Negative Feedback Feb. 13

Although we ended this class session with an introduction to motors, I want to dedicate this blog to class discussion in the first half. We discussed "trick number four" in engineering, feedback. 

Positive feedback is when you connect the output of an op amp to the (+) input . Negative feedback is connecting the output to the (-) input which is feeding back the voltage. With negative feedback in an op amp, Vout varies so that the difference between the inputs (V+ - V-) is close to zero. In other words V+ = V- since their difference is "0" only when -12V <Vout<12V. This is true because V- = Vout/2= V+. An interesting thing about the graph of negative feedback is that, unlike positive feedback, there is a slant connecting (-6V, -12V) to (6V,12V).

Using LT Spice, we made a circuit with an op amp with negative feedback. In the picture above you can see the slant.

Hysteresis: Vo (green line) switches between -12V and 12V as V- (red line) switches from 6V to -6V.

If you are still curious about motors, read my next blog!:)

Motors Feb. 16

Understanding motors means more math and abstract thinking. A basic concept about motors is that they measure speed. Speed generates a voltage and current generates a torque. In a motor the power applied is equal to the input voltage times the input current ( Pin = Vin x i in). The output of the power is T (torque) x ω (omega which is believe is a constant). You obtain the torque by multiplying the force produced to cause the rotation by the distance of the bar of the motor.

Inside a motor there is a resistor making a potential difference when the current runs through. The voltage insides the motor is the resistor multiplied by the current plus the Vemf. After passing through the motor the voltage switches from positive to negative (- resistance x current - Vemf). I am still confused about how voltage in a motor so I will further add more information about them in the future.

For our setup, we used three resistors, two potentiometers, a motor, and an adaptor. When my partner and I turned the first potentiometer we were able to see how the axle would start to spin. The motor axle would only spin when we would turn the potentiometer a little past the half way point. At the half way point the motor would stop. It was exciting to control the direction and speed of the motor.

Motor setup

Professor Mur-Miranda asked us to hold on to the wheel, turn the knob on the potentiometer, and look at the graph in the oscillator. The graph showed us how the speed was at zero but there was current still running. I was able to feel the tug of the wheel as I was preventing it from turning. The hands on motor learning helped me understand the make up and function of toys with wheel since the motor with the wheel is very similar to toy remote cars.

Motor with wheel

Capacitor Feb. 9

At the beginning of class we reviewed hysteresis which was helpful because I am able to understand the concept a bit better. Here is my attempt to briefly go over the concept. If the input voltage is greater than the out-put voltage (V+ > V-) then the out put voltage is equal to their difference (V+ - V- = 6V- -6V = Vout = 12V in this circuit). Likewise, if the input voltage is less than the out-put voltage (V+ < V-) then Vout = -12V. An op amp with positive feedback would be given by the equation Vout / 2. Thus if the out-put voltage is less than 6 V (V- < 6V) then Vout = 12V but if the output voltage is greater than 6V (V- > 6V) then Vout = -12V. This "relationship" between input (V+) and out-put (V-) forms the pattern of a hysteresis graph.

The big topic of the day was capacitors! A capacitor stores energy as an electric charge (accumulates current and is expressed as voltage). In a circuit the current most keep going so what ever current goes in must come out. As t time approaches "infinity"the current goes to 0 near the voltage boundary line. In other words, the capacitor will charge until it reaches the max voltage. The graph will rapidly increase and then slow down when gets closer to the max voltage.

Here you can see the graph (yellow line) move up and down between the max(12V) and min (-12V) as the voltage shown in blue reaches the limit.

After building the model of the circuit we were able to once again see how Vout switched back and forth between -12V and 12V. Later we added a noise piece which we were able to control with the potentiometer.

Circuit without the noise piece.

Hysteresis Feb. 6

We began class by discussing how our lack of knowledge in engineering or physics does not withhold us from understanding the concepts presented. Our professor told us to be confident in what we should know about power, voltage, and current based on previous lessons. We learned that in engineering there are abstractions. An abstraction in engineering is the concept of hiding major details so that you can focus on a few regions. In other words you do not need to know the make up or every detail about the object such as a software program, as long as you know what it is or does. An example is not knowing the complicated coding behind excel; all you need to know is that it is a spreadsheet where you to make calculations and graphs.

We learned that there is a distinction between math and reality. In the real world, a complete circuit without a resistor shortens the power source. However, in mathematical terms this circuit is incorrect since there is no voltage difference (a resistor divides a voltage). The role of a resistor is to limit the amount of current running through the circuit. It allows us to measure current.

After reviewing and going over resistors, we got down to business. We set up a circuit on a breadboard with potentiometers (blue knobs below).

circuit set up

We were able to see how the voltage jumped from +12V to -12V as the input voltage (V+) switched from greater than the output voltage (V-) to less than V-. This "motion" is known as hysteresis.

Hysteresis credited to DC/ AC Circuit Reference

Here is my attempt to explain this concept. Once the line crosses its limit at either 12 volt or -12 volts, the output voltage will jump in the opposite direction.

Week 1: Intros & Oscillator Jan. 30 & Feb. 2, 2012

     To summarize the first day of class, we discussed the reasons why we enrolled in the class or why we are drawn to engineering. The reason why I decided to take this class was to learn about the different disciplines of engineering and I was always curious about what exactly do engineers do. We got to look at different toys, and discuss the basic structure and function of them such as the talking mouse. Our professor, Oscar Mur-Miranda, told us that engineers only know the answers to a few problems and then apply what they know about the field to try to find solutions to other problems. I can agree with this concept because it is similar to knowing a certain skill and using it in multiple situations. He later told us that developing countries have three inhibiting factors: economical (how wealthy is the country), political (does the government promote technological advancement), and technological issues (are the materials and technology available).

The first lesson we had was on electric fields, voltage (volts), and current (I). While I understood that current in amps was how much charge is passing a point per time, understanding voltage was more complicated. It was difficult to understand that voltage is the difference in electric potential to do work between two points. An example for this concept is batteries. Assuming that the negative end is at 0V and the positive end is at 1.5V, the difference in electrical potential would be 1.5 V (1.5 - 0 =1.5). More volts in a wire means that there is more current flowing. Using an oscilloscope, we were able to see voltage versus time and see that there is a frequency of 60Hz for electrical appliances in the U.S.

The main idea for the following lesson was power which is equaled to the current multiplied by the voltage. Shortly after discussing power, we had our first hands-on learning with breadboard circuits. This was exciting because we had to setup the circuit by placing the pieces onto the breadboard correctly and then see the voltage of the circuit by using the oscilloscope. My partner Cailey and I were able to see the maximum voltage of +12V in the circuit and +5V. However we were unable to see the minimum voltage at -12V. We tried messing with the wires and turning the knobs on the oscilloscope but had no luck. Later Erin explained to us
that the wires connecting ground and -12V to the power source were flipped. Once we flipped the wires we were able to obtain the -12V.

I can't wait to learn more about circuits and engineering in general so that we can start building and experimenting with the equipment!