Designing a Stepper Motor and Drive

Hi VI,

I've got a design for what you've described, but all the info is at home. I'll post it tonight when I get home.

You can use a simple unipolar stepper driver circuit and motor along with a 555 IC for the timing. Alternately, you could use a PIC uController for the timing which would also give you more flexibility. The resistance safety issue would probably be simpler if it were a mechanical solution.

Talk to you later,

Mike

Rotating a number of deg over a set yet changeable time period, and stopping, with a stepper and a PIC chip and a few transistors, is very easy. With this alone you have no absolute position, that is, it will rotate 360 deg from wherever it started. If it was to miss a step at any time, unless you reset it by hand, it would be forever at a different start position. If you need an absolute position, a switch can be added so it stops when it hits the switch.

Sensing if something is in the way is more difficult, but shouldn't be too bad. You may be able to simply use the current going to the motor as feedback, although im not sure.

Alternatively, you could use a sensor optical mechanical or otherwise to see if something is in the way.

With a rotary sensor, you could also measure the steppers position, if the motor skips, you would know something blocked the rotation. You would also have to do the below which may work on its own depending on your application.

If you are just worried about the force being too strong and dangerous perhaps you could simply adjust the current going to the stepper. That is make it just strong enough to do what you need, but if its blocked, it will just start missing steps.

As to the weight, that is torque, they sell steppers in many sizes, just that price usually goes up as the torque does, depends on your application.

Good luck, not sure what your application is, but stepper motors are fun to play with.

Dear peer (Village Idiot),

This is not the complete solution, however most garage door openers have a circuit that senses an object is impeding the door, and stops then reverses. Perhaps combined with other technology this could be worked into your design and reduce cost if you happen to have an old one lying around (as I do).

CJM

Hi sir

A normal stepper does 200 steps per revolution that is 1.8 degrees per step. Aside from that you would not get 30 degrees exactly you will also have to give one step every 3.6 or 7.2 min.

You would have to go for micro stepping an/or a reduction gearbox or belt drive.

I am forever using old aborted pen plotters to drive things.

Try and get a HP A0 pen plotter. it has 2 geared servo motors. and it can be controlled from a computer via RS232 using HPGL commands.

You can drive the servos with a PIC if you have to.

Every stepper motor discussion deserves a link to "Jones On Stepper Motors". Lots of great information. This provides the theory we all need. Your question was for specifics so this great link might not be exactly what you want but other readers might find it useful.

http://www.cs.uiowa.edu/%7Ejones/step/

I am old school so be warned that there are more modern and more sophisticated ways of doing things than what I am about to propose. However, they may be more costly. First, I would use an AC synschronous stepper which is continuously rotating as long as the synusoidal wave is present (AC power applied). No special driver required. Second, I would use a cam with 12 points (spaced 30 DEG apart) placed on the shaft of the motor. This cam would interrupt the current to a motor relay by the use of a roller switch. Third, I would use a timer with momentary contacts to get the motor going at regular intervals. The timer must be able to reset itself after timing out and repeat the cycle (programmable timer) or maybe a PLC with 12 cycles programmed in. Forth, the motor relay would drop out if the motor encounters an obstruction by means of a spring loaded switch attached to the shaft of the motor and activated by the load not moving as it should around the arc. The spring tension would be your adjustment to determine how much force would be needed to activate the switch. Fifth, when this obstruction switch is activated, a second relay must come into play. As the motor relay is deactivated the second relay is activated reversing the rotation of the AC synchronous motor causing the cam and motor to rotate in the opposite direction until the previous point on the cam activates the position switch (opens contacts) and the motor stops. As far as activating the motor again in 15 seconds or so, I haven't figured that one out yet. If this apparatus is unattended, I am thinking it might not be a good idea to beat your obstruction to death by repeated activation though. You might be better activating some sort of alarm when the reverse relay kicks in and just letting the whole thing sit there until someone can intervene. I am sure by the use of an additional time delay relay that it is possible to restart the motor though at what ever interval you would require. It may be that you would prefer using a PLC, in which case I would still suggest the AC synchronous motor and cam and switches to provide input to the PLC. The cam would allow you to know where the motor is if you counted the pulses. In order to stop the cycle at the end of 360 DEG without a PLC you would need to count the pulses and send them to a count down shut off timer. Of course, encountering an obstruction would kind of mess this system up. I wouldn't mind playing with this one some if I had the time and money.

I have a cnc machine shop that specializes in grinding but I also play with these devises. www.cnczone.com and Mach3 are 2 of the best places. Mach3 software that works is free and license is about $160 per person. The people on these sites are also using cheap plc s to use logic as to the sensors.

Idea really kicks me. Possible solutions could be

1) Servo motor attached with timers to stop at intervals. To trip on load ot owith sensors

2) CAMS on shaft so that loses contact at particluar angle with a braking system.

3) RAck an pinion motion with control on linear speed.