Use Victor SP brake mode with Nidec Brushless Motor?

Hello everyone,

For weeks my build team has been asking me if we will be able to use a Nidec Dynamo Brushless motor with braking (like how certain motor controllers have a brake mode feature). We are trying to use the Dynamos for this year’s climbing mechanism. The problem is, the Dynamo comes with a built-in motor controller that has not provided whether or not the motor controller has a brake mode.

Also, I cannot seem to find the motor controller API for LabVIEW, if anybody could point to where that is, that’d be great.

Does this motor have brake mode?

Thanks,

Ian F.
5503, “Tiger Trons”

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There isn’t a break mode available with the Nidec.

The wiring and setup in LabVIEW is unique for an FRC motor.
It’s controlled via roboRIO DIO output (DIO generated PWM control signal output to the motor) and a separate roboRIO PWM connection that just Enables the motor (to handle the FRC robot Disable safely)

Here is a LabVIEW example: Team358.org - Robotic Eagles - FIRST® Robotics Competition

The wiring looks like:

A red/white connector goes to a PWM output (red=signal, white=ground) This is only used as a signal to Enable/Disable the motor with the robot.
A black/blue connector goes to a DIO (blue=signal,black=ground) This produces a PWM signal to control the direction and speed of the motor.
Bare Red/white wires connect to the PDP (20a breaker) and provide the power.
If Tach/Direction wires have been added to the Nidec harness, then they each go to signal DIO pins. No ground or power wires are required.

There is a caution with the brushless wiring.
****** If the DIO connection comes loose, then the motor will race uncontrollably while the robot is Enabled. ******

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I’ll ask the obvious question: What leads you to using the Dynamo specifically? It can certainly get the job done with enough time, but there are several other motors that I’d reach for first that offer more power, a smaller package, or simpler wiring.

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So, it’s possible that you have a good reason for trying to build a climber with a Dynamo, but I’m struggling to think of what that might be. If you look at the Dynamo’s product page, you’ll see that it has a peak power output of just 44 watts (with a rather chunky weight of 1.5lb).
image

If you compare that 44 watts to the other FRC motors listed on Vex’s website, you’ll see that even the BAG motor (popular as a “light duty” motor for low-load tasks) has a peak power output over 3x higher than the Dynamo (and at less than half the weight). Most teams wouldn’t even consider climbing on a BAG motor, let alone a Dynamo. Our climber last year ran on two NEOs, and I would recommend using a CIM, MiniCIM, NEO, or Falcon 500 to power a climber.

It’s possible that you’re just building a Dynamo climber for kicks (or for the challenge of it), in which case have fun. Otherwise I’d recommend finding a beefier motor.

Math Time!

If you look at the product page linked above, you’ll see that the stall torque of the motor is 45 oz-inches, which is 2.8 inch pounds. Given that you don’t usually want to run a motor at more than 20% of its stall torque, you only have about 0.56 in-lb to work with. Assuming that you’re using a fairly typical “winding a rope around a drum” setup with a fairly typical 1 inch diameter drum and a fully loaded 150 pound robot, you need 75 in-lb of torque on the drum, which means gearing your Dynamo down 133:1. At that gear ratio, the drum will only be spinning at 16.2 RPM. If you’re trying to reach the highest point on the generator switch and not touch the ground if it swings down, that means you need to climb 28 5/8". A 1" drum spinning at 16.2 RPM will spend 33 seconds just to make the climb. That’s a quarter of the entire match length, and it’s also assuming that your robot is perfectly efficient (which it isn’t) so the actual time will be even slower than that!

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We must be weird for climbing on 1 BAG in 2017 and 2 BAGs in 2020.

Brave souls, you are!

Assuming your climber is relatively low on the mechanical losses, and assuming you gear it appropriately, a BAG will do the job. I prefer to go overkill on motor power though. Being right on the knife edge is nerve wracking, and there’s almost no penalty to using a larger motor given how lightweight BLDCs are these days. Our process for high load things like climbers is to design the motor and gear reduction to lift 1.5-2x what we need it to using the 10V power numbers for the motor.

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I find energy equations can be a more useful comparison here, if a bit hacky and inaccurate. (Nothing ever meshes perfectly with the spec sheet) They show that the power just doesn’t exist to make the climb fast enough.

ΔGPE (J) = m (kg) x g (m/s^2) x Δh (m)

A fully loaded FRC robot is around 150lbs or 68 kg.
Technically, you only need to climb an inch or so lifting that full load in 2020, but it’s functionally closer to a foot, or 0.3 meters on average.

68 x 9.81 x 0.3 = 200 Joules of work required to climb, or 200 Watt-seconds

Divided by the 44 watt max output power, that gives you a 4.5 second climb.

Doesn’t sound that terrible, but that’s with absolutely 0 losses and assuming you’re dead on max power. With most FRC climbing mechanisms you’re looking at somewhere in the ballpark of 60-80% mechanical efficiency, often due to strange loading conditions (that number gets real bad with leadscrews ofc). You can totally hit higher than that with careful design, but it’s not worth the effort when you can just slap a 300+ watt motor on the thing and climb in under a second regardless.

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Any motor can move any load given enough of a reduction; it’s just a matter of how long it takes to do so. Your BAG motor can definitely do the job, but you could gear more aggressively with a more powerful motor. BAGs are cool though so I get the appeal.

I didn’t use the power method because paints an unrealistically cheery picture of what the climb looks like with an underpowered motor, even if it’s easier to explain. We did a cool demo last fall of hooking a CIM up to some weights and timing it lifting them with different gear reductions. When you calculate the power required, you get pretty small numbers which makes it look very doable on a little motor like a Dynamo, but it ignores a lot of “real world” stuff.

To their credit, I watched a match video and their climb is pretty impressive. I never would have guessed that they were using BAGs to do it.

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I’d consider a 4.5 second climb to be on the edge of useless, but that’s fair enough.

If you’re not 100% on how much power should be dedicated to which things, poke around how other teams did similar things. A 2016 climb has similar power requirements to a 2020 climb, and 'most everybody that got to worlds had a 775pro or more for that (all the way up to 6 CIM PTO).

There’s a reason top level teams tend to throw as much power as they can at a climber: the next best thing to an after buzzer climb is a buzzer beater.

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I don’t like any single action taking more than a second or so, so I’d agree. That said, a 4.5 second climb would have plenty of value in an elims alliance (and a 4.5 second climb is a lot better than the 30+ second climb that a Nidec would be likely to make IRL).

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