PIC Stepper Motor Tester

PIC Stepper Motor Demonstration and Test Project

• Name: PIC Stepper Motor Demonstration and Test Project
• Status: still developing, but is working
• Technology: PIC microcontroller with code in BoostC
• Author: russ_hensel ( where you can find an email address to reach me )
• Summary: A PIC16F877A project that operates a stepper motor under the control of a PC running a terminal program.

Last revision July 4 2008 – Draft, near completion except for Java control program. The C code works, but there are lots of errors in other parts of the project – they should be fixed soon, check back or email me. Archive zip file ( SerialStepperTest.zip ) includes: this doc, eagle schematic, program listing. Email me for file, will be posted eventually.

This project has several potential uses:

• Example code for stepper motor control and for RS232 communications ( interrupt driven on the receive end ) on the PIC16F877A.
• Determining which drive wire is which on a unipolar stepper motor. This is the type of motor that I have most commonly found surplus or in salvage equipment
• Determing angle per step or steps per revolution of a stepper motor.
• Determing maximum speed of stepper motor.
• Demonstrating the operation of a stepper motor.

Platform: PIC16F877A using BoostC connected via rs232 to a PC running a terminal program, or as an alternative running a Java program developed especially to control the PIC ( still under development ). The PIC chip is supplemented with a MAX232 chip and a ULN2003 driver.

The project is able to drive a stepper motor in a number of ways ( controlled by the RS232 connection ):

• Set the number of steps
• Set the direction of the steps
• Set the time delay between the steps
• Drive the wires to the stepper in any of the 6 possible connections ( permutations), this allows the user to determine which wire is which.

In general the stepper is driven by issuing several commands to set it up and then a final command to take a series of steps using this setup.

Example:

Command	Meaning
v<cr>	Get the version make sure the command interface is running, usually takes 2 tries to initialize, this is a bug which I have not fixed yet.
t10<cr>	Set the step delay to 10 ms
p1<cr>	Select permutation 1
d+<cr>	Set direction forward
g400<cr>	Go for 400 steps – full turn on many motors

Hardware

I have used a PIC 16F877A for the project, but pretty much any PIC with a uart and another 4 free I/O lines should do. To increase the drive to the motor I used a ULN2803 which is simply an array of Darlington transistors and diodes to be used as a low side switch for each motor winding. There are other similar chips around or discrete devices can be used. I run the pic, on 5 volts, and a larger voltages for the stepper, up to the limit of the driver. ( the UNL2803 is good for 50 v at .5 amp as a switch ) and the rating of the stepper motor. If you have a high power motor you may want a driver with more guts, Just put in some substitute. Coil drive is on or off there is no PWM involved here. Note that the hardware has substantial uncommitted resources. You could easily drive another motor for example. Also some of you may want to put some pull up or down resistors on some of the uncommitted resources.

Schematic

Sstest.png

Command Interface

All commands ( except stop should be terminated with a carriage return ) Note that the command interface is not very smart, giving parameters that are out of range my blow the whole program up. If so reboot the PIC. Do not send a new command ( except stop ) until earlier commands have been completed ( actually you can get ahead some if you are careful )

Command	Code	Notes, PIC Response
Report version	v	Version of the PIC software something like: Serial Stepper Test ver July 4 2008
Set direction	d+ d-	plus for forward, minus for back Direction set.
Where = Request motor position	w	Step taken since power on. Signed int.
Report on all parameters	r	Delimited by commas something like: Dir +1, Permutation 1, Stepper Pos 80, Step Delay Us = 300 .....
Go for a number of steps	gnnn	go for a number of steps ( max. about 30,000 ) Direction set with d. Responds with "g starting<cr>" when rotation begins, then with "g done<cr>" when done. May be stopped early with stop command.
Set the time delay between steps in ms ( max 255 )	tnnn	Reports delay set. nnn = 0 to 255
Micro second delay in addition to to the ms delay.	unnnn	Reports delay set. Ok to use values nnn = as high as 5000 us.
Set the permutation of the motor wires.	pn	Set the permutation, find the value that works for your motor. ( n = 0 to 5 ) Responds with the permutation set.
Spin the motor in an interesting way	x1	Motor spins responds with "x special done<cr>" when it is done. May be stopped with the stop cammand.
Seecial command 2, Vibrate the motor first a lot then less and less to stop	x2	responds with "x special done<cr>" when it is done
Stop	s	Should almost immediately stop long running commands like Go or x1 or x2. Responds with Stopped<cr> when stoped ( wich should be quick ).
Other, not understood commands	xxx	Responds with "!Bad Command = xxx" if the command is not understood.

