Stepper Motors

Stepper motors are motors which turn due electromagnetic coils pulling the axle round in a circle.  There is an amination of this on this website:

Stepper Motor

This is useful for two reasons: you can make the stepper motor turn to a particular position and it has a "holding torque" meaning that as long as two of the magnets are energised it can hold something in position.

To make the stepper motor turn you must use a particular pattern to make sure that the coils energise in a circle.  To make the stepper motor turn clockwise you type the pattern in as seen, to make it turn anticlockwise you simply use the same pattern but in reverse:

i.e.

symbol delay = b0
let delay = 100

clock: let pins = %10100000
           pause delay
           let pins = %10010000
           pause delay
           let pins = %01010000
           pause delay
           let pins = %01100000
           pause delay
           goto clock

anti:    let pins = %01100000
           pause delay
           let pins = %01010000
           pause delay
           let pins = %10010000
           pause delay
           let pins = %10100000
           pause delay
           goto anti

Repeating like this will simply make the stepper motor turn continuously.   By using b0 as the delay, we can quickly change the speed of the stepper motor by changing the value of "delay"  The smaller the number the faster the motor will turn, but if you try to turn too quickly the magnets will not have time to pull the axle round so it may not turn at all.

To make the motor turn to a specific position, we must use a FOR . . . NEXT . . . loop.  This will allow us to repeat the pattern a set number of times.  Each line of the program is 7.5 degrees, so each 4 line pattern is 30 degrees.  Therefore to turn one complete revolution we must repeat the pattern 12 times.

i.e.

symbol counter = b0
symbol delay = b1
let delay = 100

clock: for counter = 1 to 12
                  let pins = %10100000
                  pause delay
                  let pins = %10010000
                  pause delay
                  let pins = %01010000
                  pause delay
                  let pins = %01100000
                  pause delay
            next counter


The stepper motor requires a 12v supply and the microprocessor can only supply a 5v signal.  Therefore we must use an external power supply (a black box) to connect to our output driver.

Speed Control - Pulse Width Modulation

Because the microprocessor gives a digital signal out - either 5v or 0v and nothing in between, we must use Pulse Width Modulation (PWM) to control the speed of a D.C. motor.

This uses the Mark to Space ratio to vary the speed - that is the time a motor is on for (Mark) and the time the motor is off for (Space).  This is in order of milliseconds so that it happens so quickly you can't see the pulse, only a constant speed.

Examples of the programs required for these graphs:

green: high 7                                             blue: high 7
           pause 20                                                  pause 10
           low 7                                                        high 7
           pause 10                                                  pause 20
           goto green                                               goto blue

Because the motor is on longer than it is off in the green graph, this is the program which will make the motor turn faster.  You can increase the mark to space ratio to get a bigger difference in speed.

Turning a flowchart into a PBASIC program

A flowchart is the planning stage for a program.  It is important to use the correct shaped box so that it is clear when you are controlling an output, or asking a question etc.  The flowchart is to help you understand the sequence so it is written in English.

The first flowchart is to practice asking about the condition of an input:

First you must choose which pins you are going to use as outputs and which as inputs.  Looking at the board you know that pin 7 is a red LED and 5 is a green LED. I'm going to use pin 0 as my switch.

So I need to tell the microprocessor this:

init: let dirs = %10100000     'Sets pin 7 and 5 as outputs

       symbol green = 5

       symbol red = 7

Now I can write my main program.  If I want to use English words, I must first of all tell the microprocessor this as well by setting the English words as symbols.  If you prefer not to do this, use the black commands, if you would like to do this, follow the purple programming language.

To ask a question I need to use an IF . . . THEN . . .  statement in relation to my input pin.  Then I need to tell the microprocessor which bit of program to go to when my input is on, and when it is not.  If the statement is not true, it will not go to the label which follows THEN but to the next line instead.

main: if pin0 = 1 then green_on       'Check the input pin, if it is on, go to the
                                                                    'green_on sub-procedure.
          goto red_on                            'If the input pin is off, go to the
                                                                    'red_on sub-procedure

red_on: high 7           high red          'Switch on the red LED
             low 5            low green        'Switch off the green LED
             goto main                             'Loop back to the start to test the input

green_on: high 5        high green      'Switch the green LED on
                 low 7         low red          'Switch the red LED off
                 goto main                        'Loop back to the start to test the input

The second flowchart incorporates testing an input and using a counter loop.  To repeat something a certain number of times, you must use a temporary memory file in the RAM to store the number of times the loop has been repeated.  Often we will use b0 for this purpose.  Again, if you want to call b0 something in English you must use the symbol command.

The PBASIC language required to repeat a sequence is a FOR . . . NEXT loop.  For must go at the top of the sequence and next at the bottom.  Anything you write in between FOR and NEXT will be repeated.

Motors, as described earlier in the blog, must be connected using the push-pull driver on the output driver.  7&6 control one motor, and 4&5 control another motor if you require it.  It is important to use the correct pairing to ensure that your motor will turn.

