Sorry - Another stepper motor question

Using Arduino Motors, Mechanics, Power and CNC
#1

Hi,

Project background, a cart autonomously travels along rail tracks. It stops at regular intervals, turns some lights on and takes a photo. It then turns the light off and the cart moves along for another distance interval. The cart can also be controlled in manual mode with a PS4 bluetooth controller.

I have everything working reasonably well. My only issue is that it's a bit slow (approximately 0.8km/hr).

I have two 26Ncm stepper motors driving the system at the moment driven by two pololu a4988 drivers 26Ncm stepper

I tried to increase the speed by 50% by increasing the wheel size, however there isn't enough torque in the motor to get it unstuck if it runs into trouble, i.e. there's just enough torque to keep it going and if it starts rubbing the flanges on the rail tracks it sometimes misses some steps. With the smaller wheels this didn't happen.

Anyway, so I bought some bigger stepper motors, these ones:45Ncm stepper

So I did a simple test to make sure the steppers work, I've connected the arduino to the a4988 driver and then to the stepper and ran some simple code, the exact same code as I used for the 26Ncm motors, I'm 100% sure the code is fine.

The stepper spins just as expected however it's super easy to stop from rotating with a little finger pressure. I put a wheel on the motor and it spins but if I do as much as breathe on it, it starts skipping steps i.e. it has just enough torque to spin the wheel.
I've tried slowing it down but I didnt get much improvement. The smaller 26Ncm steppers work much better, at least from my experience.

So Ive done a couple of things to try and solve it but no luck:
i. I checked with a multimeter and the current sits between 50ma and 100ma, I've tried changing the voltage from 10v to 36v, I didnt get any significant difference in the torque (it remains barely being able to keep just the wheel spinning with no load on it).
ii. Tried to slow/step it more slowly
iii. I adjusted the current limiter on the A4988 all the way up to 2A, but the motor still only draws 50ma to 100ma.
iv. If I just get the coils to hold and nothing spinning, again they take such little current.
v. Out of frustration I've tried ever combination of cable configuration just to be 100% sure that there wasn't an error in the datasheet.

I don't know what else to try, has anyone seen this sort of behaviour from a stepper motor before?

To be clear, I bought two of these things and they both have the same behaviour

#2

jockwr:
iii. I adjusted the current limiter on the A4988 all the way up to 2A, but the motor still only draws 50ma to 100ma.

The Pololu A4988 web page has a lot of useful info about setting the current limit - but be careful to check if your A4988 has the same value of current-sense resistors.

Be VERY CAREFUL never to disconnect the wires between the motor and the stepper driver while the driver is powered up. The driver will be instantly destroyed.

Assuming you have not already destroyed the driver if you want to measure the current with a multimeter then you can only do so while the motor is stationary. Measuring the current while the motor is moving would require an oscilloscope because a multimeter cannot react quickly enough.

If you are in the market for a new driver I suggest you get a DRV8825 as it has a bit more current carrying capacity than an A4988 which will probably struggle to provide 1.7 amps continuously.

...R
Stepper Motor Basics
Simple Stepper Code

#3

Thanks Robin2,

I have been trying to solve this over the past week and just realised something really dumb on my part.

The product page says it's rated at 4.8v. I just connected it to my power supply at 4.8v and it now hold incredibly well and the amps have gone up to 1.6-1.7A.

I bought a pack of 10 of the stepper drivers because I knew I'd kill some. So far I've killed two doing exactly what you stated (pulling out cables while it was on). Luckily I haven't killed these ones as I can see they are still working with the 26Ncm motors.

#4

jockwr:
The product page says it's rated at 4.8v. I just connected it to my power supply at 4.8v and it now hold incredibly well and the amps have gone up to 1.6-1.7A.

You will need a much higher voltage if you want to have decent torque at speed. The important factor for stepper motors is the current, not the voltage. And the minimum voltage for an A4988 is 8v.

...R

#5

jockwr:
Hi,

Project background, a cart autonomously travels along rail tracks. It stops at regular intervals, turns some lights on and takes a photo. It then turns the light off and the cart moves along for another distance interval. The cart can also be controlled in manual mode with a PS4 bluetooth controller.

I have everything working reasonably well. My only issue is that it’s a bit slow (approximately 0.8km/hr).

I have two 26Ncm stepper motors driving the system at the moment driven by two pololu a4988 drivers 26Ncm stepper

I tried to increase the speed by 50% by increasing the wheel size, however there isn’t enough torque in the motor to get it unstuck if it runs into trouble, i.e. there’s just enough torque to keep it going and if it starts rubbing the flanges on the rail tracks it sometimes misses some steps. With the smaller wheels this didn’t happen.

Anyway, so I bought some bigger stepper motors, these ones:45Ncm stepper

So I did a simple test to make sure the steppers work, I’ve connected the arduino to the a4988 driver and then to the stepper and ran some simple code, the exact same code as I used for the 26Ncm motors, I’m 100% sure the code is fine.

The stepper spins just as expected however it’s super easy to stop from rotating with a little finger pressure. I put a wheel on the motor and it spins but if I do as much as breathe on it, it starts skipping steps i.e. it has just enough torque to spin the wheel.
I’ve tried slowing it down but I didnt get much improvement. The smaller 26Ncm steppers work much better, at least from my experience.

So Ive done a couple of things to try and solve it but no luck:
i. I checked with a multimeter and the current sits between 50ma and 100ma, I’ve tried changing the voltage from 10v to 36v, I didnt get any significant difference in the torque (it remains barely being able to keep just the wheel spinning with no load on it).
ii. Tried to slow/step it more slowly
iii. I adjusted the current limiter on the A4988 all the way up to 2A, but the motor still only draws 50ma to 100ma.
iv. If I just get the coils to hold and nothing spinning, again they take such little current.
v. Out of frustration I’ve tried ever combination of cable configuration just to be 100% sure that there wasn’t an error in the datasheet.

I don’t know what else to try, has anyone seen this sort of behaviour from a stepper motor before?

To be clear, I bought two of these things and they both have the same behaviour

are you sure about your mechnical part of your project in think you have a kind or friction or belt problem in your project if your are using 26N/cm motor your rail have to move smoothly because it have acute ande precise current you want.

Jack Arsal

#6

Îf you look at the spec of your 26Ncm motor you see:

Resistance: 30 Ohms
Rated Current: 0.4A
Rated Voltage 12V
Inductance : 37mH

This is the typical spec for a motor intended to be driven by voltage, not by current (high resistance, high rated voltage). -> Your A4988 (or any other constant current driver) will have a very hard time to drive that motor. If it works at all you need to supply the driver with the highest allowed supply voltage. Even then you will not be able to achieve high speed.

If you are looking for a motor compatible to those drivers (recommended) you need to look for low resistance (often only the rated voltage is given, then choose one with a very small voltage).If you need high speed, look for a motor with a very low Inductance (something below say 2µH).

Voltage driven motors:

  • Pro: Low current requirement, simple driving electronics
  • Con: Low speed, difficult to use with microstepping

Current driven motors:

  • Pro: Can be used with (now) standard stepper drivers, much faster
  • Con: Need much higher operating currents

https://www.omc-stepperonline.com has a lot of fully specified motors on their web site. Browsing this and comparing spec will give you a good overview of the differences.

#7