Notes on terminal program set up.

• Baud rate should be 19.2K 8N1
• Most terminal programs can be set to treat a carriage return as a carriage return line feed. Do it.

Some terminal programs will not transmit in lower case ( all our commands are lower case ) unless specially set to do so. Set it to allow lower case.

Microcontroller Program Design

I no longer have the patience for assembly language. I have moved on to C in particularly BoostC, see link below. I like this compiler it has both a free version with some restrictions and a very reasonably priced full version. Writing in C should make the program fairly easy to read. Most of the design should be evident by reading the program, however a few notes here may help.

The idea is to put each activation sequence in a table and then step through that table and activate the corresponding port bits and thus stepper coils. The four wires can be activated in a total of 6 different ways, one table corresponds to each permutation. The particular table to use is set using the permutation command ( p ).

The series of wires to energize is specified in the arrays StepperStepsN where N is the number of the wire permutation. Each step just increments its way through the array wrapping around the the beginning and the end.

Commands are received via an interrupt driven routine, the main loop checks each time around to see if a complete command has been received. Because commands are only interpreted in the main loop all commands are ignored until the program returns to the main loop. The exception to this is the stop command which will terminate a g or x command and return to the main loop quickly. RS232 transmission is not driven by an interrupt and so during transmission from the pic no stepping takes place. Commands which do not result in motion execute very quickly, most of the time is for communications.

Currently drive to the motor is half step drive. This gives twice as many steps per revolution as is labeled on the motor. I plan later to let you select half step, full step or wave drive. See the links below for more information.

Java Control Program

Rather than use a terminal emulation program you can use a Java Program that I will write. This makes it easier to issue commands and has a much lower chance of making a mistake. The interface looks like this: ( actually almost written, email me if you have a special interest ).

This basically issues the same commands as you can issue with the terminal. There are a couple of enhancements

<All> sends all the parameters then the go command, thus you can run the whole setup by typing in the parameters and pressing <All>.

Setting up the java control program.

Will have to wait until program is done.

Which Wire is Which?

Unipolar motors typically have 6 wires that come in 2 pairs. Typically the “center tap” of each winding is connected to V+ and the individual windings are grounded ( as with the Darlington array described in hardware ) to switch them on. The tester does not determine which of the wires are the center taps, but that is easy to do with an ohm meter. Each center tap will have the same resistance to far end of either of the coils connected to it. There will be twice the resistance between the ends of the coils. Sometimes the two center taps will be connected together so the motor has only 5 wires. Sometimes the center taps will be the only two wires that are the same color. Once the center taps have been determined connect them to V+ ( whatever voltage is appropriate for the motor ) and the other wires in any order to the 4 ports. Name the wires A, B, C, D ( or use the wire colors if available, adjusting the chart below for the colors ) and connect to the drivers on bit 0, 1, 2, 3. ( in the software given I have actually used bits 4, 5, 6, 7 )

Make sure the PIC came up all right and the serial connection is working – use the Version command ( v ) and see if the response is reasonable, if not try another couple of times, if it sill does not work reboot the pic. If it still does not work check over the communications parameters on your terminal program. And so on.

Set the motor to run forward, with say 50 ms delay per step. Choose permutation 0. Now try 100 steps ( g100 ) If the motor does not run try another permutation.

