[finyuk2005]

I am needing some help!
I have been upgrading an older CNC plasma table, using DMMTech DYN4 servo drives and matching AC servo motors,
For the Initial torch height sensing or material height sensing i guess, i was hoping to use the same approach as the original setup,
this was Ohmic sensing plus using the current monitoring output of the drives, the voltage output would feed an Arduino with a small program to switch an output above a certain threshold.
Unfortunately this isnt going to work with the output from the drive, as they have a range of +-10amps, the drive is 3.5A max, and i doubt i would see anywhere near that even!
What that means is the range of voltages output from the drives current monitor is only a few millivolts.

I had wondered if something like this on just one phase of the motor would work;

https://www.amazon.co.uk/AZDelivery-...6&sr=8-10&th=1

But im not sure if that would be any better! also this was one of the reviews;

" These worked perfectly, and I guess you could call them a success on their own terms. But sadly they aren’t as useful as you might think; this is no fault of the design or manufacture, just intrinsic to the way they work. They’re very sensitive to electrical noise and don’t seem to have very fast response. They have some advantages over simpler solutions such as the fact you can connect them at the positive or negative end of whatever you’re trying to measure, and also they have a very small impedance.

I was trying to use one to measure the current used by a servo motor, but in the end abandoned it and just stuck a 0.1 ohm resistor in series with the servo and measured the voltage across it. Not as elegant but simpler and more effective."


So other than having to convert my torch holder to a floating head design! Is there any ideas on how i can implement the current monitoring idea?
Plasma torch contacts plate, servo has to output more current to keep moving, above certain threshold triggers the "probe" input in mach 3

any suggestions gratefully received!!
cheers
sorry for the boring post!

[joeavaerage]

Hi,
the problem with current monitoring is that it is so coarse.

Imagine if you have a servo drive the torch downwards until it impacts the material. As the motor stalls you will see a major increase in current but by then the servo has happily
crunched the torch so bad it'll be wrecked. You could set the current sensor to be very sensitive, but the if the servo accelerates up or down as its meant to those current spikes
could be mistaken for torch contact events.

What wrong with just an Ohmic contact. ie like a probe? When the torch touches material the voltage would get pulled low whereon the servo would decelerate to a stop.
You can often have the servo stop within a few milliseconds and so the torch would 'press' down just a few um, not enough to do any damage.

Craig

[finyuk2005]

Yeah im seeing that might be an issue indeed!
The ohmic should work fine on its own most the time, but i would like a backup in case of rusty or painted metal,
the old system used a standalone torch height controller that utilised current sensing as well as ohmic, but that was a DC servo motor, so perhaps that was easier to do?

[joeavaerage]

Hi,

but that was a DC servo motor, so perhaps that was easier to do?

AC servos are more difficult to monitor current, but not impossible.

I use Delta B2 series servos which are very similar in terms of performance, quality, documentation and price to your DMM servos. Amongst the outputs of the servo are two analogue channels,
either or both of which can be programmed to reflect motor current. With the proper programming then the servo produces an analogue volte (0-8VDC) output reflecting motor current.
So in some respects the Delta AC servo is even easier than an older DC servo. Maybe the DMM servos have similar features?

Craig

[finyuk2005]

Unfortunately i dont think the dmm servo will manage,

Hi Finlay, unfortunately there is no way to increase the current resolution from the drive. The current output monitor is only meant to be used in emergency cases where the motor crashes into the end etc.

The L01 drive you are using has a current output range of -10~+10A, corresponding to -3.3~+3.3V analog output from the current output pin.

It's best to use a Z probe like normal,

Thats what i got from DMM guys.

Your Delta drives output sounds perfect for what i need!
It looks like i might just have to go to a floating head, unfortunately, as i think the current monitoring way seems like a nicer solution if i could make it work!

[joeavaerage]

Hi,
I'm not a fan of Clearpath servos, not because they don't work or their quality, but they seem very expensive for what they are. There is one feature they do have which they
have developed which might be very useful to you. In particular they can 'Home to a stop' using a some current sense setup. I do not have the details but the explanation given to me
about its operation and performance was very intriguing.

They have an excellent technical support team, by all means ask them. I rather suspect they monitor my posts as I have criticized them in the past and they always follow up with a rebuttal,
so its entirely probable they will see this post anyway.

Craig

[finyuk2005]

Thanks for all the info!, i might look into them, and the delta drives you suggested

[joeavaerage]

Hi,
for a while when I built my machine I had an analogue voltmeter monitoring the servo current (X axis). What surprised me is that 99% of the time the current was like less than 0.1A,
and only ever very VERY briefly did it ever spike much over 1A. In the end I realised that there was little for me to gain from observing motor current, the only time it draws any real current and is when its accelerating
like hell or crashed or something....hardly need a gauge to tell me I f****ked up!

Crag

[finyuk2005]

Yeah i have been very surprised at the power these wee servos have,
I was looking at the current monitor output on an oscilloscope while trying to turn the Z axis ballscrew with my fingers, and for a 200w motor, my god does it have some holding power!!!
So the torque the X axis has with a 750w motor going through a 15:1 reduction gearbox must be mental!!

[joeavaerage]

Hi,
my X axis servo is direct coupled to the ballscrew. The ballscrew is 32mm diameter and 5mm pitch. With a 200kg cast iron axis it accelerates at 0.27g at rated torque and up to 0.75 g
at peak torque. Scary bloody fast. I detune them down to 0.15g, about as much as my nerves can stand.

I do have a servo (750W Delta B2) on my trunnion table. The gear reducer for the trunnion table is 19.5:1, so the table has plenty of torque. The Fifth axis platter (C axis) has a 6.75:1
reduction so it has plenty of torque to, not as much as the table but still very useful.

Craig