Servo or stepper for a Kart electric gear shift

Hello.
As the title sais I want to shift gears electrically (using paddles and an Arduino) on my 125cc, 42 HP shifter kart.
I just don’t know what servo or stepper motor I would need to accomplish this.

I measured the force needed to change gears to be about 13 kgf (I used a spring scale to pull the leaver that changes the gear in the gearbox. When the gear changed it showed 13 kg)

What is the force in Nm that I need (I don't know exactly how to calculate it, or if I measured correctly)?
Is a servo or a stepper better?
It would also need to be fast (not long travel - only about 3 cm - but as fast as possible).
It would be awesome if it used only 5V power supply. But higher is OK too.

I saw many systems on the internet but none shower the exact model of the actuator/motor used to change gears (since they are all commercial systems).

I would like to accomplish something on the lines of ME-Shifter F1. (Of course not that sophisticated)

Any help would be very appreciated. If you need any more info please let me know.

Thanks,
Pintea

To calculate torque you need to know the distance between the point at which you were pulling the lever (to get the 13kg measurement) and the centre of rotation of the lever. I kgf is about 9.8 N.

You need to provide a diagram to illustrate the mechanical arrangement you want to create.

A servo would be much simpler than a stepper motor because a servo has a feedback mechanism so it can be moved to a particular position. A stepper motor does not know where it is, it can just move N steps forwards or backwards from wherever it happens to be. It is usual to have a setup routine that moves the motor to a HOME or ZERO position from which steps can be counted. However if it misses a step the counting will be wrong.

In my experience of driving a car it is quite common for the gear lever NOT to shift smoothly into the next position if the gears happen not to be properly aligned. The human hand and mind easily deals with that with a second try at the movement. But dealing with that in a computer controlled system will not be simple. Not least is the problem that a servo might be damaged if it cannot move and a stepper motor would probably lose steps. In many ways a simple DC motor and detector switches for each gear position might be a better option.

You can also get devices called linear actuators that you may wish to consider. However the comments in my previous paragraph are still relevant.

Then there is the need to find a servo or motor that can deliver enough torque and operate at the speed you require.

…R
Stepper Motor Basics
Simple Stepper Code

Robin2:
To calculate torque you need to know the distance between the point at which you were pulling the lever (to get the 13kg measurement) and the centre of rotation of the lever. I kgf is about 9.8 N.

You need to provide a diagram to illustrate the mechanical arrangement you want to create.

A servo would be much simpler than a stepper motor because a servo has a feedback mechanism so it can be moved to a particular position. A stepper motor does not know where it is, it can just move N steps forwards or backwards from wherever it happens to be. It is usual to have a setup routine that moves the motor to a HOME or ZERO position from which steps can be counted. However if it misses a step the counting will be wrong.

In my experience of driving a car it is quite common for the gear lever NOT to shift smoothly into the next position if the gears happen not to be properly aligned. The human hand and mind easily deals with that with a second try at the movement. But dealing with that in a computer controlled system will not be simple. Not least is the problem that a servo might be damaged if it cannot move and a stepper motor would probably lose steps. In many ways a simple DC motor and detector switches for each gear position might be a better option.

You can also get devices called linear actuators that you may wish to consider. However the comments in my previous paragraph are still relevant.

Then there is the need to find a servo or motor that can deliver enough torque and operate at the speed you require.

...R
Stepper Motor Basics
Simple Stepper Code

Thank you so much for the information.
I searched for linear actuators too. But I could not find any that would move fast enough. Do you have any in mind that would move 3 cm and back in about 0.2 sec?

I was thinking that I would program the servo/stepper to move the gear shift leaver and come back to the initial position (it is a sequencial gearbox, so the movement to change gears only has to be a linear back and forth.)

So if 1 kgf is about 9.8 N, than I woul need somthing capable of about 130 Nm correct?

Thanks.

guzu:
So if 1 kgf is about 9.8 N, than I woul need somthing capable of about 130 Nm correct?

You seem to have calculated 9.8 * 13 ~ 130Nm which implies (incorrectly I suspect) that when you measured the 13 kgf you were pulling at a radius of 1m

I have no recommendation for any linear actuator.

...R

You still haven't stated the lever length (how far is your scale connected from the pivot point?), 13 kgf * 9.81 = 127.5 N * lever length (in cm) / 100 = Nm. If lever length is 10 cm, torque will be 12.75 Nm, a BIG servo.

outsider:
You still haven't stated the lever length (how far is your scale connected from the pivot point?), 13 kgf * 9.81 = 127.5 N * lever length (in cm) / 100 = Nm. If lever length is 10 cm, torque will be 12.75 Nm, a BIG servo.

The scale is connected at 7 cm from the pivot point. Sorry for not providing this information before. I forgot :slight_smile:

Can you give me an example of a servo that would do the job please?

Thank you.

guzu:
The scale is connected at 7 cm from the pivot point. Sorry for not providing this information before. I forgot :slight_smile:

If you need a pull of 13kgf at a distance of 0.07m then the torque is 0.91 kgf.m or 8.9 Nm

I have no experience from which to suggest a servo. The Hobby King website has a huge range of them. I suggest you get one with metal gears and ball bearings, as well as comfortably more than the minimum necessary torque.

...R

Thank you all for the information.
Will post here when I chose a model.