Some advise on autonomous 2-motor tracked robot

I’m starting to work in a robotics project and need some help with a few electrical and physical considerations (I am a software person).

I apologize for the long post in advance!

The robot I am building is based on the Arduino 101 (I had one laying around) and “potentially” the Adafruit motor shield v2.3

The robot will have 3 actuators:

  • 2 dc motors for locomotion, which will need between 6v and 9v
  • 1 stepper for rotating an iPhone with a lidar type sensor. This sensor has its own battery so power for this one is not an issue at this stage.

And a few sensors (not counting the previously mentioned light sensor):

  • 4 x hc-sr04 (sound ranging) | 5V, 15mA (total 60mA for the 4 of them)
  • 1 x tr-3rc (3 IR LED/phototransistor pairs) | 5V, 50mA

Now on to my questions:

1. What minimum stepper specs would get the job done?
As in, rotating the phone + light sensor + case which attaches both.

The stepper will be mounted shaft up so that the phone + sensor pan 360 degrees. The items will weight aprox. 313g total (maybe a bit less). I looked up steppers but did not see a stall torque spec, so not sure how I should go about choosing an appropriate one.

2. Are my estimations for the robot wheel’s DC motors correct?

I estimate the total weight of the robot (excluding the stepper since I don’t have the answer to question 1 as of now) to be 607.6g

I believe what I need to look at here is dc motor stall torque to see if my motors will be able to move my robot around.

So given the 607.6g I considered this chasis (included in the weight I mentioned):

And for motors, either (also included in the 607.6g):
a. Pololu - 75:1 Micro Metal Gearmotor HP 6V
or
b. Pololu - 100:1 Micro Metal Gearmotor HP 6V

I based the motor decision on the chassis wheels having a diameter of 39mm and using:

τ= mass[kg]×9.81×radius[m]

where I used 0.7 and 1 for mass and 0.0195 for radius.

Then I converted from Nm to oz-in, which is what those motors in the links use for stall torque and got

18.97oz-in for 700g
27.1oz-in for 1Kg.

So my assumption is that the motor with a 22oz-in would be good and the one with 30oz-in would be better if I wanted to have the option to add even more stuff to the robot.

Does this make sense?

Another option is this cheapo chassis from Aliexpress:

Heavier and bigger and the motors are 9v, but they also have a quite higher stall torque and hall encoders (and more room to put things… my current chassis choice might be pushing it to hold an iphone).

Feel free to recommend another chassis too if you have something in mind. I am trying to keep it as small as possible considering the requirement that it holds an iPhone.

3. How can I power the logic + motors with the same battery source? preferably rechargeable source
I think the answer is with a regulator but I’m not super clear on how.

The chassis from Pololu comes with a holder for 4 AA batteries and the motors are 6V. The Adafruit shield connects via i2c to the Arduino and can theoretically do 3.3V logic (the 101 is 3.3V) while it would need at least 6V for the motors Vin.

So if I plugged 4 new alkaline AAs I’d get 6V, for a little bit… Then also comes the part where I’m not sure if I should plug the batteries directly to the Arduino jack and use the jumper for it to provide power to the motor shield or if I should plug Arduino and the shield both from the batteries at once.

The Pololu website mentions using a 7.5V boost regulator to power the Arduino from the same AA batteries for the motors. I looked up their regulators but they have a lot of them.

For instance would this work?

And if this works, should I only use the regulator output for the Arduino or for both Arduino and shield or use it with the Arduino and the shield jumper?

Also, does it even make sense? wouldn’t the AA’s deplete pretty fast anyway? Should I instead use 18650 batteries? Something else? I have to account for the stepper too, which the shield can drive but not sure about the 4 AAs…

4. And related to the above, if I were to use the other chasis with the 9v motors:
would I need to do the oposite in terms of regulator? meaning a step down from some 9v source to aprox 7 for the Arduino and Shield or would it make sense to use less than the motor voltage (i.e. still 6 or 2 18650’s at aprox 7.4 and step up to 9v?)

Thanks!

The robot will have 3 actuators:

2 dc motors for locomotion, which will need between 6v and 9v 1 stepper for rotating an iPhone with a lidar type sensor. This sensor has its own battery so power for this one is not an issue at this stage.

A stepper motor requires a lot of current. They are rarely the correct choice for mobile projects that need to carry their own batteries. A servo motor can probably wave the phone around just as easily, requiring far less current.

  1. What minimum stepper specs would get the job done? As in, rotating the phone + light sensor + case which attaches both.

The pink one. Which model iPhone? Which case? What does the whole thing weigh? How will it be connected to the stepper?

I looked up steppers but did not see a stall torque spec

Guess I should read ahead before typing, since you did list the weight. But, the torque that a stepper motor can generate depends on many factors - voltage, current, and speed, among others. Stepping slowly uses less current than stepping fast.

  1. Are my estimations for the robot wheel's DC motors correct?

I estimate the total weight of the robot (excluding the stepper since I don't have the answer to question 1 as of now) to be 607.6g

Interesting to see an estimate to the nearest 10th of a gram when you don't know a critical component's weight.

  1. And related to the above, if I were to use the other chasis with the 9v motors: would I need to do the oposite in terms of regulator? meaning a step down from some 9v source to aprox 7 for the Arduino and Shield or would it make sense to use less than the motor voltage (i.e. still 6 or 2 18650's at aprox 7.4 and step up to 9v?)

The Arduino has a builtin regulator. You can supply up to about 12V to the regulator, and it will output the required 3.3V to power the Arduino. The rest it will waste, as heat, which could be a problem. Having the Arduino-controlled robot stop doing anything because the robot batteries went dead is probably not a good thing. A separate power supply is.

PaulS: A stepper motor requires a lot of current. They are rarely the correct choice for mobile projects that need to carry their own batteries. A servo motor can probably wave the phone around just as easily, requiring far less current.

I guess you missed the part about LIDAR + 360 degrees (and the implication that this requires exact positioning and no servo jitter).

PaulS: The pink one. Which model iPhone? Which case? What does the whole thing weigh? How will it be connected to the stepper?

...

Guess I should read ahead before typing, since you did list the weight. But, the torque that a stepper motor can generate depends on many factors - voltage, current, and speed, among others. Stepping slowly uses less current than stepping fast.

I guess you should read ahead before typing, indeed. The whole post has a bunch of info which, if you are going to take the time to reply, you might want to read.

PaulS: Interesting to see an estimate to the nearest 10th of a gram when you don't know a critical component's weight.

Interesting to see another snotty remark which doesn't add much to the conversation or even acknowledges my calculations.

I make it very clear that 607.6g is without counting the stepper cause I need the answer to the first question. I also compute the torques for 2 motors, which I believe given the 607.6g leave me enough room for adding more things after.

It is also interesting that some components, like a qtr-3rc weight... guess... 0.6g.

PaulS: The Arduino has a builtin regulator. You can supply up to about 12V to the regulator, and it will output the required 3.3V to power the Arduino. The rest it will waste, as heat, which could be a problem. Having the Arduino-controlled robot stop doing anything because the robot batteries went dead is probably not a good thing. A separate power supply is.

I am talking about supplying either 6V with four AAs and using a step up to leave it at around 7.5V or using 2 18650s for 7.4V (and maybe just a regulator so that the 7.4 stays put when the batteries start dropping voltage).

In any case if someone else has something constructive to add please let me know. Thanks.