ARTICLES             

2001/01- What is a PLC

2001/02- What is a SCADA - MMI system

The word PLC means Programmable Logic Controller.

The main components of a PLC  are the following:

Main Rack

Power Supply

CPU

Digital Input/Output cards

Analog Input/Output cards

Special features cards

MAIN RACK

This is where the rest of the elements are connected or plugged in. It is normally screwed to the electric cabinet mounting plate. It can get a certain number of cards depending on the manufacturer and to be connected to other racks through a communication cable, being called in this way auxiliary rack.

POWER SUPPLY

Converts the input power of 110 or 220 ACV to 24 DCV and necessary current to supply the CPU and In/Out cards as well as auxiliary equipment if needed.

CPU

This is the brain of the PLC. With one or more microprocessors (depending on the manufacturer) it is programmed with a special software. Most of them are using a standard programming language like instructions, contacts, functions, sequentials, etc. They work with logic 1 – 0, this is two different states for the same bit. Normally use 16 bits word as memory basis although some new ones work on a 32 bits words basis. Depending on the CPU models, are able to manage more memory and calculation functions as well as microprocessor speed.

Nowadays the calculation power of this devices is very high and is usual to work with floating point numbers thus giving mathematical precision more than needed.

It is normal to have 10 msec. scan cycle for typical digital application and about 40 msec. for an analogue one, much more fast than any process transmitter or valve actuator.

The application  program is made in a special language, it has a beginning and an end. The time between them is called scan cycle and there is a timer inside the PLC´s CPU that look for the program to be executed from the beginning to the end before that timer ends. This is the WATCHDOG timer. If this timer ends before the scan cycle, the PLC will go to the STOP status. 

Digital Input/Output cards

They are connected to the main or auxiliary rack and communicate with the CPU across related connection. In case of digital inputs these ones transmit the 0 or 1 status of the process signals (pressure switches, limit switches, proximity switches, switches, etc) to the CPU.  In case of digital outputs, the CPU will set or reset them following the program instructions.

24 DCV for inputs and outputs are normally used, but  110 or 220 ACV inputs/outputs as well. The number of input/outputs per card can be from 8, 16 or 32 points.

Tarjetas entradas/salidas analógicas

They are connected the same way than the previous ones but in some PLCs have to be as close to the CPU as possible.

These cards receive an analogue value from field and internally convert it to a digital value to be processed by the CPU. This conversion is made by Analogue to Digital converters located in the analogue card and distributed to one for the whole card or one per channel depending on the card model (this is the fastest one) .

They are made of 2, 4, 8 or 16 input/outputs called each one “channel” and referred from 0 to (Max – 1), for example, the first channel of a 4 analogue outputs card is named 0 and the last one is named 3.

Input and output ranges varies between 4-20 mA (milliamps), 0 to 10 DVC, etc.

The most important thing to be considered when designing a control using analogue input cards is that is desirable to be galvanically isolated, otherwise will have problems with the value readings.

Special features cards

Connected in the same way than previous ones are normally used to control or monitorize special process variables like rotation speed, position, frequency, etc. They normally have an independent processor that makes the work and discharge of it to the PLC´s CPU. The following are some examples of Special cards:

Fast counting cards

Motors positioning cards

Regulation cards ...

A MMI system (Man Machine Interface) or HMI (Human Machine Interface) is the connecting point between the operator and the machine. It can be an operator panel or a Personal Computer (PC) but in both cases transmit and receive data from PLC.

The Operator panel, is composed of a screen with big or low graphics  resolution and numeric or function keys located in the panel or over the same screen (touch screen). This screen can be of monochrome or colour type and indicates the status of the different process variables with graphics, figures, or field data points being also able to introduce set points to adjust the control parameters, alarms, etc.

These Operator Panels are programmedd with a special software, similar to the PLCs but different of them. They communicate with the PLC normally with a serial port usually at a speed of 19200 bauds/sec. Some of then use faster communications.

The front of the panel use to be of a plastic material and high degree of protection, NEMA 12 or IP65, to avoid damaging when exposed to dust, water drops, etc.

The following are the functions that a standard Operator Panel can develope:

Monitorize and adjustment of process control data (read and write variables)

Process Alarms control, with help text of each one for operator.

Process Alarm history, to know when an alarm happened, was acknowledged and went off.

Alarms printing

The Portable Computer (PC), can make the same functions than the Operator Panel and moreover can make SCADA functions (data acquisition) and with the new software controls integrated under Windows OS con send an E-mail or message to a movile phone or make a phone calling under an event (process alarm).

To convert a standard PC in a SCADA-MMI system the following items are needed:

SCADA Software. This is the Software program that installed in the PC makes it to work like a SCADA system. It can be from the same manufacturer than the PLC or different one. 

PC-PLC Communication card. It is normally supplied by the PLC manufacturer but can also be from the SCADA system manufacturer. It is located in a free PC mainboard bus (ISA or PCI) and is configured  with a special software, different from the SCADA one. 

Communication driver. This is the “translator” between the SCADA-MMI system and the PLC.  It is a software program different to the SCADA and makes the PC and the PLC to understand between them via the communication card.

Basically the SCADA program generates a data base with the process parameters  (TAGNAMES) and the communication driver is in charge of read and write these data in the PLC.

It is strongly recommended to use the same manufacturer for the communication card than the PLC.

There is not any written rule about what PC is better for a SCADA application, but is necessary to clarify with the manufacturer about the compatibility of the PC and the Operating system (OS) with the communication card, specially with the plug&play and IRQ configuration of certain PCs.

A SCADA system has more advantages than an Operator Panel, one of the most important ones is the data storage in the hard disc or any other backup system for statistics and trend analysis.

With the new ETHERNET communication systems a process can be monitorized from any place in the factory or the world when using a modem.

Information:   Marcos Ubiria Manzarraga 

    BILBAO - SPAIN