Compatible motors/cabling

Ltrain · July 19, 2021, 3:35pm

Hi, I’m planning to start using the robot kit and want to do a mecanum wheel build, but I need more motors.

Is the motor cable a 6 pint JST connector?
Has anyone found a motor that works well?
I like the rev robotic motors but they are 2 and 4 pin configurations. I haven’t been able to find an adapter and I’m trying to avoid soldering my own if I can.

I was looking at these motors from servo city, they are 6 pin but I haven’t checked the pi top schematic to make sure the pins match up yet.

Anyone have any luck here? Hell can anyone point me to the motors the pi top ships with. I could always use two more, it might be easier to match the shafts with my mecanum wheels

duwudi · May 25, 2021, 12:53pm

@Ltrain it’s funny you mention this, I was just sharing a video of a mecanum build I did recently with a colleague. Here’s some photos:

For this I used two extra pi-top motors as it made it pretty straightforward to build because of the rivet system, unfortunately these are not currently available for sale but will be in the next couple of months - the only way to get them now is to have a second robotics kit. I’ve asked a colleague to see if we have any spare stock that we can sell before they are added to the website

I also wrote some of the software required for it, here’s the drive controller:

github.com

pi-top/pi-top-Python-SDK/blob/4wd_controller/pitop/robotics/drive_controller_4wd.py

from math import floor
from pitop.core.mixins import (
    Stateful,
    Recreatable,
)
from pitop.pma import (
    EncoderMotor,
    ForwardDirection,
    BrakingType,
)
import numpy as np


class DriveController4WD(Stateful, Recreatable):
    """Represents a vehicle with four mecanum wheels."""
    _initialized = False

    def __init__(self,
                 front_left_motor_port="M2",
                 front_right_motor_port="M1",

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And here’s a web interface for controlling it from a browser:

github.com

pi-top/pi-top-Python-SDK/blob/4wd_controller/examples/labs/4wd_rover_controller/main.py

from pitop import Pitop, DriveController4WD, Camera, PanTiltController
from pitop.labs import RoverWebController
from pitop.labs.web.blueprints.rover.drive_handler_4wd import DriveMode, DriveHandler4WD

rover = Pitop()
rover.add_component(DriveController4WD())
rover.add_component(Camera(resolution=(320, 240)))
rover.add_component(PanTiltController())
rover.pan_tilt.calibrate()

drive_handler_4wd = DriveHandler4WD(drive=rover.drive, pan_tilt=rover.pan_tilt, mode=DriveMode.PAN_FOLLOW)

rover_controller = RoverWebController(
    get_frame=rover.camera.get_frame,
    message_handlers={
        'right_joystick': lambda data: drive_handler_4wd.right_joystick_update(data),
        'left_joystick': lambda data: drive_handler_4wd.left_joystick_update(data)
    }
)

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Please note, none of this is in the master branch of the SDK, so you’d have to clone this branch and install it in its entirety to use.

In terms of pinout and connectors, I believe the motors you linked are the same and would be compatible - the only thing that can’t be guaranteed is the hall effect encoder system. We use 52 pulses per rotation of the motor so you may have to scale things accordingly if it’s not the same (or modify the SDK).

Ltrain · May 25, 2021, 3:10pm

The extra pi-top motor would be the simplest. (I just ordered another robotics kit, which is cool as I have 2 pitops anyway.) Are those robotmaster S1 mecanum wheels? Do they fit the Pi-top motor axles, or did you have an interface. I have a full set of rev robotics mecanum wheels I was thinking of using - but I’ll likely need some type of hub/axle connector.

duwudi · May 25, 2021, 4:13pm

Nice!

Yeah these are from the Robomaster - they fit onto the same 12mm brass hex fittings the pi-top wheels use but I had to get some longer ones because of the wheel width difference. I can measure what size I used when I’m back in the office and let you know, should be fairly easy to buy.

Ltrain · May 25, 2021, 4:27pm

Yes if you could send me the length of the fittings that would be great! Thx for the help here. Glad I posted you saved me some experimentation.

duwudi · May 26, 2021, 10:44am

Ltrain · May 26, 2021, 12:44pm

Thank you! super helpful, and will save me some time looking for the right interface.

