[Mon Feb 13] Positive/negative feedback, LTSpice, motor

First thing first, after the class, I looked up a couple terms on Wikipedia. Here is the summary:

1. Positive/negative feedback: the end result of positive feedback is to amplify/attenuate so that small perturbations may result in big/small changes. This concept apparently also applies to other sciences or even economics. In the electronics world, "if the signal is fed back out of phase, the feedback is negative and if it is in-phase the feedback is positive."

2. Schmitt trigger: a generic name of threshold circuits with positive feedback having a loop gain (the product of the gain in the feedback loop and the feedback factor in that loop, still not very clear what that means) > 1. The circuit is named "trigger" because the output retains its value until the input changes sufficiently to trigger a change.

3. Hysteresis: the dual threshold action by a schmitt trigger (under positive feedback).

These clear my confusion a little bit since I found myself losing track of all the terms that we learned in class.

Anyway, today Oscar introduced us the concept of positive feedback and negative feedback. Their meanings are clearly described by Wiki. The important thing about them is that:

1. The circuit that we built expressed a "jumping" pattern (hysteresis) because it has a positive feedback.

(From Wiki's entry: Schmitt trigger)

2. When we have a negative feedback, however, we don't see the jump. Instead, we have the equation of V+ = V- (-6V < V+ < 6V). And in the range of -6V < V+ < 6V, there is a line connecting the two end points (as shown below).

3. Conclusion: in positive feedback, we don't see V+ = V-; in negative feedback, we do.

Then we moved on and used LTSpice to simulate positive and negative feedback. Now we know that LTSpice is a cool tool that enables us to virtually build a circuit. It has all the elements in a circuit (power source, resistor, wires, capacitor, etc.) Oscar directed us to build a positive feedback circuit first and ran the running man.

Our circuit looks like this:

After fixing a couple problems such as x-axis unit, we got something like this:



(From Wiki: Schmitt trigger)

And this is our own graph:

Then we went on and switched the positive and negative and built a negative feedback circuit. We saw the pattern in the Schitt trigger picture shown above.

Then Oscar encouraged us to simulate the circuit that we build last Thursday with the capacitor. However, as we are trying to cut some devices and wires, our Mac had some difficulty problem beginning with freezing. We restarted the Mac and went into VirtureBox. That's when things went really bad and the computer spent 20 minutes deleting itself and eventually we "killed windows" - Oscar... Because of that, I didn't get any of the screenshots and saved circuits of LTspice whatsoever...

That was that...

Then Oscar introduced motor to us. We learned that when we exert a force perpendicular to a stick, we generate a torque (T= F*d, d is the length from the center of rotation to the point where the force is exerted). Then we learned that the power of this mechanical system is P mech = T * w (angular speed) (note that electric power Pelectric = V * I). Then we learned about how torque works in a motor. A motor has two ends (positive, negative). The motor itself has some resistance, therefore the electrical energy cannot be fully transformed into the mechanical energy of the motor. In fact, Vemf (electromotive force) = K * w (angular speed) and torque of the motor T = K * I of the motor (the two constants are the same because of the conservation of power).

Our job is to think of a way to get rid of the effect of the internal resistance of the motor. We have a special setup to deal with that, which I didn't understand at the time but understood it more when it was explained in detail in the next class. Therefore, please see the next entry for the explanation of the setup. The basic idea is that we will try to control the voltage by controlling the correct combination of voltage division and generate Vemf out of the box. I hope Oscar can review this part on Thursday (which he said he would) because I haven't fully understood to an extent that I can explain it here... But the big idea is: speed generates voltage while current generates torque.

That was an intense but interesting class. Excited to move on to mechanics soon.