Hi Methier,
We opted for a PLC for the control of the hydraulics and pedal control. One of the PLC inputs (physical) is an OK from the MESA card, to interlock the two together. The PLC controls the hydraulics end-to-end, so the pressure limit is a variable in the PLC that can be set via LinuxCNC or via the PLC's dedicated HMI. Pedal/hand, auto-return and a few other settings are also PLC variables. The original design was that the press should work safely without LinuxCNC. Safety is key, so the hydraulics, and the axis motors are all protected via safety relays and light curtain - these are physically interlocked into the wiring!
The X and Y positioning is done by LinuxCNC, along with the monitoring of the X and Y encoders (completing the PID loops), and the Y0 linear encoder. There's a few other IO between the MESA and PLC, and they're all physically wired. The remaining interfacing is via ModBus, for anything that doesn't need to be realtime (like setting the hydraulic pressure).
The idea:
- PLC handles the actuation of the RAM, and characteristics like pressure limit, stop/auto-return, hand/foot control etc
- LinuxCNC handles the accurate positioning of the Y axis (we have moving stoppers that govern the depth of stroke), X axis and any future backgauge axis we need.
- LinuxCNC also acts as an HMI to set/configure all of the available machine parameters.
I did pretty detailed design and documented everything (as we do with machines we design/build), so we had very few issues on commissioning. Just remember, it's a big dangerous machine, and you need more E-Stops than you think - and they need to be hardwired into a rated safety relay, that drops the power from the hydraulic circuit, motors and pump. Send a feedback signal to LinuxCNC for the E-Stop, but DO NOT use LinuxCNC to actuate the E-Stop!!
We have four layers of wiring: 400vac for motors, 230vac for switchgear, 24vac for control relays and 24vdc for IO. By cutting the 24vac, the 400vac and 230vac become isolated. We don't need to isolate the 24vdc as it's purely IO, and the only actuators are interface relays that control a contactor in an ac circuit. So as a result, if you hit the E-Stop, the machine becomes intrinsically safe - the PLC, onboard HMI, the MESA and of course all of the sensors remain on - this is really useful, as you can diagnose faults and see what's happening with the machine - importantly, you don't loose any positioning data. Avoid wiring E-Stops that shut the entire machine down - it's not useful.
We've still got more to add to the HMI, but the beauty of it, is that you can. And it's not even stuff in commercial systems that we want to add!
