PLC for monitoring a door?


I used to work at a company that manufactures automatic sliding doors for pedestrians. I found out that some of the bigger customers, such as airports and big department stores, desire to monitor each door when it gets opened, closed, or it's motion sensor actuators malfunction, floor mats malfunction, etc.

Since the door controller currently has NO interface to any network (no CAN, no ethernet, no serial cable, nothing), I heard that some customers are using PLCs.

I don't know much about PLCs. All I know is that they are industrial computers that can monitor data acquisition devices, and report to a system manager.

How then, are PLCs connected to a door controller unit? The door controller unit only has inputs for sensors and actuators. Is a PLC put in between the I/O and the door controller? Then the PLC reports all actuators and sensor events? This is the only way I see it can be done, without any connection method between the controller itself and the PLC.


Reply to
Mike V.
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A PLC may have I/O of many types and is a Programmable Logic Controller. It is a computer with a software layer that allows it to be programmed in relay ladder logic and other means like "C" and BASIC. They come with many types of comm port variations including Ethernet, CAN (and other busses), RS232/422/485 to name a few. I/O can include discrete / analog / different Buss's / and PLC specific and are configurable via add on card selection or expanders.

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You could probably connect the PLC inputs in parallel with the door controller inputs. I suspect this is not at all difficult but I would have to see an accurate schematic to come up with the right connection circuit. Controller outputs can easily be connected in parallel with PLC inputs. The trickiest part would probably be to find a convenient connection point since they may have multiconductor cables plugged directly onto a circuit board. Once you've figured it out you should be able to make a simple wire harness that can be easily plugged into the same connections as the existing.

Altogether this seems quite do-able.


Reply to
Walter Driedger

PLC's in their base form have inputs and outputs. These on medium to large PLCs come typically as groups of 16 or 32 on cards that is added in a slot that plugs onto a backplane bus connected to the CPU central processing unit. Thereby you can select any number of cards. The inputs can monitor switches and realy contacts and are typically designed for 24VDC,

110VAC and 230VAC and sometimes special levels such as 5V TTL or NAMUR for explosive atmospheres.

Smaller PLCs will probably come with only 16 inputs and outputs integrated into one package rather than add on cards.

The CPU cyclically reads the status of the inputs into a part of PLC memory called the image every 10 milliseconds or so where it can be used in logical opperations by the users prgram and combined with timers and counters to to set outputs or memory bits called flags.

Whether the PLC is small, medium or large they may run the same software though the small ones tend to be simplified.

On large to medium PLCs various communcation card can be added: ethernet, profibus, devicent, CAN, modbus etc etc.

There are functions in the PLC that can transmit the status of the inputs, outputs,counters and flags to other PLCs or to a higher level system ofen called a HMI (Human machine interface) or a SCADA package (Supervisory control and Data Acqusition)

This basically lets you draw pictures 'called screens' of the door and will allow an animation of it opening or closed (or a tank full or empty)

All HMI or SCADA manufactures eg metquips wonderware have already written drivers for the PLC anbd communication for it so you only end up dragging and dropping to program the thing.

HMI's are less capable but the graphics is just as good.

The door

The inputs of the PLC would could be connected to the same door limit switches that the doors controllers is or to spare contacts on the relays. PLCs inputs come with a variety of voltages and can be isolated so they wouldn't even haver to be spare just the same ones used.

The door manufacturers probably provide provision for this.

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FWIW, Mitsubishi FX0 PLCs are used to operate the sliding doors on train carriages in this part of the world.

Each door pair is controlled by one PLC (ie. two per carriage) with a single open/close signal linking the entire train. The Mitsubishi PLCs are compact, rugged, as easily programmed and so are ideal for a simple application like that.

Hope this helps, Cameron:-)

Reply to
Cameron Dorrough

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