Thread: driving a stepper

How much rotation do you require, how much resolution, and how much torque?

If the needs are relatively modest it would be easier to do the job with radio control type servos or sail winches. Then your pot controls a simple 555 timer circuit that outputs a variable pulse width to the servo which moves its output arm/wheel accordingly. If the load pulls the output off the required position, the servo pulls it back. Simple circuitry, cheapish output device in low power, and a simple power supply or battery to run.

To do it with a stepper is a lot more complicated because the stepper needs a computer to keep track of where it should be (hopefully is), an encoder to get from analogue input to something that the computer understands, a stepper driver to get from the computer to the motor, and a more critical power supply because you have computers in the system with narrow power supply tolerances. You also need to be able to establish a true zero position each time the unit powers up, similar to the homing sequence for a CNC machine, as the system does not incorporate absolute feedback sensing as a servo system would.

A stepper motor merely rotates a given amount one way or the other for each pulse train delivered to it, so it needs everything else to determine where it should be before any command, where it should be after the command, and to translate the command into a suitable pulse train for the motor to follow. If the pulse train is too fast, or the motor is overloaded, it can miss some pulses and will then be off position until the entire system is re zeroed at a later time.

thanks for the reply! and fast too! I do not know wheather you are familiar with rose engine lathes or not. the input end is the rosette and the output would run the compound of the cross feed back and forth. I wouldnt need to run the cross feed screw it could be set up as an arm or as a rack and pinion.

an arduino , (heaps of info on net) , you could understand the basics and cut can paste the code for it too work , C++ programming language background helps

arduino's are a usb based muilti controller , with a heap of PWM ( pulse width modulation ) pins as out puts , run it straight off your pc's USB , if you dont know c++ this is where you will learn it
you need 1 pin per speed of motor(PMW), you will also need a direction pin( Binary) , and an activate pin (binary)( you dont , but you do )

so 3 pins per motor , 3 motors easy for each ebay china rip off arduino ( under $30) , a $30 "2560" arduino comes with 16 analog pins ( binary) , 14 PWM pins

then you need a "stepper motor driver "for each motor , they sell useless pug right on " shields " that use the arduino's power to power them( good for useless sized steppers) , BYE A " BRAWL" OF GECKOs , (stepper motor drivers)+ a 48volt power supply unit if you want it to do anything ( $140 for an unbreakable Gecko off our forums sponsor > http://www.homanndesigns.com/store/ + power supply(what sized motor? )

yer a plug , but last time i purchased , homanndesigns was cheaper than ebay , compare them your self

add Ballscrews , and bearings ( all thread + washers works )
whats your budget , and what do you expect too cut with it ?
'

the cut and paste code off the arduino web site will work http://www.arduino.cc/ , but you would have to reprogram it for each job ??



have fun ken

not interested in going full cnc. I turn wood so the tool pressure is minimum. The whole lathe is home made. the cross feed is on linear rails. the servo option is interesting. i have worked with steppers before retrofiting mills and lathes but if I can skip having a computer in the shop that would be preferable.

I have considered using a teleflex cable coupled to a follower and the compound but then there is a cable strung across the floor and I am not sure how much spring it would have.

As I mentioned in the prev post for this step I want to couple an input that follows a curcular pattern as the spindle turns. typicaly on this type of lathe the head rocks back and forth. I do big stuff and moving the kind of mass (head stock tailstock and a large turning) would be rough. So I have opted to move the compound instead. the cross feed follows a template while the compound moves in and out in answer to the rosette.

Kind of a two step process if I get to the second step which would be encoding the headstock to "phase" the cuts as the cutter is moved longitudinally.

Malb, could you go into a little more detail?

RC Servos come in a range of sizes, speeds and powers. This is a fairly standard generic one costing about $35. They are also available in 'giant', minature, or micro versions.

Rotation is typically about 45 degrees either way from centre, but there are sail winch version with drums instead of arms that can operate over multiple (5 to 10) turns, but at slower angular velocities and much greater torques.

Input cable has three pins for power (4.8 - 6 or 7.2 VDC), ground and pulse. The Pulse input is the control input and is a pulse varying in width from 1ms to 2 ms typically and repeated every 20ms. Nominal center position is achieved with a 1.5ms pulse, reduce the pulse width and the servo rotates in one direction, increase it and the servo rotates the other way.

The servo consists of a housing, an elecric motor driving a gear train to an output shaft (with miniture ball race support for high power units) which is dirctly connected to a feedback pot, and a servo amplifier board. The feedback pot works as a voltage divider to tell the amplifier board where the output shaft is positioned, and the amplifier board then drives the motor to correct any load induced errors or follow changes in the input pulse width. Variation in supply voltage within the devices rated voltage range do not effect positioning but can effect speed and available torque. The speed specs for a servo are given are for no load, and torque specs are for stall torque (no rotation).

The pulse train to feed the servo can be generated from a small circuit using two 555 timers. The first timer is set to produce a narrow regular pulse every 20-30ms (not critical) while the second is triggered from that pulse and has a variable pulse width determined by the control pot of choice (rotary or linear).

I am not familar with a rose engine lathe but looked up some you tube footage which showed spindle speeds of about 2RPM and head movements of about 25mm. If this is in the order of what you require, I believe that it would be possible with a 'jumbo' or larger servo. As an indication of what these servoes can do, they control 1/4 scale aerobatic model planes with typically two servos per control surface (for redundancy), and they have been used to operate trim tabs on the control surfaces of homebuilt 4 seater aircraft.

Hope this helps you.