Let's Make Robots!

Steering servo needed for a quarter scale RC truck

 

I am building a 1:4 scale radio control truck.  I know it isn't exactly a "robot" but I started a thread on this forum looking for information about incorporating some logic into the build and have been encouraged to continue updating here.  I had made my update in the thread I had started but was suggested to move to a new thread with the new questions.

The truck will be based on a mobility scooter chassis.  I will be using mainly the rear axle and drive assembly and rebuilding 90% of the chassis.  It will have 4 wheels (not the original 3 wheels) and will have two wheel steering.  Now I am searching for a steering servo that will be powerful enough and inexpensive enough.

  Like the motor controller (SyRen 1x50), I want enough overkill that I do not need to worry about breaking the gears, but also like the motor controller, I need to be able to afford the servo >.<  I have seen many that will work, but with price tags OVER $200 for a single servo, it's not really possible right now....  I have seen several accounts of people using modified servos for position feedback while using car window wiper or power window motors, but all seem to agree that there isn't enough power or speed..

 So now I look to LMR for more advice!  I need a servo to steer this thing...  I HOPE to be under $100 for the servo...  I wont terribly mind using two servos but I am a little worried about the extra complication as well as the servos fighting each other.

 

Some more info about the truck, if anyone is interested:

 As mentioned above, the motor controller is a SyRen single-channel 50-amp controller.  I borrowed the RC out of a plane that we've had laying around and connecting the SyRen really IS as simple as DimensionEngineering makes it out to be!  I cut the pigtail off a broken servo..  black to ground, red to 5v+, and white to "S1" signal input.  + and - from a car battery to the appropriate terminals on the controller, and + and - from the scooter motor to the remaining screw terminals.  No soldering, no adjusting, no programming (had to flip one DIP Switch to put it in RC mode).  Hit the stick on the transmitter and immediately ran over my sneaker :D  Flip the servo-reverse on the transmitter and all was well!!!!

 

I did learn that 12 volts will *NOT* be enough for what I want..  it is too slow and with someone sitting on it, it struggles even in grass on flat ground ...  will have to be 24 volt (as the scooter originally was) or even 30 volt if I can source an appropriate charger without killing my budget (which isn't much).

 

I CAN load a video if anyone cares to see, but at this stage the scooter still has no remote steering what-so-ever, and with someone sitting on it, it is no more than a mobility scooter..

 

Hoping to get plenty of replies,

Mike, from DieCastoms.

 

Quarter Scale F250 sig

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get one of those fish scales that has a spring and a pointer that tells you about how much the fish weighs when it is hanging on the end of it. That will give you an idea about how much force is required to turn the wheels, I would suggest weight or someone sitting on the model when you do this and check a couple different surfaces to get an idea about max force.

At least with some numbers you are no longer saying tell me about strong servos. Instead, you are saying tell me about servos that can offer at least x amount of torque.

So, I bought a scale at Walmart that is designed for weighing your luggage ...  

Just for the sake of discussion, since the numbers will be different for the truck,

I went over to one of the random gocarts laying around our yard.  the steering arm on it is roughly 4 inches long, and it took 45 pounds to pull it to steer the gocart from full left to full right.  45 times 4 is 180.  This means I need 180 inch pounds of force, right?  Did I figure that correctly?

[edit] If math also serves me right, doesn't that also mean I need 180 time 16 = 2880 inch-ounces?

 

 

Mike.

 

180 pound x inch is about 20 Nm this is well beyond the realm of low cost servos :)

Well those figures were also for the heavy old rusty gocart, too..  I was mostly asking if I was figuring right based on the reading from the luggage scale and the length of the steering arm, both of which I mentioned in the post.  I haven't a clue what a Nm is, though I assume it is a newton meter?

 

mike