Re: Sorting out the options for a CNC mill system

Douglas,

I'm going to differ with Danny on the matter of power supplies.

First, you should use the highest voltage that your controller will
accept. Second, unregulated, linear (like the one described at the
hobbycnc.com site) are better for CNC operations.

Here is the reasoning behind my statements.

Virtually all controllers on the market today are "choppers". They
work like this:

The output of the controller is set, in amps, to the value desired
for the stepper motor being driven. (Controllers that drive multiple
motors have an adjustment for each motor.) Notice that the setting
is in amps, not volts. You can have a stepper motor that is rated at
8 vdc controlled by a controller that is fed with a 50vdc power
supply.

The controller has a feedback circuit that can "sense" the amount of
current (amps) that is being fed to the motor. Now the next part
gets a little complex. When a motor needs to step, the controller
turns on the current to that motor (feeding current into the motor so
that it moves the correct direction). But the current does not rise
immediately to the desired value. The slight delay (or ramping up)
of the current is a function of a complex relationship between the
power supply voltage, the capacitance of the circuit and the
impedance of the motor winding. You don't need to understand what
all of that means, just understand that the varying values of the
various components go into controlling how fast the current rises.
This is where the "chopping" action of the controller comes into
play. The feedback circuitry of the controller senses the amount of
current flowing through the motor at any given moment. When the
current reaches the desired value, the current to the motor is cut
off.

The controller goes through this current on, current sense, current
off sequence 1000's of times a second (usually 20,000 times or more).

Just as the current does not rise instantly, it does not fall
instantly when cut off. Again various factors (capacitance,
impedance, etc.) determine how fast the current falls (decays). The
delays in the rising and falling of the current cause a "smoothing"
effect on the current. Thus instead of "seeing" chopped current, the
motor "sees" a more or less constant current.

Power supply voltage and type (linear or switching) is a significant
factor in determining how much power a motor produces. The higher
the voltage, the more "pressure" there is to ramp up the current
quickly. (Voltage can be compared to the pressure in a water hose.
The higher the pressure, the more water comes out.) From our
understanding of how a chopping controller works, we can see that
voltage regulation (holding the voltage to a close tolerance) is not
particularly important. If a power supply rated at 60vdc actually
delivers only 58vdc, it is not going to impact motor performance very
much.

What is needed is a "reserve" of power. That reserve exists in
linear power supplies in the form of the large (10,000 mf or greater)
filter capacitor(s). These capacitors are "reservoirs" of power that
are immediately available when the current to a motor is switched
on. And during any "off" time, the capacitor replenishes itself so
that power is available on the next cycle.

Now why are switching power supplies not well suited to CNC
applications? A switching power supply operates in much the same
fashion as a chopping controller. The difference is that it controls
voltage, not current. When the CNC controller is "idling", no
reserve of power is built up. Only when the motor is switched on
does the switching power supply start to react. The demand for power
causes the switching power supply to "go into action" releasing more
power to the controller board. But because the switching power
supply has a feedback circuit to monitor the output voltage, the is a
slight delay in delivering the current to the controller. Now we
have two places where delays occur in getting the power to the
motor. Not a good situation.

Switching power supplies work well where voltage regulation is
important and the demand on the power supply is fairly constant.
They are generally cheaper than a linear power supply of a similar
rating. This is because the transformers and large filter capacitors
used in linear supplies are expensive items, especially as the power
demands increase, They are also bulkier, so they require a bigger,
more expensive housing.

One way (the only way as far as I know) that switching power supplies
can effectively be used for CNC operations, is to put a large (again
10,000 mf or larger) filter capacitor on it's output. The capacitor
again acts as a reservoir of energy that is immediately available
when the motor is switched on, giving to switching power supply time
to react to the increased demand for power.

This explanation could be 10 times longer and still not do justice to
the subject matter. I've done my best to condense it while still
giving enough information for a beginner to understand why I believe
that linear power supplies are better for CNC operations than
switching power supplies. In doing so, I have probably introduced
some vagueness and information that can be easily misinterpreted. I
apologize in advance for these short comings.

And again, I don't have a particular fondness for the Gecko drivers.
I just think that you don't get the extra value for the money. There
are thousands of hobbycnc controllers out there that drive the Taig
mill just fine.

And if anyone wishes to take exception to my opinions, feel free to
critique my work. But please don't engage in character
assassination. I'm just one guy out here, trying help my fellow
CNC'ers. I don't pretend to set myself above anyone.

Regards
Gary

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Danny Miller <dannym@...>
wrote:

> Get a 48v regulated power supply with the current greater than... I
> think it was like 70% of the sum of motor currents.