Full Circle Magazine FR

Welcome back for another entry into what I lovingly call ‘Greg’s Python Folly’. As promised, we will be working on interfacing a stepper motor to the Raspberry Pi. You will need your Raspberry Pi, a hobby stepper motor, a 4 x AA size battery pack, the L293D driver chip we used previously, a breadboard, and some jumpers. While I was doing research for this particular project, I stumbled across a tutorial at tutorials-raspberrypi.de. I was so impressed by the information at this website, I am using the majority of their information and code in this article. The website is: http://tutorials-raspberrypi.com/how-to-control-a-stepper-motor-with-raspberry-pi-and-l293d-uln2003a/. If you get confused by my explanations, you can always drop by and maybe get some clarifications. The motor I chose is a Radio Shack mini stepper motor. Basically it is a 28BJY-48 low-voltage stepper. Before you try to interface any stepper motor, please research the data sheet for as much information as you can get. In this case, the data sheet is located at: http://www.tutorials-raspberrypi.de/wp-content/uploads/2014/08/Stepper-Motor-28BJY-48-Datasheet.pdf Now, let’s examine stepper motors in general, then we’ll expand that information to the 28BJY specifically and work on interfacing it to the Pi through our L293D driver chip.

Bienvenue de nouveau dans ce que j'appelle affectueusement «La folie de Greg pour Python». Comme promis, nous allons travailler à connecter un moteur pas-à-pas à un Raspberry Pi. Vous aurez besoin de votre Raspberry Pi, un moteur pas-à-pas de loisir, 4 piles AA, la puce de commande L293D que nous avons utilisée précédemment, une platine d'expérimentation, et quelques cavaliers.

Alors que je faisais des recherches pour ce projet là, je suis tombé sur un manuel sur tutorials-raspberrypi.de. J'ai été tellement impressionné par les informations de ce site web que j'utilise un majorité de leur renseignements et de leur code dans cet article. Le site web : http://tutorials-raspberrypi.com/how-to-control-a-stepper-motor-with-raspberry-pi-and-l293d-uln2003a/. Si vous êtes perdu par mes explications, vous pouvez toujours y aller et obtenir quelques éclaircissements.

Le moteur choisi est un mini-moteur pas-à-pas Radio Shack. En fait c'est un moteur basse tension 28BJY-48. Avant d'essayer de connecter un moteur pas-à-pas, merci de vous procurer sa documentation et autant d'information que possible. Pour mon cas, la documentation est ici : http://www.tutorials-raspberrypi.de/wp-content/uploads/2014/08/Stepper-Motor-28BJY-48-Datasheet.pdf

Bon, regardons d'abord les moteurs pas-à-pas en général, ensuite nous préciserons cette information pour le 28BJY spécifiquement et nous le connecterons au Pi via notre puce de commande L293D.

Stepper Motors Stepper motors are used in robotics and in CNC type machines where you want the ability to move an item to a specific location easily. There are two basic types of stepper motors, one called Unipolar and one called Bipolar. The difference will become obvious as we go through this tutorial. The 28BJY is a Bipolar motor and also has a gearing system. In both models, there are multiple electromagnetic coils that are turned on and off in a sequence to make the motor turn. Each time we apply power to one of the coils, the motor rotates a small amount (if powered in the correct sequence for the motor), called a step, hence the name stepper motor. Unipolar Motors Unipolar motors have coils that are powered in only one direction, hence the UNI in Unipolar. The rotor of the motor is controlled by powering the various electromagnetic coils on and off in a specific sequence for a certain amount of time. In a simplified version of this model, let’s look at the following diagram… Turning on each coil one at a time will cause the magnet in the rotor to turn toward the proper coil. Using a clock face as a guide, turning on the coils in the sequence of 12 o’clock, 3 o’clock, 6 o’clock, 9 o’clock and then again at 12 o’clock will cause the rotor to turn clockwise one full rotation. This requires 4 “steps” to make one rotation. This is called the Unipolar wave. If we go a bit further, we could make a more granular movement by alternating the coils from a single coil turned on and then turning on the next coil as well, which makes the rotor turn in an eighth turn when both coils are turned on. The sequence would then be: 12, 12 and 3, 3, 3 and 6, 6, 6 and 9, 9, 9 and 12, and then finally 12 alone again. This then is 8 steps per rotation which is called half stepping. To make the motor reverse (counter-clockwise), we simply reverse the sequence. This is a VERY simple representation, and many stepper motors have a resolution that can be as high as 200 steps per revolution.

