Quote:
Originally Posted by adciv
I'm not sure if I missed it but I have one basic quesiton
How do you determine the model of the Motors and other devices which you have to purchase? To be clear, I'm wondering about the equipment which you aren't designing such as CIMs.
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The quick answer is your model is generic, and takes in parameters which describe your physical system. When you want to model something you purchase like CIMs you simply use the spec sheet, or measurement tools to determine the proper value of the parameters which get plugged into your model.
The long answer:
A DC motor is an electo-mechanical device which converts electrical current to mechanical torque. In order to fully model the DC motor you need to model the dynamics of both the mechanical side, and the electrical side of the motor.
What links the electrical model to the mechanical model is current. The torque provided out of the motor is proportional to the armature current determined by the electrical model and input voltage.
The electrical side of the DC motor can be idealized as a simple RL circuit. The mechanical side can be idealized as a spinning inertia load. You can also expand your model to include a gearbox on the mechanical side of your model.
When you create the mathematical model, which is just a set of math equations, you create it generically, where it is not tied to a specific design, but rather based on certain parameters which you can measure from your design and plug it in. Similar to the example I explained above about the projectile in 2 dimensions. The math equations are based on mass. It is not tied to a specific object, you can plug in the mass for any object and have a working model of that object under the same assumptions. This makes a robust model you can re-use over and over again, by just tweaking the parameters to match your new physical system. We do something very similar for the DC motor model.
The most basic models of DC motors all yield the same results and are based on the following physical parameters:
%Mechanical Parameters
(J) moment of inertia of the rotor kg.m^2
(b) motor viscous friction constant N.m.s
(Kt) motor torque constant N.m/Amp
%Electrical Parameters
(Ke) electromotive force constant V/(rad/sec)
(R) electric resistance Ohm
(L) electric inductance Henry
This model can be used to model any DC motor with the same simplified assumptions. If you want to apply this to a CIM motor, you can pull all of these values from the spec sheet or by directly measuring them from your motors in hand and plugging them into the model. Once you have those parameters, you have a model of your CIM motor system. If you want to add a model of a gearbox, simply add a proportionality constant which modifies the output speed/torque by the gearbox ratio.
Here is where I get most of my specs for FRC motors from, I usually measure the R and L parameters with a handheld meter:
http://banebots.com/p/M4-R0062-12
Also here is a rudimentary explanation of the math to create a DC model from MathWorks.
http://www.mathworks.com/help/contro...r-control.html
I have a whitepaper I made a few years back which I can upload which goes into further detail about modeling a DC motor with gearbox and dynamical load, and accounting for torsional stiffness, and load dynamics, yada yada. I'll try to find it, polish it, and upload it to chief.
I hope this helps,
Kevin
Edit: I wish there were a way to write proper equations on chief Delphi, I could just write out the math behind these explanations.