Has anyone tried to make PLC/PCS using Embedded boards like Raspberrypi / arduino? You can manipulate the signals without complex hardware.

sure you can do that even with high voltages ! using conductors and relays you can convert voltages and such embedded solutions will work fine with you and be sure they are FASTER than any PLC ! 

even for hardware and software design they will be easier , one problem you may have if you used to deal with is for analog sensors that are not working with low voltages , like distance sensors working with 24 volt analog signal and in this case you need to use a NPN transistor to make voltage scaling .

if you are working with complex operations needed sharp response time why dont you use FPGA merged with embedded board like ardunio and Xilinx Coolrunner II .

Sure you can

I made control unit for many machines using arduino and boards designed by me included atmega and atmega

The problems you will find is that most industrial analog sensors is0-v and4-ma so you  need to use some shields or to use converters

You can do it but it is dependent on the comlixity of the application. for some applications PLC is more suitable and easier to use.

it should not be that hard. you just need  bunch of5v mechanical relays and resistors, diodes for echo relay, and transistors or optocouplers. the hardware should very simple, usually the "big deal" is the software, though in your case it should be complex unless you plan for full user interface.

the internet is full or resources about arduino. below are some examples i found USE THE EXAMPLES ON YOUR OWN RISK, I DID NOT TEST, OR VERITY ANY OF THEM. 

check the schematics below

http://cdn.instructables.com/FOL/EWR3/G1QWARRC/FOLEWR3G1QWARRC.LARGE.jpg

http://www.electroschematics.com/wp-content/uploads/2013/07/arduino-control-relay-schematic.png

src1: http://www.instructables.com/id/Arduino-Christmas-Light-Controller/

src2: http://www.electroschematics.com/8975/arduino-control-relay/

That depends your application, if your working with low voltages devices you can use Arduino or Raspberry, but if your working in a factory it is better not to try that.

PLCs usually are used in plants or factory and works with24V, in the case of Arduino or Raspberry pi we are talking about low voltage devices which work with5V and these devices are not adviced to be used in a place that consists high power machines like a factory, because it will be affected by the electrical or/and the magnetic perturbation.