Ltrain · May 26, 2021, 1:08pm

If anyone is going down this path here is the coupler.

https://www.amazon.com/Pieces-Flexible-Coupling-Coupler-Connector/dp/B08P89YD11/ref=sr_1_22_sspa?dchild=1&keywords=12mm+brass+hex+coupler&qid=1622033842&sr=8-22-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEzSzBCQTBPU1pPTTJIJmVuY3J5cHRlZElkPUEwMzY4OTg5OVJJM01HMTlPUllRJmVuY3J5cHRlZEFkSWQ9QTA0NDA3MTExUE5YV0s5UlY2UDBGJndpZGdldE5hbWU9c3BfYnRmJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==

duwudi · May 26, 2021, 1:16pm

No worries! We’re hosting pi-top Sessions this Friday and it’s a Robotics Kit special, if you have time it might be interesting for you to come along as there’s a lot of other cool stuff being worked on - here’s the signup link, it’s at 5 PM GMT

Ltrain · May 26, 2021, 2:55pm

Thanks - I’ve signed up for the event.

tlfong01 · June 30, 2021, 3:33am

I have a couple of these little N20 gear motors lying in my junk box for some years now, collecting dust.

I once used Raspberry Pi and the bulky L298N motor driver board to control it and found it OK. Now I am thinking of using Rpi Pico with a smaller motor controller.

My long term objective is to make a robot car as small as possible.

Update 2021jun30hkt1130

I am thinking of using Rpi Pico to control the N20 motor, as described in the following Tom’s Hardware article.

How to Use Your Raspberry Pi Pico With DC Motors - By Les Pounder, TomsHardware, 2021jan30

duwudi · July 5, 2021, 2:02am

Nice idea! I look forward to you sharing the progress on your build in the future

tlfong01 · July 5, 2021, 8:15am

Thanks for your encouragement. I am going as slowly as possible, so to learn more things, and remember them harder. Cheers.

mannadu · July 5, 2021, 2:19pm

Isn’t the power supply from the battery underpowered - if one wanted to get maximum performance from these motors?
The pi-top lithium battery is rated in specs :

38.85 Watt-hour - (11.1V, 3500mAH)

Each of the n20 Gear is rated:

Voltage (Nominal) - 12V
Current (Stall @ 12VDC) - 1600mA

I’m assuming this means max possible wattage for each motor at max would be

12V x 1.6A = 19.2 watts

With two of these, we’re already at 38.4 watts.
With 4 motors it would be 76.8 watts of power… which is much more than the pi-top battery is rated.

How could we provide motors with enough juice to get maximum output possible?

Ltrain · July 5, 2021, 2:29pm

Can’t you hook up an external power source via a USB C connector? I recall seeing posts about people taking on anker batteries.

Ltrain · July 5, 2021, 2:31pm

Yup checked the rack sheet. You can draw an additional 15v.

https://f.hubspotusercontent40.net/hubfs/4368942/WebsiteV3/Robotics%20Kit/PT_[4]_FactSheets_RoboticsKit_16122020_V2%20(2).pdf?__hstc=225167877.3379bc76d34f5ca55dee110d31de5562.1625495414236.1625495414236.1625495414236.1&__hssc=225167877.1.1625495414236&__hsfp=26556968&hsCtaTracking=c68deb3d-72a9-4aae-ac04-516fe36b9756|f1ac27e7-41e3-4ca5-8dff-998717b07a37

duwudi · July 5, 2021, 2:32pm

Those are the pi-top [3] specs, pi-top [4] ones are here

In terms of the maximum power, you are mixing watt-hours and watts. I believe we limit the maximum current from the battery pack at 2C, which means 2 multiplied by the Ah rating. This would give a maximum current output from the pi-top [4] as 2.6 * 2 = 5.2A @ the battery voltage. Taking the nominal battery voltage to 7.4V you can get 38.48 watts

duwudi · July 5, 2021, 2:33pm

And yes you can hook up an external battery pack to the Expansion Plate’s USB-C connector, the motors would then draw directly from this power source

tlfong01 · July 6, 2021, 3:10am

Well, the stall current of my N20 gear motor is only around 500mA. Can you give us a link of your N20 gear motor?

References

(1) N20 Gear Motor Spec - Amazon

mannadu · July 6, 2021, 6:19pm

Thanks for catching that and recalculating, I’m hopeful again for my use-case!