If the motor is good you should find 2 configurations that work, one forward, one back. ( In some cases I have found more than 2, I do not quite understand this, look for the smoothest running – russ test some more with more motors ) The jerky motion of the shaft can be deceiving, I add a pointer to the shaft, one that will not slip. This chart then tells you the “standard” identification of the wires:

Wire ID	Perm. Pattern 0	Perm. Pattern 1	Perm. Pattern 2	Perm. Pattern 3	Perm. Pattern 4	Perm. Pattern 5
1	A	A	A	A	A	A
2	B	B	C	C	D	D
3	C	D	B	D	B	C
4	D	C	D	B	C	B

Discussion

Which wire is which depends in part what you consider a standard coil activation sequence. From my reading I believe that this corresponds to the table Zero_StepperSteps in the code. Your standard may differ, if so you can modify the code. The code uses a so-called “half step” drive. You can contact me russ_hensel if you are having trouble figuring this out. Google will link you to a huge amount of information on steppers.

Stepper Test Form

I have developed a form for testing motors that I find useful. It looks something like this ( and is included in the file download package ):

In use:

Entry	Use
Test Date	Test Date.
Motor ID	Something to identify the motor, including anything written on it.
Appearance	Generally what the motor looks like.
Table	Enter the wire colors in 1...6 then measure the resistance between all pairs. The table is symetric so you need to measure only one side of the diagonal.
Electrical data	Anything you know or can figure out about the voltage, current, resistance.... of the motor.
PO rot =	What the motor does when driven by permutation 0. And so on through P5 rot.
Diagram	Draw in a schematic of the coils, lable A...D with the wire colors ( this is based on the resistance chart )
cw on P0	List the wire colors for 1...4 and common to drive the motor clockwise on permutation 0. ( to be sure about this I change the connections and test that permutation 0 actually works that way. The chart above for the identification of the wires helps with this.
Half steps per rotation	Make sure by rotating the motor at least 10 whole turns and verify that it returns to the starting position. Remember that the tester uses half steps not whole steps.
Minium Step Time	Smallest time ( set both the ms and the us ) at which the motor rotates reliably and with enough torque.
Max Rev	Compute from Minium Step Time

Example, filled out:

Screen Shot

Other Things to Try

• How fast can your stepper go?

Run it faster and faster ( shorter time delay ) until it fails. You may want to turn it 400 turns to clockwise fast, then 400 counter clockwise slow. If it does not end up where it started then it skipped a step or more. If you know how many steps per rev then you need not reverse it just ask it to go a round number of revolutions.

• How many steps per revolution for your motor?

Guess, try to make it turn 10 revolutions fairly slowly or at least slowly enough so that it is not skipping steps.. If you number is right it will end up at the same angle it started. If not try a new guess.

• Demo the motor with Special Commands x1 and x2

x1: spins the motor one way faster and faster, then reverses and returns to original position. x2: motor vibrates faster and faster through smaller and smaller arc, then stops where it started.

Possibly useful links

This program uses my: Serial Communications Library -- BoostC and 16F877A

More information on serial communications with microcontrollers: Microcontroller Serial Communications Articles

Info on steppers: Stepper motors http://www.allaboutcircuits.com/vol_2/chpt_13/5.html

More info on steppers: Basic Stepping Motor Control Circuits http://www.cs.uiowa.edu/~jones/step/circuits.html

A free terminal program, I like this much better than hyperterminal: Welcome to our Free Download/New Products Page! http://www.rs485.com/psoftware.html

BoostC – I think the free version is enough to compile the program: SourceBoost Technologies http://www.sourceboost.com/

Some may considered this project for just determining which stepper wire is which overly complicated: there are some simpler methods ( for example: http://www.doc.ic.ac.uk/~ih/doc/stepper/others/ ). However for one reason or another this has not always worked for me, this test is definitive and fun.

Modifications You May Need to Make

As they occur to me.

Download

Comming, until then email me, see russ_hensel

Comment, Questions, Contributions?

Email me russ_hensel, or use the talk page for this topic. All feedback is welcome.