Bipolar Motors The 28BJY, as I stated earlier, is a Bipolar motor. In this case, the coils can have their current reversed and two coils are powered at any time. This creates a situation where the switching is more complex, but the amount of turn force (power) of the rotor is increased. A simple block diagram of the 28BJY is shown below. The numbers shown with the colors of the wires are for the 28BJY and yours may be different. The wire connector (if there is one) might differ from unit to unit. You can use an ohmmeter to verify the coils. The Wiring A couple of words of warning on this before we start. First, do all of your wiring BEFORE you power on the Raspberry Pi. We are working with an external power source, so you want to make sure that you don’t short any wires or apply the battery power to the wrong pin. Second, BE SURE of your wiring before you power on your RPi. If you get the wiring confused, at best your project will not work and the motor will just sit there and buzz. When you look at the fritzing drawing, it looks fairly simple (and it is). I made sure that the wiring from the RPi to the driver chip were the same color as the intended segment of the motor. We will be using only 4 of the 5 motor wires. The red one (if yours has a red one) is not connected for this project. Since the central component in this project is the L293D driver chip, here is a quick breakdown to try to make things easier for you…

L293D Pin 1 → Pin 9 Pin 2 → Pi GPIO 6 Pin 3 → Motor Pink Pin 4 → Breadboard Negative Rail Pin 5 → No Connect Pin 6 → Motor Orange Pin 7 → Pi GPIO 5 Pin 8 → Breadboard Positive Rail Pin 9 → Pin 1 Pin 10 → PI GPIO 23 Pin 11 → Motor Yellow Pin 12 → No Connect Pin 13 → No Connect Pin 14 → Motor Blue Pin 15 → Pi GPio 24 Pin 16 → Pi +5VDC If you follow this, you should have no problems with the wiring. The Code As always, I will discuss the code in blocks. So let’s get started. import RPi.GPIO as GPIO import time GPIO.setmode(GPIO.BCM) GPIO.setwarnings(False) coil_A_1_pin = 6 # pink coil_A_2_pin = 5 # orange coil_B_1_pin = 23 # blue coil_B_2_pin = 24 # yellow

Here we are simply defining the imports, setting the GPIO mode, and setting the warnings to False so we don’t get any nagging notices about already initialized pins. We also define which GPIO pins control the motor coils through the driver chip. # adjust if different StepCount = 8 Seq = range(0, StepCount) Seq[0] = [0,1,0,0] Seq[1] = [0,1,0,1] Seq[2] = [0,0,0,1] Seq[3] = [1,0,0,1] Seq[4] = [1,0,0,0] Seq[5] = [1,0,1,0] Seq[6] = [0,0,1,0] Seq[7] = [0,1,1,0] Now this is the key to making our project work. This motor wants to have 8 steps (internal) per revolution of the motor (per the data sheet). We also define the sequence of which coil(s) are energized per step as a series of lists. Each sequence array explains which coil(s) is energized at any given time. GPIO.setup(coil_A_1_pin, GPIO.OUT) GPIO.setup(coil_A_2_pin, GPIO.OUT) GPIO.setup(coil_B_1_pin, GPIO.OUT) GPIO.setup(coil_B_2_pin, GPIO.OUT) Here we are going through the setup steps, defining each of our pins used as outputs. def setStep(w1, w2, w3, w4): GPIO.output(coil_A_1_pin, w1) GPIO.output(coil_A_2_pin, w2) GPIO.output(coil_B_1_pin, w3) GPIO.output(coil_B_2_pin, w4)

This subroutine is called each time we want to step the motor and we pass a 0 or 1 to each coil wire port on the driver chip to energize or deenergize the various coils to turn the rotor. And finally our “main” routine which loops over and over again asking the amount of the time delay and the number of steps in that given direction. For my motor, it takes 512 steps to make close to a full rotation. On my system, with my motor, a time delay of 1ms works well. However, you might have to add a few milliseconds to yours for it to work. Notice I stated that it takes 512 steps to make CLOSE to a full rotation. This motor has a 64:1 gearing ratio, which leaves a rather ugly fractional step angle. But for the purposes of this tutorial, it works pretty well. If you want to learn more about stepper motors, adafruit.com has a very nice article on the subject. Hopefully you have enjoyed the series so far. Next up will be learning to use the Arduino microcontroller board. We’ll use this information in the third section of the series where we control the Arduino with a Raspberry Pi (or other computer). So, that having been said, you should be ready and have an Arduino (Uno or Mega) ready and dust off the components we used in the early part of this series for next time. Until then, keep learning and above all, HAVE FUN!

Encadré de la page 19, texte en noir : These two routines allow for easily commanding the motor forwards or backwards a specific number of steps in the proper direction.