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Unread 02-01-2011, 17:47
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Modeling motor control

I'm trying to calculate the torque of a motor based on the voltage, speed, and current.
I'm actually interested in the torque at the output of the transmission, not simply at the output of the motor.
This means I need to know the no-load current of the motor.
I was thinking no-load current was proportional to voltage. Apparently it's not.

Here's a graph:


This is measured with a Jaguar over CAN, though I've confirmed the approximate current trend with a Fluke 179. (I didn't care to take 10,000 samples by hand)

Some curious things about this:
There is a sudden drop in current at 10v (about 16800 RPM on the motor shaft)
At this drop, the sound of the motor changes. Before, it is somewhat buzzing, and afterwards it is quiet and smooth.
After the sudden drop, current continues to decrease.
Speed remains proportional to Voltage, regardless of current.

Some more about my setup:
I'm using an rs550 motor with a Banebots 20:1 planetary transmission. This is attached via chain and sprockets (at a 2:1 ratio) to a small wooden wheel. (The wheel serves as a temporary until I get around to making mecanums for my half-size testing 'bot.)


I'm guessing this particular point is specific to this motor, but has to do with the speed of the motor (as opposed to the duty cycle of the Jaguar).


Have you ever seen this before?
What do you think is happening?
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Unread 03-01-2011, 18:11
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Re: Modeling motor control

Here's a drill motor, with no gearbox or pinion attached . I think it's from a 6v Ryobi.

Unfortunately, there's no encoder on it, so I'll have to assume the speed is proportional to voltage.
The current curve seems a lot more regular; there's less noise, and it looks like a continuous function [perhaps 1/x*log(x) ].
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Unread 03-01-2011, 18:19
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Re: Modeling motor control

i think that the current has that weird curve is because there is not enough load for it to run at near no load, but there is not enough load for it to run consistently at multiple speeds, try adding some load to the motor and see if that does anything.
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Unread 03-01-2011, 18:39
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Re: Modeling motor control

Quote:
Originally Posted by kamocat View Post
Here's a drill motor, with no gearbox or pinion attached . I think it's from a 6v Ryobi.

Unfortunately, there's no encoder on it, so I'll have to assume the speed is proportional to voltage.
The current curve seems a lot more regular; there's less noise, and it looks like a continuous function [perhaps 1/x*log(x) ].
Why is the plot of current a series of horizontal lines?




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Unread 03-01-2011, 18:59
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Re: Modeling motor control

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Originally Posted by Ether View Post
Why is the plot of current a series of horizontal lines?





account for error i believe.
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Unread 03-01-2011, 19:01
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Re: Modeling motor control

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Originally Posted by Hawiian Cadder View Post
account for error i believe.
If he is setting the voltage and measuring current, I would have expected the error bars to be vertical.



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Unread 03-01-2011, 19:10
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Re: Modeling motor control

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Originally Posted by Ether View Post
If he is setting the voltage and measuring current, I would have expected the error bars to be vertical.


that is a good point, perhaps it is just a method for filling in an area, but i would think that would make it a solid color.
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Unread 03-01-2011, 19:25
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Re: Modeling motor control

I suspect the horizontal lines are due to quantization error in the current measurement and exacerbated by a large number of data points.
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Unread 03-01-2011, 19:40
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Re: Modeling motor control

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Originally Posted by Joe Ross View Post
I suspect the horizontal lines are due to quantization error in the current measurement and exacerbated by a large number of data points.
Good point Joe. That would explain it.

@Marshal: can you confirm this? Is the resolution of your current measurements ~130ma ? Also, is there noticeable change in the sound of the motor as the peak of the current curve is reached and the current starts to decrease?



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Unread 03-01-2011, 19:49
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Re: Modeling motor control

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Originally Posted by Ether View Post
Good point Joe. That would explain it.

@Marshal: can you confirm this? Is the resolution of your current measurements ~130ma ? Also, is there noticeable change in the sound of the motor as the peak of the current curve is reached and the current starts to decrease?


Yes, that is the resolution. I can't get any data between those increments.
(By the way, there's 10,000 points of data on these graphs. They're in power increments of 1/1000, with 10 samples at each increment)

The data in these graphs have not been processed, aside from scaling speed to account for the gearbox ratio and encoder placement.

If you're talking about the second motor, yes there is a change in sound, but it's a lot harder to hear because there's no sudden change.
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Last edited by kamocat : 03-01-2011 at 19:55.
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Unread 03-01-2011, 20:28
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Re: Modeling motor control

Quote:
Originally Posted by Hawiian Cadder View Post
i think that the current has that weird curve is because there is not enough load for it to run at near no load, but there is not enough load for it to run consistently at multiple speeds, try adding some load to the motor and see if that does anything.
How should I create additional load in a repeatable way?

What would this tell me?

EDIT:
Are you saying the motor speed isn't repeatable?
Based on the first graph, we saw that the speed is directly proportional to current, with fairly tight control. It's only the current that is doing funny things.
I can tell you that at full voltage, it's operating pretty close to no load, under peak efficiency. Below that speed, I have no data. That's why I'm doing this test.
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Last edited by kamocat : 03-01-2011 at 20:41.
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Unread 03-01-2011, 20:45
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Re: Modeling motor control




Marshal, I know you have read this thread, but for others who have not, it provides some additional background discussion and links to external info about no-load current vs voltage.


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Unread 03-01-2011, 21:05
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Re: Modeling motor control

I'm not sure why my data differs so vastly from those assumptions.
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Unread 03-01-2011, 21:06
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Re: Modeling motor control

Quote:
Originally Posted by kamocat View Post
I'm not sure why my data differs so vastly from those assumptions.
Perhaps because you are comparing apples to oranges.

Do you have an FP or a Globe motor, like the ones referenced, that you could run on your test setup ?


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Unread 03-01-2011, 21:11
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Re: Modeling motor control

I have a globe motor, but the current is too small to measure with a Jaguar.
I don't have any Fisher Price motors (but plenty of different drill motors).
I'm going to try to get ahold of a CIM with a toughbox and encoder, but I may not get that for a few days.
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