Re: [CAD_CAM_EDM_DRO] Re: E-stop relay welds contacts for load dump resistor

Dan,

I can see how in an industrial situation, it would be entirely valid to
engage the emergency stop circuit as part of the lock out - tag out
protocol. However, I would expect that said machine would already be
stable, stationary and powered down with discharged power supplies
before the E-stop would be engaged. The E-stop would only be engaged to
prevent the power supplies from being charged up and thus permitting
unexpected machine movement.

I've been doing electronics for 45 years, and built my own CNC equipment
(mill and two lathes (larger is 12" swing with 96" travel)) using both
Superior Electric steppers and Yaskawa servo drivers. Not as sexy as
current production Gekko drives, but quite adequately functional.

My stepper machines are driven with both linear and switcher supplies.
(Big lathe has 5 and 10 HP 3 phase Yaskawa AC servos) When powered off,
the stepper supplies discharge within one or two seconds. The holding
currents of the motors drain the supplies very quickly after the line
current is interrupted.

None of them have had dump circuits in their power supplies. In all of
the machines, the systems actually lose control if you dump the power
supplies, as the control circuits loose their ability to hold the motors
stationary, but instead allow them to freewheel.

IMO, the best function for an E-stop circuit would be solely to
disconnect all of the step signal lines from the PC interface and force
them to ground, without interfering with the power supplies. That would
instruct the stepper drivers to hold the motors at their current
position, rather than allowing them to freewheel to a stop. If the
motors were running at high speed, they would probably lose a few steps,
but I'd bet that they would stop a whole lot faster than they would if
unpowered.

I don't know of any common stepper drivers that are capable of
initiating continuous motor motion autonomously if they receive no step
signals. Even if there was a failure in the bridge circuitry of the
stepper driver, it would leave one or more windings shorted or
deenergized, but the motor wouldn't spin out of control.

Unless convinced of a valid reason to the contrary, I would omit the
power dump circuit entirely, and insert a 3PDT switch or relay into the
step lines between the computer and stepper drivers to use as an
E-stop. I would just be sure that I have powered the control computer
and started the control program before powering the motors, and
conversely, power off the motors before the computer.

Dave

On 1/30/2011 5:08 PM, kzf817 wrote:
> The E-stop circuit is, at the very least, energized when the machine is powered up for a working session, and dumped when the machine is shut down at the end of the evening (home hobby workshop). Under normal circumstances, yes, it is not activated frequently. As I finish configuring this machine, I may have to go into E-stop three or four times on a Saturday to unplug motors for troubleshooting, or to make adjustments on things. Have you seen any sample schematics, or do you have any methods for safely de-energizing the drives and power supply by not passing power through the load dump resistor? I have worked with many other industrial machines in the past where engaging E-stop is a required part of lockout/tagout procedures, and is never considered an extraordinary, or "disaster only" event that risks causing damage. Large production machines like printing presses running at high speed are excepted, of course. We only E-stop them to "safe" the machine when idle. Thanks,
>
> Dan.
>
> ==================================================================
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, David Speck<Dave@...> wrote:
> I'm a little puzzled about the whole issue. The dump circuit should
> really be reserved for rare emergency situations, not for routine
> shutdown operations. Perhaps there is a bit of a learning curve being
> addressed?
>
> Dave
>


[Non-text portions of this message have been removed]