Lecture 9 Uli Raich UCC semester 2017/2018 Stepping Motors Lecture 9 Uli Raich UCC semester 2017/2018

The 27BJY-48 stepper motor an its ULN-2803 driver module

Stepping motors versus DC motors While DC motors simply spin when they are powered, 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 28BJJ-48 data sheet

The driver The maximum current that a GPIO pin on the Raspberry Pi can deliver is 60 mA 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.

Connecting the Driver to the Motor As 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

Movement Single Phase Forward Coil 1 Coil 2 Rotor North Pole

Signal Table for Single Phase Forward We have seen in the last slide how we have to power the coils to 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: Step 1 Step 2 Step 3 Step 4 In 1 coil 1 + 1 In 2 Coil 2 - In 3 Coil 1 - In 4 Coil 2 +

Other modes of operation Single Step forward (and backward) works fine but we can do better. 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

Movement Double Phase Forward Coil 1 Coil 2 Rotor North Pole

Double Step Forward Table Of course our signal table must change correspondingly, our program to send out the signals for one full cycle stays the same however. Step 1 Step 2 Step 3 Step 4 In 1 coil 1 + 1 In 2 Coil 2 - In 3 Coil 1 - In 4 Coil 2 +

Programming double step forward mode For this, nothing much is to be said: We change the table, but since the program is independent from the contents of the stepping table, the code stays strictly the same

Position of rotor in doubleStepForward mode An important point to note is that the rotor moves half a step further than in single StepForward 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.

Movement Half Step Forward Coil 1 Coil 2 North Pole

Half Step Forward Table In 1 coil 1 + 1 In 2 Coil 2 - In 3 Coil 1 - In 4 Coil 2 +

Programming Half Step Mode Again, apart from the fact that we must now pass through 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.

The pigpio library Up to now we always used wiringPi to access our GPIO pins. I explained in the first lecture on the Raspberry Pi that there are 2 independent libraries available: wiringPi wigpio Why would we be interested to use a different library?

Advantages of pigpio The pigpio library comes in 2 versions: 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

Accessing the daemon The deamon is started once before we make any hardware access: 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 There is a library version that runs on the 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

Documentation of pigpio The WEB page of pigpio can be found here

Types of functions in pigpio Going through all the function in pigpio during the lecture is impossible. I have not used the all neither! 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

Features of pigpio

This is how to use the library Please convert this to what will be required in your source code.

Initializing pigpio You must include #include <pigpiod_if2.h> … and the you can happily use pigpio functions

Sending the pulses to the hardware Now all we have to do is to go through this table and, for each step, send a high level to the GPIO pin to which we connected the coils.

Going through one step cycle

Reading command line arguments You say: This we have seen before and you are right. 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:

Using getopt_long First we include <getopt.h> Then we create a table of options (last entry must be all zeros:

Call getopt and interpret

Write the stepper program With this information you should be able to write a program where the user can 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