6/30/2020 12:45 PM | |
Joined: 4/13/2019 Last visit: 6/9/2022 Posts: 1 Rating: (2) |
I was asked to look at an ancient underfloor heating system for a local church. The underfloor heating elements were fine but the control was well past it’s sell by date. I’ve been retired for almost 10 years so I fell back on control I was confident with which when I was working was S7 – 200,300,400 series. During preparation of the FSD someone asked if I could provide system entry from Google calendar as the church used this function to log one off events like weddings, funerals, concerts etc. To provide a multizone adaptive heating system I settled on an S7 – 1215 and initially included a serial interface to link to some as yet undefined interface at that time. The church could take over 30 hours in winter to get up to temperature so one-off events had to be interrogated up to 2 days in advance. The main PLC control was installed in autumn 2019 and throughout the winter quarter gave greater than a 25% saving on the church’s heating bill, enough to pay for all the new hardware. It was just after the initial installation whilst looking through the Siemens TIA portal that I came across the application example https://support.industry.siemens.com/cs/ww/en/view/109748872 and started to become interested in MQTT as a means of messaging. I then looked at an interface to interpose between the PLC and the Google calendar. Whatever it was, it had to have no moving parts, fans or magnetic disks, to be robust enough to last for many years. When I was working, I tended to use mini ITX PC’s with custom software written in VB which is what I initially considered. But when I looked at the implementation of MQTT and the calendar interface on the PC, the path ahead was not straightforward. I then began to look at the problem from the aspect of well supported software with many examples and was surprised to find that the Raspberry Pi fitted the bill better than anything else. Single PC, no moving parts, Mosquitto broker and client were easily installed https://randomnerdtutorials.com/how-to-install-mosquitto-broker-on-raspberry-pi/ (one of many) and a well-documented API from google with examples provided the interface for the calendar. https://developers.google.com/calendar/quickstart/python The only drawback for me was that I had to learn a new operating system, Raspbian, and a new programming language, Python. But with plenty of examples and help on the internet for a 70-year-old like me, all the programming cards fell into place. https://www.learnpython.org/ Having chosen a simple Ascii protocol I put the control of the messaging under the control of the S7. The PLC programming was very straightforward with a plug in module and excellent application support from Siemens. Status messages and calendar queries were generated by the PLC to the Pi. A simple GUI on the Pi displayed this status information and when it received a calendar query for a specific church area it would look up to 2 days ahead for any entries. One of the inbuilt functions of the Pi is VNC which not only allowed remote viewing of the status GUI but also allowed me to access the PLC Web pages/watch tables using the browser on the Pi. This proved invaluable during commissioning and early trials. Initial entries are made in the calendar specifying start and stop times, area of church to be heated in the location box and the initial entry colour was set at yellow. When the entry was accepted by the PLC the colour was changed to red by the Pi and then every 15 minutes it would update the description box in the calendar entry with time, area temperature and control status. Finally, when the event was finished the calendar entry would be turned to grey by the Pi. The last security feature was to interpose a hardware watchdog using a 74HC14 in the Pi supply and triggered from a spare GPIO even though the Pi has an onboard software WD. Comments on the internet about the onboard watchdog were mixed so I opted for a true hardware solution and manipulation of the relevant GPIO was embedded in the python program using just 1 instruction. All in all a very worthwhile and absorbing project for a has-been engineer. N.B. You will need V16 of TIA portal to undertake fully the PLC programming side. |
Last edited by: Jen_Moderator at: 06/30/2020 13:52:37Optimized link. |
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