Dc motors for wheels

Panzotom

Hi everybody !

I'm a french market farmer, and I'm working on a small "robot" tool for transplanting vegetables on my farm.
I have a Duet 2 Maestro card that I would like to use for creating this, because I think it's more reliable and easy to program than others devices.
I would like make 3 movements with this robots : 1 on the Z axis to up and down the plants, 1 on the Y axis to create a "charger" which contains all of the plants plugs falling down into the Z axis, and 1 on the X axis to move 30cm diameter wheels... for moving the robot on my land (to transplant every 15cm for example).

By reading the Duet documentation, and because the robot will be heavy (maybe 40 kg to move), I understand that I can't use the x-axis stepper motor traditionnal PIN on the board because the voltage, the wheels and the weight needs more powerful DC motors than a NEMA 17. I would like to use this kind of motor

So my question is : the documentation mentioned that it's possible to wire DC motors on the heating bed PIN. Do you think it's possible to wire this 2 wires DC motor directly to the card for moving wheels and robot, and controlled by the Duet card, without burn it ? Or do I need an expansion card to control it ?

Thank you for your help, and if you have another ideas, don't hesitate !

droftarts

@Panzotom The motor you have linked appears to be an 24V, 18A, 300W motor. The bed output on a Duet 2 Maestro is rated at 18A (though is generally fitted with a 16A fuse), and the extruder heater outputs are rated at 6A; see https://docs.duet3d.com/Duet3D_hardware/Duet_2_family/Duet_2_Maestro#operating-limits. There is also no flyback diode on the heater outputs, which prevents the motor supplying current when it is spinning free, like a generator.

So I think just one of these motors would be pushing the bed heater connections to the limit, and would more than likely cause a failure of the board. However, you could use a suitable relay to switch the motors on and off, though I have no specific recommendation for a large DC motor.

For connecting (generally, smaller) DC motors, see https://docs.duet3d.com/en/User_manual/Connecting_hardware/Motors_servos#connecting-dc-motors

Ian

infiniteloop

@Panzotom Nice project! Adding to the comment by @droftarts, which is as always substantial, you can in fact use a DC/DC SSR (solid state relay), with two caveats:

all of these SSRs switch to GND, you can’t control the Vss line.
Chinese Amperes are somehow short in stature, for a motor rated at 18 A, better buy an SSR capable of 40 or better 60 A. Some come equipped with a flyback diode - if not, add one externally. Oh, and always provide a suitable heat sink as well.

Just out of interest: How do you intend to steer the robot? Or has it just to go straight?

o_lampe

@Panzotom Your project sounds interesting, but are you sure you can control the robot with only one motor?
Even when you use the chain sprocket to drive a two-wheel chassis with a third (caster-) wheel, it won't drive a straight line on rough ground.

If I'd build this, I'd use two independent drives at least, maybe controlled like a tank.
I have experimented with brushless motors from used hoverboards. They are cheap and have hackable controllers, which also work with step/dir signals like a stepper.
The main controller could be a Duet board with extension port (Duet2 WiFi would be my choice)

The hoverboard tires can be replaced with tires for rough terrain. ( If I find the links, I post them later)

A good starting point might be EFeru-github or SimpleFOC (they have implemented a step/dir interface)

Panzotom

@droftarts Thank you for your reply !
I was thinking of doing something like that. Thanks !

Panzotom

@infiniteloop Thank you for the details ! I will try this soon !
In fact, I think it will be a "semi-automatic" robot, I will drive him manually during the planting session, but I'm working on permanents beds so it's quite straight

Panzotom

@o_lampe Thank you for this idea !
Yes, I would like using only one motor with a chain, cause even if the ground it's not perfect, it will planting very slowly.
I imagine this more like a semi-automatic transplanter, so I will be close to it to drive him by hand.