7.24.3 Shielding and Grounding

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-Separate voltage levels, and AC/DC wires from each other when possible.

•typical sources of grounding problems are:

-Electrostatic

-Magnetic

-Electromagnetic

-Resistance coupled circuits

-Ground loops

•Shielded wire is one good approach to reducing electrostatic/magnetic interference. The conductors are housed in a conducting jacket or the circuitry in housed in a conducting metal cabinet.

•Resistance coupled devices can have interference through a common power source, such as power spikes or brownouts caused by other devices in a factory.

•Ground loops are caused when too many separate connections to ground are made creating loops of wire that become excellent receivers for magnetic interference that induces differences in voltage between grounds on different machines. The common solution is to use a common ground bar.

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7.24.4 PLC Environment

• Care must be taken to avoid certain environmental factors.

Dirt - dust and grime can enter the PLC through air ventilation ducts. As dirt clogs internal circuitry and external circuitry, it can effect operation. A storage cabinet such as Nema 4 or 12 can help protect the PLC.

Humidity - Humidity is not a problem with the modern plastic construction materials. But if the humidity condenses, the water can cause corrosion, conduct current, etc. Condensation should be avoided at all costs.

Temperature - The semiconductor chips in the PLC have operating ranges where they are operational. As the temperature is moved out of this range, they will not operate properly and the PLC will shut down.

Ambient heat generated in the PLC will help keep the PLC operational at lower temperatures (generally to 0°C). The upper range for the devices is about 60°C, which is generally sufficient for sealed cabinets, but warm temperatures, or other heat sources (e.g. direct irradiation from the sun) can raise the temperature above acceptable limits. In

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extreme conditions, heating or cooling units may be required. (This includes “cold-starts” for PLCs before their semiconductors heat up).

Shock and Vibration - The nature of most industrial equipment is to apply energy to exact changes. As this energy is applied, there are shocks and vibrations induced. Both will travel through solid materials with ease. While PLCs are designed to withstand a great deal of shock and vibration, special elastomer/sprung or other mounting equipment may be required. Also note that careful consideration of vibration is also required when wiring.

-Interference - Discussed in shielding and grounding.

-Power - Power will fluctuate in the factory as large equipment is turned on and off. To avoid this various options are available. Use an isolation transformer. A UPS (Uninterruptable Power Supply) is also becoming an inexpensive option and are widely available for personal computers.

7.24.5 SPECIAL I/O MODULES

•Counters

•each card will have 1 to 16 counters generally.

•typical sample speeds 200KHz

•often allow count up/down

•the counter can be set to zero, or up/down, or gating can occur with an external input.

•High Speed Counter - When pulses are too fast to be counted during normal PLC ladder scans, a special counter can be used that will keep track of the pulses.

•Position controller - A card that will drive a motor (servo motor or stepper motor), and use feedback of the motor position to increase accuracy (feedback is optional with stepper motors).

•PID modules - For continuous systems, for example motor speed.

•There are 2 types of PID modules. In the first, the CPU does the calculation; in the second, a second controller card does the calculation.

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-When the CPU does the calculation, the PID loop is slower.

-When a specialized card controls the PID loop, it is faster, but it costs

more.

• Typical applications - positioning workpieces.

•Thermocouple - Thermocouples can be used to measure temperature, but these low voltage devices require sensitive electronics to get accurate temperature readings.

•Analog Input/Output - These cards measure voltages in various ranges and allow monitoring of continuous processes. These cards can also output analog voltages to help control external processes, etc.

•Programmers - There are a few basic types of programmers in use. These tend to fall into 3 categories:

1.Hand held units (or integrated) - They allow programming of PLC using a calculator type interface. And is often done using mnemonics.

2.Specialized programming units - Effectively these are portable computers that allows graphical editing of the ladder logic, and fast uploading/ downloading/monitoring of the PLC.

3.PLC Software for Personal Computers - They are similar to the specialized programming units, but the software runs on a multi-use, user supplied computer. This approach is typically preferred over 2.

•Man Machine Interface (MMI) - The user can use,

•touch screens

•screen and buttons

•LCD/LED and buttons

•keypad to talk to PLC

•PLC CPU’s - A wide variety of CPU’s are available and can often be used interchangeably in the rack systems. The basic formula is price/performance. The table below compares a few CPU units in various criteria.

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Legend:

prop. - proprietary technology used by a single vendor option - the vendor will offer the feature at an additional cost

•Specialty cards for IBM PC interface.

-Siemens/Allen-Bradley/Etc have cards that fit into IBM computers and will communicate with PLC’s. Most modern PLCs will connect directly to