RS422 incremental encoder on Siemens PLC

[userxyz]

I allready worked with incremental encoders. 24V encoders with an A and a B signal. I allways used the 313C CPU todo such things in stand alone applications.

But I do have a problem now.

On a machine we need to change the CPU to Siemens. But I see that all 4 incremental encoders work on 5V:




When I look in the documentation of the encoder I can see that there is spoken about RS422. Also on the old encoder cards of the previous PLC.

View attachment INKREMENTALNI%20SNIMAC%20IRC3x5F%20A%20IRC3x8.pdf

So I guess I will learn something new. I tought RS422 was a communication protocol, but it seems that the incremental differential encoders on TTL voltage can also be called RS422 incremental enconders.

What does this mean ?

What do I need ?

FM350-1 ?
This is good for 1 channel, 4 of these baby's would cost me 2400 euros...


Pls help,
Thanks,
Combo

 

[bernie_carlton]

Refer to BEI Encoders conversion module. It is available in a variety of voltage formats.

 

[BryanG]

Various people seem to do this same thing but with their own part number, here it is PU204.
http://www.motrona.com/level_converters.html

The only bit that is a pain is the incoming and outgoing signals are via DB9 connectors rather than screw clamp.

You might associate RS422 with data transmission, but when you think about it data transmission is just a series of 1's and 0's and when you then look at an incremental encoder it is also just a series of 1's and 0's. If you look at an ASCII table 01010101 translates to capital U, so an encoder just sends a string of UUUUUUUUUU.

Bryan

 

[Peter Nachtwey]

EIA-422 encoder interfaces are common in motion control and preferred over open collector or even TTL. EIA-422 is more noise immune and faster because the voltages don't need to swing over a wide range like open collector interfaces must do. I am surprised that Siemens doesn't have an encoder interface board or a high speed counter card that can interface the encoder using EIA-422.

 

[userxyz]

Encoders

Mostly 24V encoders are used in industry I think, because this is the first time that I see these RS422 encoders. I still don't understand why they used these encoders, all movements are pretty slow...

EIA-422 encoder interfaces are common in motion control and preferred over open collector or even TTL. EIA-422 is more noise immune and faster because the voltages don't need to swing over a wide range like open collector interfaces must do. I am surprised that Siemens doesn't have an encoder interface board or a high speed counter card that can interface the encoder using EIA-422.

 

[Peter Nachtwey]

Mostly 24V encoders are used in industry I think,

Perhaps so in the industries you have seen but I doubt you have seen them all. 24 volt encoders are slow. That is fine when used with slow PLCs. It the motion control world EIA 422 is quite common. It is the standard way our product connects to an incremental encoder.

because this is the first time that I see these RS422 encoders. I still don't understand why they used these encoders, all movements are pretty slow...

In your application that may be so and you can get by with slow encoders and PLCs too. My customers are always demanding more speed, more accuracy and smoother motion. This requires high resolution encoders and high speed counters 10-12 MHz. These rates can't be achieve with 24 volt encoders because the voltage swing is too large.

The modern trend is to use low voltage high frequency lines. ATA interfaces are now replaced by SATA drive interfaces. PCIe has replaced the old ISA parallel bus. The list goes on. If you do a search for LVDS
http://en.wikipedia.org/wiki/Low-voltage_differential_signaling
You may not seen this but it will come.

So why bother? If you want to measure the velocity over one millisecond the input speed from the encoder is only 1000 counts per second then the motion controller will see only 1 pulse per millisecond on the average. Due to sample and mechanical jitter this means the controller will see 0 or 2 counts sometimes but 1 count most of the time. The controller can not calculate accurate velocities and the derivative gain can't be used. Now if the encoder has 1000 times the resolution and can go 1000000 counts per second then the controller will see about 1000 counts change per millisecond and every once it a while the counts will be 999 or 1001. This means the error is only 0.1%. Now the derivative or even second derivative gain can be used.

If you want to improve the performance of your system one of the first places to start is with the resolution of your feed back and high resolution feed back will probably need to be EIA422.

 

[userxyz]

tnx

Thanks for the info.

Perhaps so in the industries you have seen but I doubt you have seen them all. 24 volt encoders are slow. That is fine when used with slow PLCs. It the motion control world EIA 422 is quite common. It is the standard way our product connects to an incremental encoder.


In your application that may be so and you can get by with slow encoders and PLCs too. My customers are always demanding more speed, more accuracy and smoother motion. This requires high resolution encoders and high speed counters 10-12 MHz. These rates can't be achieve with 24 volt encoders because the voltage swing is too large.

The modern trend is to use low voltage high frequency lines. ATA interfaces are now replaced by SATA drive interfaces. PCIe has replaced the old ISA parallel bus. The list goes on. If you do a search for LVDS
http://en.wikipedia.org/wiki/Low-voltage_differential_signaling
You may not seen this but it will come.

So why bother? If you want to measure the velocity over one millisecond the input speed from the encoder is only 1000 counts per second then the motion controller will see only 1 pulse per millisecond on the average. Due to sample and mechanical jitter this means the controller will see 0 or 2 counts sometimes but 1 count most of the time. The controller can not calculate accurate velocities and the derivative gain can't be used. Now if the encoder has 1000 times the resolution and can go 1000000 counts per second then the controller will see about 1000 counts change per millisecond and every once it a while the counts will be 999 or 1001. This means the error is only 0.1%. Now the derivative or even second derivative gain can be used.

If you want to improve the performance of your system one of the first places to start is with the resolution of your feed back and high resolution feed back will probably need to be EIA422.

 

[userxyz]

Maybe solution

I may have found a solution after seeking a lot...
ET200S has modules that can read RS422. They actually have positioning modules. I think I will go for that. I agree with the interfaces (TTL to HTL) in case of using the compact CPU's with their HTL inputs. The interfaces are around 250 euros... the function modules for reading TTL on ET200S also are around that price...












I tested it in the ET200 configurator and it validates Okay.

Only question that I have...
Should be no trouble to connect an HMI to this ET200S I guess ? Because The ET200S is not a DP-Master.

?

Regards,
Gerry

 

[rantao00]

RS422 can be 5V(TTL) 5-30V(HTL).5V use generally
just be two modules.maybe you need to change encoder or give a 5V from other place to encoder
www.simre.com.cn encoder manufacturer

 

[userxyz]

5v

5V comes from external supply.
What do you mean, only 2 POS U cards needed ?

RS422 can be 5V(TTL) 5-30V(HTL).5V use generally
just be two modules.maybe you need to change encoder or give a 5V from other place to encoder
www.simre.com.cn encoder manufacturer