Match programmable logic controller (PLC) devices with their application

Programmable logic controllers (Plcs) are software-based equivalents of hard wired relay logic control panels used to electrically control a machine or process. Plcs are composed of an input interface (or input module), output interface (or output module), logic processor unit with memory storage (cPu), power supply, and programming device. Programming devices may either be an integral part of the unit or separate so they can be used to program other units of the same type. the components of a Plc are shown in figure 7-6.

referring to figure 7-6, the input interface, which is hard wired to the machine, receives signals or commands from the machine in either one of two forms:

1. digital signals - on/off or closed/open. examples include push buttons, limit switches, and position encoders, or:

2. analog signals, which are a variable voltage or current signal. an example of an analog signal is the variable input to the amplifier card of a proportional valve. a digital-to-analog card converts the digital output signal from a Plc into the analog signal required by the valve’s amplifier card.

the logic processor of the Plc is commonly referred to as the central processing unit (cPu). the cPu analyses and integrates the input data received from the input interface based upon the instructions that have been programmed into memory. When the various preprogrammed conditions have been met, the cPu will command the output interface to send a signal to the appropriate output component. an example of how the cPu executes a series of instructions is shown in figure 7-7.

note that in the diagram in figure 7-7, items a, B, and c are shown as brackets. these symbols may be shown instead as vertical lines. also, item d is shown as parentheses. Item d might have been shown instead as a circle. the difference in symbols is based on whether or not the ladder diagram was generated by a typewriter or a text based computer program rather than a program dedicated to generating ladder diagrams.

1. the cPu will scan the input interfaces to determine the status of a, B, and c.

fig. 7-6: Programmable logic controller components.

7 -  • Hydraulic Specialist • Study Guide HS Manual # 40 - 07/0/06

Interface Hydr aul Ic Instrumentat Ion and control systems

2. Based upon the program logic, if input a and input B, or if input c is true, the cPu will turn on output interface d.

3. the cPu will continue to scan the input interface and will act on the status of the inputs based upon the logic determined by the program.

the cPu also has memory storage capabilities. the program stored in memory tells the cPu what to do. the program is the logic of the Plc. the program is stored in memory by either the manufacturer (oem) of the machine or by the end user of the machine. most Plcs use random access memory (ram), programmable read only memory (Prom), or erasable programmable read only memory (eProm).

random access memory (ram) stores information in a solid state chip. the information is retained by supplying a constant electrical supply. therefore, a backup battery is required in order to retain the memory storage in case of a power failure. this is currently the most commonly used type of memory.

Programmable read only memory (Prom) consists of solid state chips with information permanently stored in them. Prom chips do not require electrical power in order to retain memory. normally, Prom chips are not re-programmable.

erasable programmable read only memory (eProm) is electrically erasable. as is the case with Prom chips, an electrical supply is not required in order to maintain memory storage.

all of these memory types file fixed format instructions. the fixed format consists of the instructions the Plc will process. the program’s instructions are read sequentially, one instruction at a time, line by line.

the time required to read the memory one time is called the memory scan time. one instruction takes up one unit of memory. the total memory capacity is the maximum number of instructions or data that the memory can store.

the power supply converts alternating current into the direct current that the processor needs for operation.

a typical power supply will convert 120 vac current into +5 and +15 vdc current. some Plcs do not have integral power supplies. If such is the case, an external power supply must be used.

a program may be uploaded into a Plc by a variety of means including, but not limited to a dedicated keypad, a special application programming device, a laptop computer, or a wired connection to a central computer system.

Programmable controllers have almost completely replaced the use of hard wired relay logic to control machines. there are several reasons for this. first, generally it takes less time to install and wire a Plc

fig. 7-7: Processor logic.

Hydraulic Specialist • Study Guide • 7 -  HS Manual # 40 - 07/0/06

Interface Hydr aul Ic Instrumentat Ion and control systems

than it does to mount and wire a group of relays. secondly, a program may be easily revised and uploaded to the Plc. to make changes in a relay logic system requires rewiring the relays. also, small, low cost Plcs are available, making it cost effective to use a Plc rather than even a small number of relays. finally, a Plc based system requires less panel space than does an equivalent relay logic system.

electromechanical functions, such as timers and counters, are easily duplicated in Plc logic, negating the need for the additional cost and space for these components. often, such components are configured for a specific adjustment range, such as 0 to 1 minute, 0 to 10 minutes, or 1 to 100 counts. If a different range is needed, the component needs to be changed out. In contrast, timer and counter functions programmed into the logic of a Plc do not have any limits on their ranges, and have no wearing parts, as do electromechanical components.

another advantage of Plcs is that they can be used to perform arithmetical and logic operations. Plcs can store data collected from sensors, such as pressure, position, velocity, or temperature vs. time information.

some Plcs include self-diagnostic capabilities.

a Plc may be used to control just a segment of a machine, such as an electrohydraulic servo system, or it can control an entire machine. furthermore, Plcs can be networked together.

the inputs to the Plc typically consist of all of the physical input devices. each device is wired to a different input connection and is given its own unique number or label. the labels are used in the program to identify the input device. In the wiring diagram shown in figure 7-8, input contacts mr-1 and mr-4 do not need to be wired to the input connections on the Plc circuit. Instead, these contacts, and their associated relay coils, are mimicked in the Plc’s program using logic relays. the Plc inputs can be specified in rating for many voltage and amperage configurations: 12 vdc, 24 vdc, + or - 10 vdc, 120 vac, 240 vac, 0 to 20 ma, or 4 to 20 ma, for analog inputs. digital inputs are acceptable as well. each input is generally wired to a small light emitting diode (led) so that one may visually determine whether the signal from the input

fig. 7-8: Plc logic Wiring diagram.

7 - 0 • Hydraulic Specialist • Study Guide HS Manual # 40 - 07/0/06

Interface Hydr aul Ic Instrumentat Ion and control systems

device is “on” or “off.”

the Plc’s instruction manual details specifics related to the input devices such as the allowable voltage rating of the input devices, how the inputs are wired, and the numerical input address of the terminal connection numbers. the input address is the electronic memory address where the Plc stores the value (0 or 1) of the input device.

In many ways, Plc outputs are similar to Plc inputs. the outputs are physical devices that are wired to the Plc’s output terminals. these terminals are numbered, as are the inputs. the output must match the rated voltage of the output device, for example, 120 vac. this is generally accomplished by an output relay device. figure 7-9 illustrates a Plc output strip with variable outputs.

Plc outputs are special in that they have a current rating. consequently, each output device must not draw more current than the rating of the output strip. to protect the output circuit, some manufacturers provide fusing. this wiring diagram has a common voltage source connection provided for each group of four outputs. this configuration allows for the possibility of using one voltage configuration on the first group of four outputs, and a different voltage output for the next group of four outputs. this method is commonly uses groups of relay contacts rated for a specific output voltage and amperage range. many times, the “common” connection is dedicated to only one output rather than to multiple outputs. the Plc does not supply the voltage or current to the loads.

fig. 7-9: Plc output strip with Variable outputs.

Review 7.2.1.1: analog output signals from a Plc