From: "Roger Hamlett"
Subject: Re: Stepper motor question
X-Newsreader: Microsoft Outlook Express 5.50.4807.1700
Date: Sun, 12 Jan 2003 14:51:08 -0000
NNTP-Posting-Date: Sun, 12 Jan 2003 14:52:06 GMT
Organization: ntl Cablemodem News Service
"Frank Bemelman" wrote in message
> "Esoteric" schreef in bericht
> > Hello all,
> > This is my first time working with a stepper motor and related drive
> > circuitry.
> > I am using an optical encoder w/ 360 pulses/rev to generate a quadrature
> > waveforem (2 bit grey code). This signal goes through logic and a Step
> > pulse & Direction are generated as outputs. These are fed into an
> > 5804. The 5804 is set up for full step with option for one or two
> > The motor is a 6 wire unipolar motor. 12V 75ohm
> > The system works fine while turning the dial on the encoder. The issue
> > that if I turn the dial too fast the stepper motor stops rotating and
> > "buzzes" until I slow the ecoder down again. This is regardless of one
> > two phase operation on the Alegro.
> > The Alegro 5804 docs state that a minimum step input pulse width is 3us.
> > get nowhere near that with manual rotation. This condition occurs with
> > step input of around 10ms as seen on my scope. So I have ruled that
> > Is this motor resonance? What is causing this and how do I get rid of
This is nothing to do with motor resonance, but all to do with the way that
stepper motors behave. The coils on a stepper, display high inductance. Now
the rate at which the magnetic field builds when power is applied, depends
(primarily), on the voltage being applied, and the inductance involved (once
the current rises, the resistance in the loop becomes significant, but
initially, it makes little difference). The problem is that if you have
(say), a '12v' nominal stepper motor, then this means that it is rated to
have 12v across the coils without overheating, but if you step it using a
12v supply, the field will build very slowly, and the torque will be very
limited, except at slow step rates. The solution to this, is to drive the
motor off a much higher voltage, but to limit the current, so that when the
motor is stationary, the voltage across the coils does not exceed the rated
value. Hence it is normal to use (say) a 60v supply to feed a 12v motor, and
then have either (simplest), a current limiting resistor for each winding,
with a resistance equal to 4* the coil resistance, in series with each
winding, or to use a current limited drive chip, set to limit the drive to
the motors rated current. When the power is applied to the winding, the
initial current now rises at 5* the original rate (since the end voltage is
now 5* the original value).
Given you probably don't want to increase the voltage, you may well have to
use a lower voltage motor (for instance, the motors commonly used in
printers, are usually operating off 24v supplies, but are 5v motors).
> > I do not need extreem speed on the motor. It is being used to manually
> > control a butterfly valve on a propane fired foundry. The stepper
> > geared down such that it requires about 1000 steps to go from open to
> > closed, this was done to increase resolution on controlling the valve.
> Your motor does not develop enough torque. Try increasing the 12V to 15V,
> see if it can go a bit faster. When you go faster, the windings get
> pulses. It takes a while before the current reaches a certain level, and
> with short pulses the current drops. The result is that your torque drops
> and drops, and when too low, the motor stops. You have to find a way to
> increase the current, increasing the supply voltage is the only option.
> Beware that at low speeds the current gets larger, and a too high supply
> voltage may overheat the motor.
> Unipolar, 12V, 75ohm seems a rather small steppermotor. The UCN5804 is a
> very simple stepperdriver. What you may need is a more sophisticated
> controller with current control (pwm) and a higher voltage. Possibly also
> a motor with lower resistance windings.