voltage "clamp" when input analog signal removed

[Gryd3]

Not a problem Skippy.
As a software guy you don't usually need to know or care about hardware... but from the sounds of things, your software will interact with the real-world through hardware. In my opinion, it is very important that the software guy knows as much as he can about the hardware that he will essentially be controlling. I am a CNC programmer and I feel this is essential.
If you ask the questions to learn how it fails, (open circuit, or frozen at 0V) you could take care of error handling and perhaps program an additional output that is used as a safety.

Can you answer a couple other questions for me?
Is this machine automated, or is there an operator working on the machine?
Does the Shaft Transducer run at full speed during any of the jobs it is supposed to carry out?


As it stands, it does not sound as though there are many options to put a safety here...
We do not know how the line fails... so putting a pullup resistor on the analogue input for the MMP may not do anything depending on how the RMC fails. This would need to be tested and would require the tester to cause a failure on the analogue circuit or the RMC that would be identical to how it would normally fail... simply disconnecting the line will not cut it. (a Pullup resister will however prevent the motor from running full tilt... but ONLY reliably if the line is open or disconnected.)
One option is to provide some kind of lock-out to disallow running the device at full speed unless an operator is present, but this would not be ideal if this device is supposed to be automated.
Another option would be to provide some kind of 'keep-alive' output from the RMC, and if this output fails to respond or hangs in either a low or high state then the MMP can be shut down.

I would like to avoid suggesting feedback from the motor to the RMC... as if the RMC fails we are in the same boat. I did however notice a 'fault' output on the RMC, this may be exactly what is required depending on how it functions... (What would the state of the RMC be if the computer froze or shutdown?)

 

[skippyV]

Gyrd, you and I are on the same page. How can I write software that fires off warnings or even shuts down the system - unless I have a clear and in-depth knowledge of that system? That is a big peeve I have. I got to gather "intel" in a manner that doesn't provoke anyone. It's harder than you can imagine.
Anyways...
An operator does (or will) control the machine through my application.
The transducer running at full speed would be an ideal scenario - but unlikely at this time due to unrelated circumstances. Meaning we have to go slow until we figure out other environmental aspects.
You see the machine is connected by a tether - but a very long tether. This tether provides power to a power-box (like a power strip) that resides on the machine (with the spinning shaft). So in the original configuration - a signal from my app tells the SBC to output a voltage to the MMP. But the SBC would periodically lose its connection with my app. So if the operator had a situation where he needed to suddenly turn off the shaft - and at the same time the SBC lost the connection - then as RoboCop said "there will be trouble".
Now the new design will have the RMC (located with the control person and with the PC running my app) pushing that analog signal all the way down the tether to drive the input of the MMP. At this point we aren't concerned with failure of the RMC (its a pretty slick device!), but there certainly is a chance of the wire carrying the analogue voltage of breaking. If the power line to the machine itself broke then everything would stop. But having power at the machine and suddenly no signal to tell it to stop seems like a bigger potential hazard.
And you are right about the RMC. It does have capabilities for detecting many events. I'm still absorbing it's abilities. Perhaps it will provide a solution.
The take-away lesson I got from all this is to just get the right device and get it working the way it should - as you designed in the beginning.
There has been time to send the MMP back and get a replacement. That first deadline is long gone.
By continuing to use it because of... some perceived convenience is beyond me. Just complicates the design unnecessarily.
Anyway - time to get off my soap box now.
Skippy

 

[Gryd3]

Skippy,
Due to the expected use of the equipment and capabilities of the new RMC, I think you would be covered by using a large resistance pull-up resistor. This will cover you in the event of a wire breakage. Pick a larger value though so you do not alter the signal too much. (This is essentially making a voltage divider, so you want a larger value for your pullup than what the RMC's equivalent resistance is).
Additionally, wire in one more more E-Stop switches. (Normally closed, all connected in series... if one is pushed, it creates and open circuit).
If this was the type of machine that was run unattended for prolonged periods of time, I would not recommend the E-Stop switch... as I have seen guys run toward a dangerous situation to hit the off switch rather than let another safety feature handle it..