Siemens S7 spec

Hi,

I have to spec a new system using Siemens S7 PLCs, I have not done this before and was hoping for a bit of help.

The system in question is a rotating wheel with hydraulic controls. I need to measure speed, tension and have various alarms and ESTOPS.

Looking at the S7-1200 I think it may be a bit limited. My load measuring device ideally outputs 4-20mA. As far as I can see the S7-1200 only accepts 0-10V AI?

The S7-300 looks like a very costly option in comparison. To me it seems sensible to install two S7-1200s and have one controlling the wheel and the other monitoring my tension/speed and taking the inputs from the ESTOPS. Are there any obvious pitfalls with doing this?

The total system needs:

1 x (4-20mA) AI
1 x (0-10V) AI

8 x DI (7 BOOL and 1 square wave)

4 x DQ (audible alarms)

I will also need to connect a HMI screen with some control and display features (all to duplicate what would be on a physical control panel).


Any advice/obvious pitfalls you could advise me on would be appreciated.

Thanks,

Hi!

The 1200 series does support 0/4..20mA, but you might have to get a separate module for that.
https://www.automation.siemens.com/...atic_st70_chap03_english_2013.pdf?HTTPS=REDIR

There is also a 1500 series, it could be cheaper then the 300 series.

I can recommend the 313C, it offers about the in and outputs you need, including pulse counters and the price is pretty reasonable. Think the list price is about 1000 euro's for the basic kind.

SIMATIC S7-300, CPU 313C, COMPACT CPU WITH MPI, 24 DI/16 DO, 4AI, 2AO 1 PT100, 3 FAST COUNTERS (30 KHZ), INTEGRATED 24V DC POWER SUPPLY, 128 KBYTE WORKING MEMORY, FRONT CONNECTOR (2 X 40PIN) AND MICRO MEMORY CARD REQUIRED

Looks like you could do all this with one S7-1200. If all signals are physically close, I don't see any advantage to using multiple S7-1200 and one will make the programming much easier. If some of the signals are a long ways away from some of the other signals (for example, the hydraulic controls are at the pump and the wheel is a couple hundred feet away), it may be cheaper to use two S7-1200s so you only have to run an Ethernet cable between them but the programming will be more difficult. I usually only use multiple S7-1200's if I have too many signals or need to add redundancy per customer specification.

The 4-20mA input can be handled with an optional 4-20 module (as mentioned in another response) or you could put a 250 ohm resistor across your 4-20mA device at the PLC to convert the input to 1-5 V (or a 500 ohm resistor to convert to 2-10V). Verify your resolution and accuracy requirements since the built-in voltage inputs have lower resolution and accuracy specs than the optional built-in analog inputs.

If you are using the 4-20mA input as your speed input, you also may be able to measure speed with one of the high speed digital inputs instead. Use an encoder to a high speed input and do your math to ensure you do not exceed the high speed input frequency. I've done this many times and it makes for an excellent speed input - its about selecting the right encoder PPR and single-ended output capability.

The S7-1200's performance is excellent for the price.

Take a look on the new CPU module for Siemens ET200SP distributed I/O serie.

http://w3.siemens.com/mcms/distributed-io/en/ip20-systems/et-200sp/Pages/default.aspx

You get a compact solution with better IO than S7-1200 and more power in the CPU.

If you want to use the S7-1200 series, then I would say the following components should be used:
Controller
CPU 1214C 6ES7 214-xxxxx-0XB0 (xxxx = depending on the type of IO)
DI 14x24 V DC
DO 10x24 V DC or 10xRLY
AI 2x10 bit 0–10 V DC

Analog input card
SM 1231 AI 4 x 16 bit analog input (voltage/current) 6ES7231-4ND32-0XB0