Linear actuators

Hey all
I have just finished - or at least 80% finished my first 2dof rig with 2 nema 34 steppers and 20:1 gearbox. I’m experiencing some jogging / stepping movement and can see that this not an uncommon thing. Have tried a lot of stuff tweaking simtools and the arduino sketch, but can’t quite get rid of it.

A couple of questions:
- are there some “standard” arduino sketch you are using for motor control. I have made mine myself and are thinking that that might be the issue?
- I had tons of fun making it and seeking knowledge but are considering shifting from the rotational stepper actuators to linear actuators. How fast do the need to be and what torque / nm ? As far as I can tell this might fix the issue?
Best
Jacob

So far as far as I know there is no standard stepper motor sketch. There is not enough info in your post to understand what problem you are having.

The standard sketch is SMC3 which supports 3 servo gear motors at a 4khz update rate. Generally the servo motors are gear motors used with lever arms. For a rocker aka ujoint in the middle you need > 25Nm at the motor. That also depends on the length of levers arms etc. So that may involve a range of 50:1 to 30:1 and a top speed of 30-100 RPM.

As for a linear actuator there are lots of options. Look at https://opensfx.com/documentation/ for a good tried and true example of that kind of system. Most people here use AC servo motors for linear actuators. That can get expensive but not as expensive as it used to be maybe $150 per motor/amplifier.

you need scn5 or scn6 linear actuators really but theyre not cheap
let us know what problems youre have with the motors in more detail and should be easy to iron out

Instead of a fluid motion they sort of step a bit, making small movements that I can feel in the rig. Its not too bad, but I expected a smoother ride. If I test movement in Simtools its the same result, so I guess its either just the way the steppers behave or something in the code. The code is largely done with ai but without SMC3 - so I will look into that option.

I can send som pics of the rig when I get home if you guys promise not to trash it ;-)

Yeah send some please there are many janky builds here including my own
The issue is bound to be software related unless you have an unbalanced rig

Hello again
Here are some pictures of the rig. It has kind of been a matter of work fast, fail fast approach. Im not an engineer, just a simple IT guy, so I have been struggling with the actual contruction which I grossly underestimated the difficulty of. I thought that would be the easy part. Took me almost a month to find a u-joint. I should probably rebuild the whole thing, but I enjoy the process of learning - so if im to rebuild anything, its going to be with linear actuators.

Well - its actually moving the rig ok, but I just feel an annoying stepping sensation which I blame on a combination of stepper motors and maybe something in the code. I will rewrite the code tonight.

Any other suggestions are welcome.

Enjoy you evening / morning depending on where you are in the world
Jacob

That's not a bad first attempt
I would firstly ask about the balance while seated as if its front or rear end heavy that could cause a stuttering effect
Second how have you tuned the pid loops? That's quite an important part
And thirdly id would personally put the motors closer to the chair as it will give you more movement
You really don't need to be an engineer to make one
Trial and error is a lot quicker and easier if you just get to grips with the basic principles

Also take those bungee straps off the back they are not doing any good

Will do. When you say PID loops (had to look it up) is it in SimTools or in the Arduino code you mean?

I have just done a version where the arduino consumes the values from SimTools and converts it to raw movement without any filters. Didnt do any good... The big movements are ok, but with the smaller adjustments in the plane you clearly feel the stepping.

I fiddled a bit with the SimTools values. The limits and percentages on Roll and Pitch works fine, but I dont feel any difference when changing Profile intensity.

Best
Jacob

PID loops are fundamental to servo control and pretty much meaningless for steppers. Filtering is another matter.

SimTools will give you a value at some constant interval you set. The game will give SimTools a value on some different interval maybe 30ms. That makes filtering difficult. Maybe we can change the way SimTools works.

For now let’s pretend that you get a new position every 30ms. I don’t know how fast your steppers move so I will make up numbers. Let’s say that your steppers can move 300 counts in 30ms. If you get a position that’s 100 steps away. In that case the motor will move 100 steps in 10ms and stop for 20ms. If the game is moving slow and steady your rig will start and stop every 30ms. What you want is to move 1 step every 3ms. That way the rig will move slow and steady.

If you have a CPU with actual memory (Uno is 4k of RAM) then you can do this. I have the same issue with my AC servos. These are pulse-direction motors where the pulse rate is the velocity. Basically you want to do velocity control in addition to position control.

I have been meaning to make more general velocity control post because I want to be able to refer to it. I will post a link here maybe soon. If you don’t see it post here to remind me I promised.

What he said