Instrumentation Sensors Book

Figure 14.3 Computer-based digitally controlled process.

the digital output signal is fed to the actuator through a demultiplexer and converted to an analog signal by a DAC. The processor also will have mass storage for storing process data for later use or for making charts and graphs. The process also will be able to control a number of peripheral units and sensors as shown, and perform alarm functions [4].

14.3Controller Operation

The central processing unit can be divided into the processor, memory, and input and output units or modules, as shown in Figure 14.4. The units are interconnected by a two-way 16-bit data bus, a one-way address bus, and a one-way enable bus. The address and data buses are common to all units, and the enable bus will select the individual units being addressed and connect the unit’s data bus drivers or receivers to the data bus. The enable and address buses are controlled by the processor, which uses software instructions for its direction. When addressing an input module, the module is selected with its enable bus code. The address bus then can be used to select which external input data is to be put onto the two-way data bus. This data is then transferred to memory to wait for the next step from the software instructions. The output modules are addressed and selected in the same way as the

Data bus

Address bus

Enable bus

Figure 14.4 Block diagram of processing unit.

AC Input

Figure 14.6 Block diagram of an input module with ac or dc input.

uses a bridge rectifier to convert the ac to dc, then uses the same circuit blocks as in the dc module. The LED is used to indicate the input logic level of the input signal. The input level LED indicators are normally located above the tag strip.

14.4.2Analog Input Modules

Typical analog input modules are used to convert analog signals to digital values or words. Analog signals are derived from temperature, pressure, flow, position, or rate measurements. Analog signals can be single-ended or differential. Figure 14.7 shows the block diagram of a differential analog receiver. The signal is converted into a digital word. The resolution of the ADC is normally 305.176V when using voltage input, and 1.2207A when using a current input. The ADCs are 14 to 16 bits in length. The inputs are floating, so that the output from the ADC uses an optoisolator to reference the digital signal to the ground level of the controller.

Analog input applications are temperature, humidity, pressure, flow rate, load, thermocouple, RTD, magnetic, and acceleration sensors. The typical input voltage levels are from 50 to 500 mV, up to ±10V, and current ranges from 20 to 50 mA.

14.4.3Special Function Input Modules

To satisfy some special cases, a variety of discrete modules are available to process or interface special signals. A list of these functions is given in Table 14.2.

Figure 14.7 Analog input module block diagram.

Status

LED

Output

Controller Logic Optical

isolation

Status

LED

Output

Controller Logic Optical

isolation

Power

Figure 14.8 Discrete ac output module.

14.4.5Analog Output Modules

Analog outputs from the PLC drive analog meters, chart recorders, proportional valves, and variable speed drive controllers. They also are used for current or voltage to pneumatic transducers. The voltage and current output ranges are the same as the input ranges. Figure 14.9 shows the block diagram of an analog output stage. The digital output from the controller is fed via an optical isolator to an ADC to reference the signal to the ground of the peripheral device. The analog output of the converter is amplified and fed to a voltage or current driver, which can have a single-ended output or a differential output. The output signal also will meet the standard voltage or current control ranges.

14.4.6Smart Input/Output Modules

A number of specialized modules have been developed to interface to the controller. They normally contain their own processor and memory, and can be programmed to perform operations independent of the central processor. These modules are categorized in Table 14.4 [6].

Serial and network modules are used for data communication. The serial modules communicate between other PLCs, message displays, operator terminals, and intelligent devices. The network modules are used for LAN and WAN. The Manufacturing Automation Protocol (MAP) is used for communication to robotic devices and a variety of computers of differing manufacturers that can support MAP [7].

The coprocessor modules are basically used for housekeeping functions, such as math functions, algorithms, data manipulation, outputs of reports, outputs to

Status

LED

Figure 14.9 Analog output block diagram.

printer, displays, and mass data storage, using basic programming language functions that would be hard for the main processor to perform with ladder logic.

Intelligent modules performing closed-loop control algorithms are required for PID functions, such as maintaining temperature, pressure, flow, and level at set values. However, the introduction of smart sensors reduces the load on the processor and communication to the processor in this case is via the Fieldbus.

The internal control function of the PID module using an analog loop is shown in Figure 14.10. The analog temperature signal from the furnace is the input to the module, where it can be converted to a digital signal and recorded in the computer memory, fed to the PID controller via a gain control. It is then sent to an analog differencing circuit, where it is compared to the set point signal from the processor. The furnace signal and set point are subtracted, giving an analog error voltage. The error voltage is fed to the PID controller. The controller then produces an analog

Figure 14.10 Supervisory PID control using an analog control loop.

14.5Ladder Diagrams

The ladder diagram is universally used as a symbolic and schematic way to represent the interconnections between the circuit elements used in programming a PLC. The ladder network also is used as a tool for programming the operation of the PLC [10]. The elements are interconnected between the supply lines for each step in the control process, giving the appearance of the rungs in a ladder. A number of programming languages are in common use for controllers, as follows:

•Ladder;

•Instruction list;

•Boolean flowcharts;

•Functional blocks;

•Sequential function charts;

•High-level languages (ANSI, C, structured text).

14.5.1Switch Symbols

Figure 14.11 shows the typical switch symbols with switch number used for the elements in a ladder diagram. A number of momentary action switches are shown. These are, from top to bottom: a push to close [normally open (NO)] and push to open [normally closed (NC)]. These switches are the normal momentary action panel mounted operator switches.

Position limit switches sense the position of an object, and close or open when a desired position is reached. Pressure, temperature, and level switches set limits, and can be designed to open or close when the set limits are reached. The level switch shown is the lower limit switch (LLS), The upper limit switch (ULS) or full switch (FS) are not shown. The output from the most common flow meter produces pulses that go to a counter. The symbol for the meter is shown.

Figure 14.11 Switch symbols in use for ladder diagrams.

14.5.2Relay and Timing Symbols

Figure 14.12 shows the symbol for relays, timers and counters. A control relay is a circle with the designation CR, followed by the number of the device to distinguish between the various relays used. This is shown with the symbols for its NO and NC contacts. These contacts will carry the same CR and number as the relay. A timer relay has the designation TR in a circle with a number, and its associated NO and NC contacts will be likewise named and numbered. A timer has two sets of contacts:

(1) the On-delay contacts that operate after a time delay, and (2) the Off-delay contacts that operate as soon as the delay is initiated and reset at the end of the set time period. The delay time also will have the delay time specified. A counter has the designation CTR with its number. A third lead on the counter is used to activate the count, and to reset the counter after the count has finished. The counter will specify the number of counts.

14.5.3Output Device Symbols

Symbols for output devices are shown in Figure 14.13. A circle with the letter M and an appropriate number represents a motor. A circle with radiating arms and a letter to indicate its color (e.g., R = red, B = blue, O = Orange, and G = green), plus its number represents an indicator. A solenoid that is used to operate a flow valve or move an object, or for a similar, unspecified, use, is designated SOL with number. Other output elements without specific symbols, such as alarm, heater, or fuse, are represented by boxes, as shown, with the name of the element and its number in the box to distinguish between similar types of elements used in different places.

Figure 14.12 Symbols for control relays, timers, counter, and contacts.