6.111 Motors

Slide 1

Many of you will want to use motors in your project. Projects are much more appealing if they do something other than sit and blink lights. Motionis eye catching, whether it be a moving car, a pointomg device, a robot, or something else.

Slide 2

Often, it is easy to move something, one simply generates a force in the desired direction. However, it is hard to control the actual amount moved of the velocity of movement. For the moment. we will concentrate on controlling position,though a similar approach can be used for controlling velocity, etc.

The use of negative feedback is a powerful method for controlling a variable. One measures the cariable and subtracts it form the desired value and uses the resulting diference as an error signal to drive the system towards the desired value.

We see a block diagram of a negative feedback servo system along with its transfer function. To be useful, we require the transfer function to be stable. That is the zeros of the denomination must be in the left hand plane. If some zeros are on the j omega axis the system will oscilate. I they are close to the j omega axis then there will be a lot of ringing to a step change in the input position, x. We design such a servo system by controlling the position of the dominant poles and zeros, taht is the ones closest to the j pmega axis.

A desirable charactistic of this negative feedback system is that the output, y, settle down to the input, x, after a little while. This design goal is reached by putting an integrator into H(s).

Slide 2

Perhaps unfortunately, we do not have control of all parts of the servo. The system dynamics, resulting from the masses, springs, and losses of the system are often beyond our control. Some people actually tune these parameters, such as changing the springs or shock absorbers used in a car, but most of us digital designers are stuck with what we get.

We can, however, control the characteristics of a large amount of the servo system as indicated by the dotted area of the black diagram. Earlier, analog circuits (an even mechanical systems) were used to implement servo systems. We will implement our servop systems digitally within our FPGA. We must be aware of the fact that digital implementations can have significant delays, indeed these delays can sometimes be stochastic.

There are many different ways to measure the position of something.

Slide 2

There are two main analog ways to measure positon.

A variable resistor can be used. This resistor can be linear or rotary and it can vary linearly or logarythmically with position. This is a very simple method. The accuracy is determined by the potentiometer. The noise of the resulting signal is influenced by the power supply noise. Naturally, one should include a calibration step.

Another method is termed a resolver. Two sinusoidal signals are produced and are used as input to a system which then calculates the position. The resolver cosine and sine signals can be realized magnetically. This puts little load on the system being measured. Resolvers are not often used in 6.111 projects.

Slide 2

A common digtal way of measuring position is to use an LED, a phototransister, and a sensing ring.