Constant Current Drive - was: something else, can't remember.

> 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