Homebrew PCBs

hello all

A few questions on fuser feed rate and temperature sensing.

I have pullrd apart an old HPLaserjet5L and have the fuser attached 
to the stripped down chassis.

I recovered the stepper driver chip from the pc board and am 
currently successfully driving it with a PIC micro. The rollers will 
accept a 1.6mm (1/16 inch) board with a gentle push to engage it in 
the rollers. What feed rate have others used?

A general stepper question - Should steppers be driven with a short 
pulse then allowed to free-wheel. It seems that this might make the 
motor turn more smoothly. How would I work out how long the pulse 
should be?

I have heard that the temperature sensor is a thermister. Can 
thermisters withstand this temperature without damage? I intend to 
control the temperatue via an analogue input on the PIC. My sensor 
measures 500 k ohms at room temperature, which is too high to use as 
one leg of a voltage divider to dive the analog input (10k max 
source resistance). What sort of resistance would I expect at fuser 
temperature? I know I can buffer the voltage divider with an op-amp, 
but would rather not.

To get the right temperature I was going to use a trial and error 
method - estimate a value to switch at (intentionally too low to 
start with) and increase it until it works. My digital multi-meter 
does not have a temperature function.

thanks

pete

On Fri, 07 Apr 2006 23:41:56 +0200, petere_au <peter.e@...> wrote:

> What feed rate have others used?


About 30cm per minute (one A4 page).


The thermistor will work just fine. You will probably want to use a  
thermocouple or something to calibrate the thing.

ST

--- In Homebrew_PCBs@yahoogroups.com, "petere_au" <peter.e@...> wrote:
>
...
> A general stepper question - Should steppers be driven with a short 
> pulse then allowed to free-wheel. It seems that this might make the 
> motor turn more smoothly. How would I work out how long the pulse 
> should be?

Steppers don't freewheel, they must be driven continuously. I'm not
sure you understand how stepping motors work. You might Google if you
are not familiar with stepping motors.

Steve Greenfield

At 14:07 06/04/08, you wrote:
>A general stepper question - Should steppers be driven with a short
>pulse then allowed to free-wheel.

A stepper motor has positive indexing, it doesn't 'free-wheel'
in normal use. That's why it's called a stepper motor. ;-)

It has a higher holding torque whilst energized: if you
need high holding torque then power it continuously.

If you don't need much holding torque, it's up to you how
long the drive pulse is, as long as it contains enough
energy to step the motor and it doesn't exceed the
voltage/current/power ratings of the windings.

Note that the windings are highly inductive so a high
voltage is necessary for high stepping rates but static
high voltage drive will lead to large ohmic losses.
So consider what happens if there is a software error ;-)

>  It seems that this might make the
>motor turn more smoothly.

Not even wrong, as Don Lancaster might say.
When you pulse it, the motor will step: that's
what it is designed to do. If one step is too
coarse for you, learn about micro-stepping.

>How would I work out how long the pulse
>should be?

Options:
(a) calculate from the parameters of your motor
(b) reverse-engineer your LJ-5L (or measure a working one)
(c) suck-it-and-see (probably easiest in the circumstances).

Basically, with insufficient drive the motor won't step
reliably, especially under load, but too much won't make
much difference until you hit the max ratings. So find out
what works and add a safety margin.

There's _lots_ of stepping motor info on the Web - go Google.


Regards, LenW
-- 
   Quote little, say much - as a general guide,
   your reply should be bigger than your quote
   (and following it, of course,)