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servo direction change on limit switch?

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servo direction change on limit switch?

Postby nix » Fri May 11, 2012 9:19 am

I thought I'd ask about one of my areas of work-
string winding.
I would like to oscillate between 2 limit switches to flat wind
the roll.
This carrier goes in the transverse machine direction and feeds
the string onto the core.
I was wondering if SM caters for 1/4hp-180 watts servo motors?
Also I was wondering how much this would cost me-
not including FS.
What hardware do I need?
I have done this once before-
it was Baldor's solution, and cost $5000au.
It is just theoretical questions at this stage
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Re: servo direction change on limit switch?

Postby DSP » Sat May 19, 2012 5:24 am

FlowStone can communicate with all types of servos it doesn't know how big they are as it's just data to FlowStone.

Have you any hardware in mind?
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Re: servo direction change on limit switch?

Postby fixstuff555 » Mon May 21, 2012 5:55 pm

For hobby level servos, I have 4 wheelchair motors on a robot, that are 350W brushed DC motors. I am using 4, Pololu 24V23 motor amplifiers which can handle up to 23A continuous without a heat sink. There are several interfaces you can use on this board, and the one I am using is the USB to serial. For you, the most important thing is the serial interface capbility. Flowstone can communicate to a servo with a bunch of different interfaces, including the serial ports, analog commands, etc. but it requires 3rd party hardware like Phidgets, or Pololu, or whoever. I chose the Pololu due to its current rating, but I could have easily chosen a Dimension Sabertooth module for another example. There aren't large numbers of amplifiers out there that can handle that power, that are hobby level affordable.

For industrial grade servos, which are typically brushless DC or similar performance, such as a Baldur, Parker, Yaskawa or whatever, I would recommend a model that uses an analog command for simplicity's sake. This would require you to purchase for example a Phidget 1002_0 or similar board. This board, has 4, 12 bit analog output channels that would allow you to run an industrial servo using their analog interface.

For that, I would also recommend purchasing an 3 port isolation device of some sort, such as a Phoenix Contact MCR-C-UI-UI-DCI-NC, since the Phidget board is non isolated. Interfacing a non isolated analog output to a servo module potentially could suffer ground loop issues and could either fry the amp board, your servo, or have bad noise problems. They are expensive, but are generally available in surplus, such as Ebay or something, lots of manufacturers make isolation modules, or you could even make a PCB with your own isolation amplifier chip, such as an Avago HCPL-7840 or similar chip.

If the drive has a serial interface, that would give you the most capability, but you would have to probably use a serial converter if making a long run from the serial port on the PC to the drive. Noise can be a big issue here as well, since the serial interface, will probably be ASCII, and not be encapsulated in a protocol with error checking. If you use serial, I would recommend seeing if there is a checksum option, and use it. This requires a bit of extra code, but it would allow the system to throw out bad data and not do something crazy with your motor.

FInally, if your drive has the capability of interfacing to ethernet, Flowstone has both client and server TCP/UDP capability. There is even a Modbus TCP program in the examples folder, that you could modify for whatever data you want to transfer.

There isn't any sample programs written to cover any other fieldbus protocol that I have seen other than the Modbus TCP, although theoretically, you could simply use an RS232 to RS485 converter, and write a program to talk to whatever fieldbus protocol you need (within the serial data rate limits of Flowstone). Not sure how difficult that would be but it would not be trivial. Potentially Flowstone could handle it, its just a matter of how much trouble you want to do it.

If you really wanted to get crazy, you could potentially talk to any fieldbus you want with Flowstone using a serial to fieldbus gateway module. For example, an HMS Networks X-Gateway module would allow you to take Flowstone and talk to any fieldbus out there. The cool thing about this, and is the reason why I pushed a bit to get the Modbus TCP interface, is the serial side of the gateway module uses Modbus RTU. Its basically set up as a dual port RAM setup, you feed it data on the serial side, and it spits out that data on the fieldbus side. This would actually be an awesome module for Flowstone to support, but the need has to be there for someone to create it. Heres the link for that module. They make some really cool products, I have used the Anybus IC version for getting Profibus into a product I built a few years back.
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Re: servo direction change on limit switch?

Postby nix » Mon Jun 25, 2012 10:35 am

Thanks so much folks,
I will read again.
Sounds a little complex,
but as i can use fs,
I may try and control servos.
It's great that we can use powerful machines.
Much appreciated,
edit 2014:
This is making more sense now- I'm not that quick(and I have neither experience nor
training in electronics).
So much thanks for all the detail!
Hopefully I can do this simple task with RC components to start with,
and continue to refer to your great response FixStuff.
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