Arvest

you have deduced already pretty much what we are trying to do, but let me elaborate: We are building a printer for 3D printing concrete using powder bed technology. It has a XY gantry (although the X axis is restrained to only 10 mm  of motion) with as a Z axis a platform (the bottom of the container in which we print) that can be lowered (hydraulic system). The array of nozzles is mounted along the X axis. The motion that the machine makes is relatively simple: the Y axis makes scanning motion, and at the end of a scan, the X axis moves 2.5 mm to the side, and the Y axis returns. This is repeated again, as the nozzles are 10 mm apart. Then the Z platform (the print bed) moves down 2.5 mm, another layer of dry material is applied through the U-axis, and the process is repeated. As said, we use an array of 88 nozzles to which just eject water. We use so many nozzles in a row to speed up the process. Our resolution is 2.5mm, and the volume we print is roughly 1x1x1 meter, which means 400x400x400=64 million points. (Also you are right that 16 bit is not sufficient for addressing 88 valves. I mispoke, it of course needs 11 bytes to encode it). But basically it all boils down to roughly 64M / 8 = ~8Mbyte, plus some overhead (other equipment also needs to be controlled during depositing of the layers). The valve behavior look-up table is generated from some slicer software, which is written by ourselves, so we have full control over the output format.  

In the mean time I have gone through a large part of the documentation and tutorial videos on the Schneider Electric Modicon M241, but am unable to find a description of how to implement my use case, as I described in my initial post: download a program/instruction set (which contains the information on when which valve should open), and have the PLC program look up the data based on a encoder value. I can find things about 'recipes', but these seem to be mainly used as varying a few variables, like the number of holes you want, or the time to mix something. The program I need to load will be about 15-20 megabytes, consisting of a lookup table to look up a 16-bit value for each X,Y,Z value. Nowhere I can find anything on using a PLC like that. Would you have any pointers where to find more information? How is this normally implemented?

Thanks for the reply. I still need to figure out how to get that pattern values into my PLC program, but that is (in this case) a more specific Modicon question. You are right about the Modsbus RTU being too slow. I have used Modbus in the past to provide corrections to a robot arm in real time, but that was Modbus TCP. I mixed the two up. Modbus TCP should be fast enough, depending on how effecient the PLC is at getting that signal out there. Direct connection to the PLC would not be the preferred option because of all the cabling involved. A RTU would be a much lighter and simpler option. Regarding the coils: currently we use a normal flyback diode, and this achieves a 7 ms closing time. I will investigate using a MOV or Zener in the design, but I believe not the coil, but the spring inside of the valve is the limiting factor wrt the closing time.

Hello, quite new to PLC's, currently figuring out if / which PLC is the way to go for my application: I'm construction a machine with a two axis (X/Z) with 88 valves mounted on them. The X axis makes a scanning motion, and after every pass, the Z axis moves down 2.5 mm. When the X-axis moves, the valves need to switch in a certain pattern, opening / closing with a resolution of 2.5 mm, based on the current X and Z-position (i.e. encoder values). My goal is to move around 0.25m/s, so they would switch around 100 times per second (i.e. every 10ms). I want to be able to upload the pattern via Ethernet connection from my PC. The valves will most probably be switched through a Modbus RTU digital I/O unit(s) I ended up with the Modicon M241 as a nice PLC with the right I/O capabilites and features for my application. However, I don't know if it is suitable, as I am unable to find out how the whole uploading a pattern to memory, and then retrieving values one by one from memory during operation would work (i.e. which instructions should I use in my PLC program?)