Re: [CAD_CAM_EDM_DRO] Safety -- servo vs. stepper

Jon,

|At 12:47 PM 5/20/2004 -0500, you wrote:
|
|Carl Mikkelsen wrote:
|
|<snip>
|>Under hardware fault conditions, it seems that steppers are safer.
|>In particular, most failures in the high-power drive system will
<snip>
|A single faulted axis on a rigid hexapod will not cause movement,
|it will
|blow fuses. That is not true on a cable hexapod, of course.

Perhaps my hexapod geometry is different from others, but a single
faulted axis will move the platform, in some positions quite
radically. Where it moves it, and how far, is determined by exactly
where in the operating envelop the platform is at the time. The
most likely outcome is that the head will move a few inches in X-Y,
a inch or two in Z, and tilt violently until a joint angle is
exceeded. If it will stop, great. If not, then it will be a
contest to see if the joints, actuators, or motors will fail first.

|I have been using servos on a cartesian machine since 1998. I
|built my
|own servo amps, and have had a few runaways. In general, the
|machine only
|gets about 1/4" before the amp trips offline or the computer
|commands an
|E-stop. In my case, I have two forms of feedback - DC tach and
|encoder.
|The tach feeds back to the servo amp, and the encoder feeds back to
|the
|computer. If either fails, the system is still marginally
|controlled.
| In the
|case of an encoder failure, the computer has no idea that motion is
|occurring, it thinks that motion is NOT happenning, and commands
|more
|velocity from the servo amp. This will usually trip the servo amp
|offline in a very short time, maybe 50 mS or less. You can verify
|this
|response with just the motor and no hexapod attached.
|
Building my own servo amps might be the answer. I don't see how I
can get feedback from a Gecko drive to help me know that a problem
is occurring. I'd need encoder feedback to the software, a model of
what the drive signal should be, and possible also tachometer
feedback to give me a redundant way to detect motion.

I have some nifty quadrature decoder, pwm generator, A-to-D
converter, CPU chips available for the servo smarts, but I'm not
really comfortable building the high current, high voltage H-
bridge. What did you do for your home-built servo amps?

|So, in about 6 years, I have NEVER had a true runaway with the
|machine
|screaming toward the limits. I have had my share of loose wires,
|dirty
|connector contacts and similar stuff, and a number of e-stop events
|when
|the machine was not responding correctly, but never a servo runaway.

|But, you HAVE to make sure your E-stop system really stops motion,
|no ifs ands or buts! And, that means some effective way to remove
|power from the servo amps and connect a braking resistor.

|Jon

With a good E-stop circuit, is it customary to have a fault on one-
axis stop motion on non-faulted axes? [sounds like from Leslie M.
Watts' emails that ALL faults trigger a e-stop on all axes]

It seems valuable to have several features in a servo drive:

1) over current detection, where over current is a dynamically
changed quantity based on the current acceleration and static load
models.

2) redundant velocity feedback, matched against the current expected
velocity

3) a true closed-loop control, rather than a step-and-direction
interface, so that faults which result in unexpected performance
problems can be sensed by the control software

4) limit switches -- if only I could find a way to implement them.
For a hexapod, these would need to sense all conditions what would
occur as the result of a single or possibly dual axis failure. With
the kinematics of my hexapod, this would most likely result in a
joint angle error before it would cause an actuator length error.

5) sensing in the servo drives to assure that the host is actively
generating control inputs. This could be through a watchdog time or
other means.

The thoughtful responses have gotten me thinking more intensely
about how to design for safety.

Are there multi-axis commercial servo amplifiers that approach Gecko
pricing?

Does anyone have a proven H-bridge circuit that takes direction and
PWM input, plus an analog signal representing a dynamically
adjustable current trip point, and delivers motor outputs and a
fault signal?

-- Carl