Modeling motor control

[kamocat]

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?

[kamocat]

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) ].

[Hawiian Cadder]

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.

[kamocat]

Originally Posted by Hawiian Cadder

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.

[Ether]

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.

[kamocat]

I'm not sure why my data differs so vastly from those assumptions.

[Ether]

Originally Posted by kamocat

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 ?

[kamocat]

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.

[Ether]

Originally Posted by kamocat

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.

If you run a CIM with a toughbox, what do you expect to compare that data to? There is no manufacturer's data for this. Still apples and oranges.

I have a globe motor, but the current is too small to measure with a Jaguar.

If you run the CIM w/o the toughbox, the no-load current is too low to measure with any accuracy with the Jag, isn't it?

[kamocat]

The current just needs to be greater than 1A to be measured.

The point of measuring a CIM with a toughbox is to see if it still has these strange current characteristics.

[Ether]

Originally Posted by kamocat

The current just needs to be greater than 1A to be measured.

Per the datasheet, the accuracy of the Jag's current measurement is +/-2 amps for currents less than 8A. The no-load current for the CIM is 2.7 amps.

The point of measuring a CIM with a toughbox is to see if it still has these strange current characteristics.

The point of measuring something for which you have manufacturer's data is to validate your test method. It is possible that the funky data is a problem with your test method.

Just sayin'

[theprgramerdude]

Would it be better to hook up (not really) a magnetic current monitor outside the wires to measure current that way? It might have a more accurate current reading, if you can find a model that can handle such low currents and ignore any interference from the motor.

[Ether]

Originally Posted by kamocat

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?

[Hawiian Cadder]

Originally Posted by Ether

Why is the plot of current a series of horizontal lines?

account for error i believe.

[Ether]

Originally Posted by Hawiian Cadder

account for error i believe.

If he is setting the voltage and measuring current, I would have expected the error bars to be vertical.