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Now I am the one that is confused. We started out looking at linearity and then progressed to start configuration and simulations.
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Ok.. lets regroup.
My objective was to describe what causes the JAG to be linear.
I first conjectured that it was probably due to the commutator segment seeing more JAG pulses than the Victor.
The first excel I did tried to model the segment switching and showed that there was no difference between the Victor and JAG... but I goofed and assumed that the currents headed for zero rather than -Vemf/R in OFF phase. This was an important oversight and made all cases look like the victor.
In rethinking the problem, it became clear that the diode in the OFF phase was allowing the inductor to "run dry" (I think these were Eric's words) for the Victor but not for the JAG. This is because L/R is short with respect to the Victor period and the current decays and is zero for most of the OFF phase. The JAG on the other hand looks like a pure inductor because the frequency is so high (in an A/C ckt its impedance would dominate the R) and just integrates the voltage pulses . The current doesn't decay much in the OFF phase so the current builds up until the motor moves. Vemf comes into the picture and there is a transient but the current never goes negative and the diode is always conducting. This led to my two little equations:
i_free*R = (12-Vemf)*duty for the Victor (behaves like a diode in OFF phase and all current comes from ON phase duty cycle)
i_free*R = 12*duty -Vemf for the JAG. (behaves like a short in OFF phase
so current can remain in the inductor and we can model it as a simple linear low pass ckt with a chopped 12 v and DC Vemf)
These equations match the measured performance amazingly well!!!
My DOOH moment came when I assumed the Victor was running in a coast mode (with diodes) and the JAG was running in the brake mode (shorted)during the OFF phase since this seemed to match the results we were getting.
You reminded me that there is no difference between the coast and brake mode under normal operating conditions. So... rather than go back to the excel and try to model a complicated switching problem I just wrote a basic LR switching simulation to see how it would match the results of the simple equations. The first LABVIEW program showed speed vs duty for Victor 120hz,2k hz and JAG 15k hz. The plots looked realistic and I was happy(
http://www.chiefdelphi.com/forums/at...&d=1242511867) To make these plots, I ran the time simulation to steady state. I had to fool with L but the final value was within reason with old CD posts.
Eric wanted more explanation so I modified the program to output the waveform time history and it really helps others visualize what's going on. It hasn't helped you since my LABVIEW choice wasn't good for your team. I could have done it five times faster in C or MATLAB!!
It is important to note that my model has only one diode in the OFF phase and it returns current in a ground loop involving the motor and the diode.
Enter Al... seemed almost all your comments involved two diodes and a 12 v return path for the current and it would never jive with what I was showing. Of course you educated us on many other things along the way as well. But this was the biggest issue in my eyes.
To clarify the circuit, I generated some flow graphs in the ppt showing my high side switching model with the one diode return loop plus an "Al" model that had two diodes and 12 v current return path hoping to nail this down. You responded by elaborating on the "AL" configuration and said nothing on the single diode return so I assumed that this was your Hbridge return model.
We have yet to agree on this unless I'm missing something
I am not naive enough to believe that you can simulate any real Hbridge without taking into account all the effects you mention. But, it is my assertion that all these effects are secondary to the problem of linearity and the simple LR switching model does a good job in explaining the phenomenon and thats all you can ask of a model.
This is why I encouraged you to do the simulation. My simulation code is basically written in C and wrapped in a LABVIEW while loop(see spoiler in
http://www.chiefdelphi.com/forums/sh...9&postcount=35.) I was hoping you would examine the differential equations , which include inertia dynamics and coulomb friction effects and run something similar in C , however, you'll have to substitute for the nice LABVIEW graphics. We would all like to see some real lab data in addition to the Beach Bot data I posted which showed the simplified model matching to within 5% by adjusting i_free to match installation friction. Hopefully, things will settle down in your work and give you some time to tinker and that this post didn't confuse things further.
Post note: I was never really concerned about the start transient. All my simplified equations address the steady state. After I posted my LABVIEW plots showing how well the steady state simple LR model worked, you posted your start up excel transient. This was perfectly accurate but the plots seemed sooo misleading because there was a linear one for the JAG and a nonlinear one for the Victor. Although you did not claim that that was the reason for the phenomenon...it just looked like it was key. So I jumped on it and pointed out that the main reason for the phenomenon was the difference in the OFF phase currents which were not represented there.