Aprendizaje ubicuo: el aprendizaje ubicuo (Cope y Kalantzis, 2010) representa un nuevo paradigma educativo que, en buena me-

One thing that happens when you take a motor and stop the rotor is a huge increase in current. Depending on the motor and your design, this could be more current than your control can handle. In this case you might need a cur- rent-limiting circuit.

This is a circuit that monitors the current used by the motor and, at a preset level, scales back the output, limiting the current available to the motor.

In a DC PM motor, when you control the current, you control the torque. Don’t believe me? Flip back a few pages and look at the torque constant equa- tions. The units are in oz per amp. This is a linear relationship—the more cur- rent through the motor, the more torque at the output shaft.

What all this means is that a constant current supply will create a constant torque when hooked up to a motor. This is essentially what happens when a control hits its current limit. The control goes from being a constant voltage supply to a constant current supply. This protects the motor and the control from damage.

Braking

Imagine careening down a hill on your electric scooter. “ Gosh, ” you think to yourself, “it would be nice to use some of the energy I’m wasting to slow this vehicle down. There ought to be a way to make it recharge the batteries.

G H IR Comp Measure Current Desired Speed FIGURE 4.15

Block diagram of IR comp feedback.

CHAPTER 4 The Real World

Hey, I’m an engineer, ” you say to yourself, “Why don’t I design a regenerative brake?” Just such a thought has come into my head and I have been able to ignore 13 _{it quite effectively until now.}

Some time ago I was asked to design a motor control with a regenerative brak- ing circuit. Having done several controls, but none with regenerative braking, I started by perusing the Internet. I don’t follow Star Trek’s creed to boldly go where no man has gone before on a whim. That is to say, if someone has been there already, I would sure like to know the path he or she took. Once the end of that path has been found, I will then venture into the unknown.

Anyway, in this case, several hours of searching were somewhat futile. A simple and concise explanation and possibly a schematic (particularly for a PM DC motor) were all I needed. There were reams 14 _{of information explaining what it}

does but not much was there showing exactly how it was done. Alas, my effort to fi nd the simple explanation was to no avail. Maybe it was out there some- where, but I got sick of all the pop-ups.

As you might have guessed by now, I take such a lack as a personal affront that I must correct. The following is what I have pieced together in my own mind, dis- tilled down to my level of intelligence (the longer I spend in management, the lower this level seems to be), then ousted to my readers in a form I hope is easy to understand. After I looked at the best idea since raw toast and the nice read about the Honda Insight’s regenerative brake, the following is what came out.

No More Secrets!

One place I found said that regenerative braking is the well-kept secret of motor control. However, when I learned the truth, I think it is just poorly explained. Let’s start with Figure 4.16 (see next page), a diagram of a simple PWM controller for a DC PM motor.

A PWM is fed into a switch, such as a MOSFET, at a frequency that is high enough to keep current fl owing in the inductor inside the motor, not at all unlike a switching power supply. When the PWM shuts off, the current fl ows through the diode (sometimes referred to as a freewheeler diode). That part I could understand, but the question that I kept asking myself was how do you

13 _{I fi nd it very easy to ignore such thoughts when I am playing Nintendo (or Xbox 360).} In fact, back in my college days, I had to redo an entire quarter of school due to a severe Nintendo addiction (except for one class that I passed due to a very persuasive paper on said topic). But we’ll save that story for some other time.

14 _{Can you use the word reams when referring to the Internet? After all, it isn’t really on} paper, is it?

get a motor, which is spinning at a lower voltage than the output of the battery, to push current back into the battery?

Let’s start with a small change to our earlier circuit, as shown in Figure 4.17 . We will replace the diode with a synchronous switch that goes off when the primary

PWM I2 I1 Battery FIGURE 4.16 PWM motor control. PWM Battery Inverter FIGURE 4.17

PWM motor control with FET in place of diode.

CHAPTER 4 The Real World

goes on, and vice versa. For the purpose of this discussion we will ignore the fact that the FETs need particular driving methodologies for the high side and the low side of a motor.

I had read about this topology many times. It is usually brought up as a way to make your controller more effi cient in terms of heat loss. This is because the FET has a signifi cantly lower voltage drop across it than the diode does. I had no idea that it also functions as a regenerative brake, until I fi gured it out for myself. Here is how it works.