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Mixing Jaguars and Victors
Is there any danger in using one Jag and one Vic to control a pair of CIMs that are connected to the same ToughBox? Using C++ four-motor Tank Drive from WPILIB.
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Re: Mixing Jaguars and Victors
Bad idea. The jaguars and Victors have different output curves, and the Victors are much more non-linear, so the motors will not be running at the same speeds at low speeds. Software makes no difference.
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Re: Mixing Jaguars and Victors
That was my concern, I will find a work-around. Thanks!
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Re: Mixing Jaguars and Victors
Actually, I don't think this would be as bad as you think. The motors are mechanically coupled in the toughbox, so they will run at the same speed regardless. The only issue would be unequal sharing of the load, but I suspect that due to differences in motors the load is never perfectly balanced, even with identical speed controllers. They key obviously is to have the Victor and Jaguar run the motors in the same direction :)
On that note, I would say that the way things are done in FIRST is far from ideal. In industry, you would typically see a single motor sized large enough to handle the load. Or you might come across multiple motors connected in parallel to the same driver. Or motors electronically synchronized with encoders. But having separate drivers control two mechanically coupled motors is really asking for trouble. All it takes is a wrong connection or incorrect code to run the motors in opposite directions, which would stall both of them. And when motors stall, bad things happen. |
Re: Mixing Jaguars and Victors
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Re: Mixing Jaguars and Victors
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I'd recommend against the Jaguar + Victor on one toughbox scenario. If you command e.g. 50% forward and hit free speed, you will have a motor/generator situation with potentially large currents. I don't believe it will destroy either controller, but it will be extremely inefficient. |
Re: Mixing Jaguars and Victors
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Re: Mixing Jaguars and Victors
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He's not talking about FRC applications. What if you had two starter motors for your car, say, and they had slightly different turn speeds? |
Re: Mixing Jaguars and Victors
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Ideally, you would match the speeds of the motors and let the loads balance themselves according to output torque at that operating point on both motors' curves. However, that is easier said than done (due to difficulties in measuring each motors' free speed at a given voltage, for example). If permitted, one way to do nifty load balancing is to put both motors in a series circuit - when one motor gets overloaded, its back EMF decreases and more electrical power becomes available to the other motor (an electronic differential). |
Re: Mixing Jaguars and Victors
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Connecting two 12-volt motors in series, and then powering them with a 12 volt supply reduces the efficiency of the motors. ~ |
Re: Mixing Jaguars and Victors
They are always at the same speed, yes. Jared was talking about efficiency, not necessarily speed.
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Re: Mixing Jaguars and Victors
One little item no one has mentioned here is the switching frequency of the individual controllers. The Victor is 150 Hz while the Jaguar is 15 kHz. At 50% throttle the Jaguar would switch an additional 50 times during the period that the Victor is turned off. Although the motors are linked and there would certainly be some flywheel action provided by the transmission and coupled mechanism, I would suspect this would be bad for the Jaguar. The net result on the mechanical output is dependent on the stored mechanical energy in the rest of the system. As to differences in linearity, the two motors are coupled and the result would be a combined speed slower than expected. As the two motors both will supply power, it is a matter of which is providing more power. In this case I believe the Victor supplied motor will be contributing the majority of the power below about 75% throttle. At full throttle there is little difference.
In the case of the series wired motors, the current is theoretically the same in each motor. However, the production variations in inexpensive motors will affect the output power vs input current. The result is that the power output of the transmission is never fully shared by each of the motors. One is always providing more power than the other. |
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