The program for this flowchart would therefore be:

init: let dirs = %11000000             'sets 7 and 6 as outputs
       symbol counter = b0

main: if pin0 = 0 then main     OR    main: if pin0 = 1 then motor   'both of these statements check
                                                                    goto main                       the input.
motor: for counter = 1 to 5           'Starts the loop to repeat 5 times
              high 7                              'motor clockwise on
              pause 10000                    'wait 10 seconds
              low 7                               'motor clockwise off
              high 6                              'motor anticlockwise on
              pause 10000                    'wait 10 seconds
              low 6                               'motor anticlockwise off
          next counter                        'If it has not been repeated 5 times, loop back to motor
          end

The third flowchart is in two parts.  This shows that the sequence uses a sub-procedure.  Where "alarm" is in the main flowchart, it means that the whole of the alarm flowchart fits into that box.  So from the main flowchart, the sequence goes to the start of alarm, and at the end of alarm it returns to the main program and follows the arrow back round to the start.

Here is the program:

init: let dirs = %10000000          'sets pin 7 as the output (buzzer)

main: if pin0 = 0 then alarm        'if the door is opened go to the sub-procedure alarm
          if pin1 = 0 then alarm        'if the window is opened go to the sub-procedure alarm
          if pin2 = 0 then alarm        'if the door mat is stepped on go to the sub-procedure alarm
          goto main                           'go back to the start and test the inputs again.

alarm: high 7                               'buzzer on
           pause 500                         'wait half a second
           low 7                                'buzzer off
           pause 500                         'wait half a second
           if pin3 = 0 then alarm      'test the reset switch, if it is not pressed then continue sounding the alarm
           return   /   goto main        'return to the main program.

Usnig Inputs

The next thing we need to learn about is how to use inputs in microprocessor control.

In real life there are a variety of different inputs we could test - switches, temperature sensors, light sensors, strain gauges (to measure how much something has been bent) etc.  In class we are going to start by using simple switches:

Using the connections given we can only use pins 0-3 without adding any extra wiring.

Now that we are dealing with both inputs and outputs we have to make sure we set up the DDR (Data Direction Register) correctly:

init: let dirs = %11110000     'Sets pins 7, 6, 5, 4 as outputs, and 3, 2, 1, 0 as inputs

To "test" an input, we must ask about its state, i.e. whether it is on or off.  In a flowchart we must do this using a rhombus box.  Each question must be YES/NO.

Here is a flowchart using inputs, for a toy train which should travel along the track until it reaches the end.  This system uses a start switch (pin 0) and an "end switch" (pin 1) as its inputs and a motor (pin 7) to drive the train.

This can be turned into a PBASIC program using "IF . . . THEN . . . "

init: let dirs = %11000000     'Sets pins 7 and 6 as outputs, and 5, 4, 3, 2, 1, 0 as inputs

main: if pin0 = 1 then train    'tests the start switch
          goto main                     'if the start switch is not pressed, go back to main

train: high 7                            'switches on the motor
         if pin1 = 0 then train     'tests to check if the motor has reached the end of the track
         low 7                             'switches off the motor.
     
         end

If the IF . . . THEN . . . statement is true, it will go to the label stated.  (following the then MUST be a label - another part of the program to go to).  If the statement is not true, it will skip to the next line of the program.  You can ask if the pin is '1' or '0' depending on what your flowchart says.

Microcontroller

First we looked at the architecture of a microprocessor:

ALU: Arithmetic Logic Unit: The "brain" of the microprocessor, reads the program and carries out the mathematical calculations necessary.

ROM: Read Only Memory - where the program is stored.  The type of ROM we are using is Electronically Erasable Programmable Read Only Memory, which means that to reprogram the microprocessor, all we have to do is connect is back to the computer and write over the old program with a new one.  Other chips may be once use only, meaning that if you make a mistake, or need to change a part of your program, you need to get a brand new chip.

RAM: Random Access Memory.  This is memory the microprocessor uses whilst the program is running.  The parts of the RAM are called b0 - b13.  You can ask it to remember any number, add and subtract, or count how many times a part of the program has been repeated.

Clock / Program Counter / Timers: Controls the speed of the mathematical calculations

I/O Ports: Connect the microprocessor to the real world so that we can control outputs and test inputs

Buses: Groups of wires which transport information from one part of the microcontroller to another.

Then we looked at the "Stamp Controller" we will be using for our programming.

To be able to use outputs with the Stamp Controller, we needed to add an output driver to boost the current from the microprocessor enough to drive output transducers like buzzers, motors and lamps.

There are two types of output driver.  The Darlington Pair at the top, which is used to drive anything which requires on/off control - lamps, buzzers; and the Push Pull driver at the bottom which is used to drive motors so that they can turn in both directions.  This means that a buzzer needs to be connected to one output pin, and a motor to two output pins - one to make it go forwards and one to make it go backwards.

Today we looked at pnuematics Revision. The one we did is under January.  Here is another examlple circuit to answer questions on:

1. Give the full name of all the valves/components.
2. What function do the three highlighted sub-systems perform?
3. Describe the operation of the circuit.
4. If the double acting cylinder has a diameter of 30mm and a rod diameter of 5mm; and the supplied air pressure is 5N/mm, calculate:
      a) The outstroke force
      b) The instroke force