Another stepper drive type was Re: Constant Current Drive

Hello,

There IS one more stepper drive type which has found good success and
is not often mentioned on this forum.

It is the Bi-LEVEL drive; and Anaheim Automation is one supplier.
Hundreds of these were used in Digital Tools' Dr series of routers in
the early 90's...

This drive type has the advantage of not "chopping", and therefore is
both electrically and audibly quiet.

The way it works is to use TWO power supply voltages. The higher is
called the "kick" voltage, and it performs as does the higher voltage
in other drive types. However, after a short burst of this high
voltage the drive switches to a lower voltage to maintain the holding
torque whilst not overheating the motor. Two levels of voltage,
making a bi-level drive. Properly adjusted, performance is similar
to "chopper" types.

Hope this helps,

Ballendo

--- In CAD_CAM_EDM_DRO@y..., Alan Rothenbush <beer@s...> wrote:
> > Constant current works by using higher voltage (like 80V) on the
> > windings, and no resistors. The current ramps up faster thru the
> > motor windings because of the higher voltage. When the current
> > reaches a maximum, a monitoring circuit removes the voltage.
>
> This is not correct. What is being described above is a chopper
drive,
> not a constant current drive.
>
> Let us step back a minute.
>
> Stepper motors are current devices. As such, the spec sheet for a
stepper
> motor always has a current rating. It usually has a voltage rating
as
> well but does not NEED one ( except in one special case ) .. only
the
> current rating is of interest to us.
>
> This single current rating, assuming typical engineering, is
usually two specs
> in one. The first is the current required for maximum power, the
second is
> the maximum current without overheating.
>
> Let us take a theoretical stepper motor rated at 1 amp 1 volt.
>
> There are four basic drive circuits we can use to make this stepper
move.
> In all cases, we want to supply 1 amp of current. More and it might
> overheat, less and we won't get all the power we paid for.
>
> The first circuit is the special case I mention above. Create a 1
volt
> power supply. Connect it to the motor ( through a suitable
switching
> arrangement ) and the motor will turn. It will have as much holding
> power as any other circuit. And it will have the worst acceleration
> characteristics and lowest maximum speed of any of the drive
circuits.
>
> The reason it will have the lowest speed is because the low voltage
will cause
> the motor coil to "fill up" slowly. Since the coils fill slowly,
the motor
> will turn slowly.
>
> This is the simplest possible drive circuit, but it will work and
it will
> hold just fine.
>
> The next drive circuit is what you are using now, an RL drive.
This circuit
> always uses a voltage higher than the "rated" motor voltage.
Without anything
> else in the circuit, the motor would quickly overheat as a result
of too
> much current. The resistor serves to limit the current to a safe
value. The
> trick here is that "extra" current will flow when the stepper coil
is first
> energized because an unenergized coil looks like a short circuit.
As the
> current in the coil rises, it looks less and less like a short
circuit until
> eventually some equilibrium is reached, with the resistor serving
to limit
> the current to the safe value.
>
> This circuit has the same holding power as all the others, but
somewhat
> better acceleration and maximum speed than the circuit above
because of the
> "extra" current that flows.
>
> The fourth circuit ( I can count .. # 3 will be described last ) is
a chopper
> and is what was accurately described above. In such a circuit the
current is
> turned on and off at a rapid rate such that the AVERAGE current is
some
> appropriate value.
>
> No resistors are used, but a high voltage IS used. As such, the
motor "fills
> up" with current very quickly. This "quick fill" results in
excellent
> acceleration and the highest possible top speed. The holding
torque is same
> as the first two because the average current is the same.
>
> This is how all modern stepper drives are built.
>
> The third drive design is a constant current source, and is similar
to the RL
> drive. In this case, the current limiting resistor is replaced by
a bit
> of electronics. These electronics are designed so that some
constant current
> will flow no matter what the conditions. Even if the motor is
replaced by
> a short circuit, the same current will flow.
>
> Think of this circuit like a light dimmer with a person at the
control
> trying to maintain a constant light level. When the sun comes
up, the
> person turns the dimmer down. Clouds roll in, the dimmer gets
turned
> up a bit. Night comes and the dimmer is turned up full tilt.
With
> enough range and a fast enough person at the helm, the light stays
> constant no matter the conditions.
>
> The constant current source works the same way, keeping the current
at
> precisely the same value, no matter the conditions.
>
> This is a wonderfully simple way to drive a stepper motor and has
> performance as good or better than the chopper drive with
considerably less
> complexity. Again, holding torque is the same because the current
is the
> same.
>
> The one problem with a CC source, and it's a big one, is the heat.
The CC
> source does its magic by converting the "extra" power to heat, just
as the
> R in the RL does. But the performance difference between the RL
and CC drive
> is HUGE.
>
> Hope this greatly simplified description helps.
>
> Alan
>
>
>
> --
>
> Alan Rothenbush | The Spartans do not ask the number
of the
> Academic Computing Services | enemy, only where they are.
> Simon Fraser University |
> Burnaby, B.C., Canada | Agix of
Sparta