Mariss? Dan M? Re: Bill W's question?

Ballendo,

Sorry, I must have missed the question. Here goes:

STEP MOTORS:

I work with the premise that anything that "breaks" with a step motor
system makes the motor stop. The exception may be a PC that goes off
to "La-La Land" and continues to spew step pulses.

An E-Stop would simply disconnect the +5VDC connection to the STEP
and DIRECTION interface. killing the step pulses. Fuses protect
against more catastrophic failures.

DC BRUSH SERVOS:

Anything that "breaks" here has a tendency to make the motor take-off
full tilt. This is a harder problem. The first thing is to get a
handle on how much energy must be removed. That will determine if you
can simply shut off the power as recommended for steppers. You can
skip to the conclusion if you trust my math.

CONCLUSION:

I would recommend using a DPDT relay for an E-stop. Use one pole to
kill 115VAC to the power supply. Use the other pole to place a large
power resistor across the DC side of the supply while the AC power is
off. The relay coil should be energized to enable the supply.

Use a resistor that will carry 10A at the power supply voltage(R =
V/10). That means 7.5 Ohms for a 75VDC supply and so on. Use at least
a 20W wire-wound resistor.

This will take care of multiple axies. Again, fuses will protect
against more catastrophic failures.


THE MATH (yawn):

You mention 100 IPM @ 100 lbs. This requires about 19W to move (1HP =
550 ft-lbs/sec, so 100/550 * 1.666/12 = .025HP, 1HP = 746W, so .025 *
746W = 19W)

Assuming the worst case where there is no friction to help slow
things down, applying 19W would result in a 1G deceleration rate. The
time to stop at that rate would be .066 seconds (A = 1G = 32
ft/sec/sec, velocity = .139 ft/sec, so t = square root of V/A,
or .066 seconds). The total energy (watt-seconds or joules) to be
removed becomes 1.25 joules (.066 sec * 19W = 1.25 watt-seconds).

1.25 joules is a tiny amount of energy (I hope I did the math right).

Next, how much energy is stored in the power supply capacitor. Let's
say the biggest single-phase 115VAC supply is 1,500W. The stored
energy will be about 60 joules using a 10% ripple voltage capacitor.
Say you have a 20A, 75VDC (1,500W) supply. A capacitor that would
have 10% ripple (7.5V) at 20A would be 22,000uF. Energy stored in
that cap would be 62 joules (22,000uF * 75V^2 / 2). The majority (62J
versus 1.25J) is stored here and must be removed during an E-stop.

If a 7.5 ohm resistor is used with the above example then the power
supply will discharge (and stop the motor) in 0.56 seconds (T =
3*R*C, so 22,000uF * 7.5 ohms * 3 = 0.495 seconds + .066 seconds for
the inertia). This method also makes for a very effective bleed
resistor. The rsistor will see 62 watt-seconds of dissipation. A 20W
wire-wound resistor is rated for a 300% over-dissipation for 5
seconds, which is 300 joules (5 seconds*20W*300% = 300J).

Other methods like disconnecting the motor or DC power from the
drives may result in damage due to stored inductive energy in the
motor armature inductance that would have no place to go.

Mariss

PS What's with Yahoo? I have noticed up to 18 hours of latency
between the time I post and when it appears on the list.

--- In CAD_CAM_EDM_DRO@y..., "ballendo" <ballendo@y...> wrote:
> Hello,
>
> Did I miss the answer(s)?
>
> A bit ago Bill W (list mom) asked you two what I thought was a very
> good question: (paraphrased below)
>
> "What is the best way to handle you drive(s) in an estop situation"
>
> I'd add: Let's assume there is some mass, say 100 lbs. plus. Let's
> also assume this mass is currently in the middle of a multi axis
move
> at something around(at least) 100IPM.
>
> So the question is, What are your recommendations? How do we wire
for
> estop so that we can meet safety standards, while not destroying
> Your/the drives in the process(if possible)?
>
> Thank you both in advance,
>
> Ballendo
>
> P.S. Although Bill W named Mariss and Dan Mauch specifically, I'd
> appreciate input/answers from others who are selling drives... Dave
> R, Dean, Bill, Doug(your non-geckos, obviously), etc.?