Lecture 9

Uli Raich

UCC semester 2017/2018

stepping motors can be moved in defined steps and thus positioned very precisely.

They contain 2 coils which can be powered in positive or negative

direction making the current flow in normal or reverse direction

and thus creating magnetic fields of opposite polarity

The rotor has a series of magnets (16 in case of the 28BYJ-48 which we are using)

with alternating opposite polarization.

A north pole followed by a south pole followed by a north pole and so on.

The motor base has 2*16 teeth which can be polarized as

north or south poles depending on the direction of the coil current.

Here is the data sheet of the 28BYJ-48

which is insufficient to power the motor coils.

We therefore need a driver circuit (Darlington stage) to amplify

this current. In our case we use the ULN-2803 chip

The little PCB has 4 LEDs on it to show which of the 2 coils is

powered and in which direction. This is very useful to demonstrate which signals

are sent to the motor and it illustrates nicely the functioning of the motor.

We can see from the connections that when powering the pink line

the coil current is flowing in one direction while when

powering the orange line it is flowing in the opposite direction

south pole and the second one a north pole, which

makes the rotor (whose north magnet is considered) moves to the first tooth.

Then we switch off the first coil and power the second coil such that

the first tooth of the lower row has a south pole.

This makes the rotor move to this tooth.

Now switch off the second coil and switch the first one on again,

however this time with reverse current. The second tooth of the

upper row now has the south pole

(the poles are inversed with respect to the first step).

Now we switch on only the second coil with inverse current to make

the rotor move to the forth tooth. From now on the whole cycle repeats.

make the stepping motor move by 1 cycle, which is 4 steps in case of

Single Phase Forward stepping

How does this translate into a program?

We connect the 4 phases (coil 1 forward, coil 2 backward, coil 1 backward, coil 2 forward)

to 4 GPIO pins on the Raspberry Pi cobbler

Then we create the following signal table, which is a 2-dimensional array of booleans:

Up to now we have always powered only one coil at a time.

It is however possible to power both coils at the same time.

This will draw of course more current but the motor will get

a higher torque and can therefore handle bigger loads.

our program to send out the signals for one full cycle stays the same however.

from the contents of the stepping table, the code stays strictly the same

further than in single Step Forward because its north pole

is now attracted by the top and the bottom south poles.

By first powering only the upper coil, then powering both coils

then powering again only one pole of the lower coil it should

be possible to move the motor by half steps only

and by doing so increase its resolution.

This is what we call half step mode and it is a combination

of single and double step forward modes.

The torque of the motor is less than in double step forward

and the speed is only about half because now we need 8 steps for a full cycle.

a table of 8 entries instead of 4 the program does not change.

If we use the signature of all bits being zero

as end of table indicator then no change at all is required.

I explained in the first lecture on the Raspberry Pi

that there are 2 independent libraries available:

• wiringPi
• pigpio

• The standard one where calls to the library code
  results in direct access to the hardware
• The daemon version where first a daemon (pigpiod)
  is started and then the hardware access is only made in this daemon

sudo pigpiod

After this all hardware access can be make by a normal user

The program communicates with the deamon through library calls

which connect to the daemon through

• pipes (when running on the Raspberry Pi) or
  
  
• through sockets when running on a remote system
  like our Ubuntu PC

We can write C programs on the PC with hardware access

to our devices on the bread board through pigpio calls to the

PC version of the library which communicates

with the pigpio daemon on the Raspberry Pi

• There are functions to
• Initialize the library and close it
• Control gpio lines and read from or write to them
• Get a call-back when a gpio state changes
• Access routines to the I2C bus
• Access routines to the SPI interface
• PWM functions
• Functions for use of a serial device

Please convert this to what will be required in your source code.

send a high level to the GPIO pin to which we connected the coils.

However, some commands use very complex command line arguments

and the system provides a parser for it.

Have a look at getopt_long, which helps you in figuring out what the used wants.

The stepper motor program had these options:

• Select the movement mode
• Select the movement speed
• Select if you turn clockwise or counter clock wise
• Select how have to move of to which angle