Is there a motor limit? The guidebook is kind of vague

What is the motor limit. Is there none? I know there no limit on the amount of each type you use, but is there still a total limit which we must be below

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The number of motors you can power is limited by the number of channels on your power distribution, because there’s a limit of one motor controller per PDH/PDB channel (R621) and whether you’re controlling one or two motors per motor controller (see R504; roughly, if a motor’s max power is less than 1/4 HP you can put two on a controller). The number is dynamic based on whether you’re using PDB or PDH and what other things are using PD channels.


The “total number of motors you can use” question is a phenomenal example of how many constraints on your robot are spread across many rules. This is not always obvious at first read.

Read, re-read, and study the whole manual!


You may have as many motors on your robot as fit within your volume and weight constraint.


16 for PDP
20 for PDH


Nope. You might want to re-read the rules.


Now remove any ports required for other required equipment and multiply by 2. This is one possible correct answer for “how many motors”

Now dump a robot volume of motor-shaped ballast in. This is a different answer, and will vary based on which motors you use as ballast.


Yes, that’s exactly why I used the phrase “The number of motors you can power” in post #2. It isn’t clear whether unpowered MSOs* are legal, though I’m thinking it is, given that R605 explicitly prohibits BSOs* from being ballast and there’s no corresponding rule for motors. OBTW, unpowered MSOs can also be used as sensors or brakes or decoration (likely others) - it’s unclear whether they may be so used. It’s even more unclear whether you can use an object shaped like a motor not listed in 501 in a non-actuator capacity (e.g. as ballast, sensor,brake, or decoration).

* MSO: Motor Shaped object; BSO: Battery Shaped object

@OP For all the comments which are some flavor of “read the rules” - I wanna clarify.

  1. I promise, it’s not just because we’re lazy, or think you are lazy
  2. please please please please read the rules again and again and thuroughly.

Case study: In 2019, I thought I read the rules. I was wrong. Don’t risk having to throw out something you’re proud of because it’s legal.

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There have been quite a few people suggesting you re-read the rules. I would suggest you go further and think about how the various rules may “interact” with each other and discuss this with your teammates. Sometimes, this leads to “hidden constraints” like a few people have hinted at, in jest. Sometimes, “hidden freedoms” can be discovered that make solving the challenge easier. People can tell you what the basic rules are but you and your teammates are the ones who have to understand the implications of the rules.

A few years ago, a few other mentors and I brainstormed on solving a question of how few motors a robot could have and still be effective. This resulted in coming up with some creative power transmission designs which allowed just one or two motors to engage handfuls of robot mechanisms (simultaneously, in some cases). It was a fun thought exercise, but I don’t recommend it…

For most use cases, you can have 1 motor controller per PDP/PDH slot. Big motors like the NEO550, NEO, Falcon, 775s, and CIM can be powered at a rate of 1 per motor controller. Some of the smaller motors like BAGs can have 2 on the output of 1 motor controller, which is handy. See the rules for the exact implementation guide.

I recommend planning out which PDP slot will go to which actuator on your robot. There are a limited number of 40A slots for high-power mechanisms and you should allocate them carefully. Reserve a slot for custom circuits like a vision coprocessor.

Lego Technic vehicles do this all the time. They are also toys and don’t need to support load.

In addition to the PDP/PDH slots, you can install servos powered directly from the roboRio. If you count continuous servos as motors, I presume you can have 20 more as well (10 in the existing PWM slots and 10 more in the expansion slot)

It may not just be about the rules. We had 16 motors last year and ran into reliability issues that were hard to diagnose. Overheating of controllers, over heating of drive motors during practice and battery power issues for speed of the shooter later in the match… wasted a lot of time diagnosing intermittent issues, rewiring, adding cooling, reworking electrical panels and replacing motors.

then the limit will probably come down to weight of the lightest servos you can find, because there aren’t any restrictions about teeing PWM signals, nor any limit I can find as to how many REV servo power modules you can put on a single breaker*. Eventually the PWM signal will be spread too thin to be effective, but that’s still not against any rules I could find.

* In investigating this edge case, I realized that the REV servo power module is not addressed in R621, nor indeed anywhere in the rules but R505, which simply allows it as a way of powering and controlling servos.

The limit is your PDP you can put motors until you don’t have place in PDP and it is 16 motors.

Feel like this thread is going to keep getting bumped by people ignoring the rest of the thread and responding with 16 or 20.

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Or 20 if you have a PDH instead of a PDP, or a lot more than that if you note which motors are a 2-for-1 special on slots, or a lot more than even that if you use the motors as ballast.