to-center function. I´d rather suggest making a simpler steering without steering racks and gears, just a white clutch + lever + rubber bands.

Edited December 9, 2022 by brunojj1