Related Literature:

In order to attain the target device for double dead meat detection, it is

therefore an object of this invention should provide the applications of sensors,

controllers or if possible sensor systems.

A sensor is defined as a device that receives and responds to a signal or

stimulus.(Handbook of Modern Physics, Designs and Applications: Jacob

Fraden, 2004). Moreover, a sensor responds with an electrical signal. The

purpose of a sensor is to respond to some kind of an input physical property

(stimulus) and to convert it into an electrical signal which is compatible with

electronic circuits. We may say that a sensor is a translator of a generally

nonelectrical value into an electrical value.

The term sensor should be distinguish from transducer. The latter is a

converter of one type of energy into another, whereas the former converts any

type of energy into electrical. Transducer may be used as actuators in various

systems. An actuator may be described as opposite to a sensor, since it

converts electrical signal into generally nonelectrical energy.

Two types of sensors includes direct and complex sensors. A direct

sensor converts a stimulus into an electrical signal or modifies an electrical

signal by using an appropriate physical effect, whereas a complex sensor in

addition needs one or more transducers of energy before a direct sensor can be

employed to generate an electrical output.

A sensor does not function by itself; it is always a part of a larger system

that may incorporate many other detectors, signal conditioners, signal

processors, memory devices, data recorders, and actuators.

Any material may become a subject of some kind of a measurement.

Data are collected from an object by a number of sensors. In Fig. 1, sensor 1

perceives the object without a physical contact and, therefore, is called a

noncontact sensor. Sensor 5 monitors the internal conditions of a data

acquisition system itself. Some sensors (1 and 3) cannot be directly connected

to standard electronic circuits because of inappropriate output signal formats.

They require the use of interface devices (signal conditioners). Sensors 1,2,3

and 5 are passive. They generate electric signals without energy consumption

from the electronic circuits. Sensor 4 is active. It requires an operating signal,

which provided by an excitation circuit. This signal is modified by the sensor in

accordance with the converted information.

Electrical signals from the sensors are fed into a multiplexer (MUX),

which is a switch or a gate. Its function is to connect sensors one at a time to

an analog-to-digital(A/D) converter if a sensor produces an analog signal, or

directly to a computer if a sensor produces signals in a digital format. The

computer controls a multiplexer and an A/D converter for the appropriate

timing. Also, it may send control signals to the actuator, which acts on the

object. The system contains some peripheral devices (for instance, a data

recorder, a display, am alarm, etc.) and a number of components, which are not

shown in the block diagram.

Fig. 1. Positions of sensors in a data acquisition system. Sensor 1 is noncontact,

sensors 2 and 3 are passive, sensor 4 is active, and sensor 5 is internal to a data

acquisition system.

All sensors may be of two kinds: passive and active. A passive sensor

does not need any additional energy source and directly generates an electrical

signal in response to an external stimulus; that is, the input stimulus energy is

converted by the sensor into the output signal. Examples are thermocouple, a

photodiode and a piezoelectric sensor. While, active sensors require external

power for their operations, which is called an excitation signal. That signal is

modified by the sensor to produce the output signal. Examples are thermistors,

and resistive strain gauge.

Depending on the selected reference, sensors can be classified into

absolute and relative. An absolute sensor detects a stimulus in reference to an

absolute physical scale that is independent on the measurement conditions,

whereas a relative sensor produces a signal that relates to some special case.

Another way to look at a sensor is to consider all of its properties, such

as what it measure (stimulus), what its specifications are, what physical

phenomenon it is sensitive to, what conversion mechanism is employed, what

material it is fabricated from, and what its field of application is.

Modern industrial control systems are microprocessor- based

programmable systems containing hardware and software for direct control,

distributed control, programmable control, and PID action. The systems are

designed not only for continuous monitoring and adjustment of process

variables, but also for sequential control, which is an event-based process, and

alarm functions (Introduction to Instrumentation, Sensors, and Process

Control; William C. Dun, 2006).

The processor in a PLC system has software that is easily programmable

and flexible, making the initial program, updates, modifications, and changes

easy to implement. Because of the complexity and large number of variables in

many process control systems, microprocessor-based PLCs are used for

decision making. The PLC can be configured to receive a small number of

inputs (both analog and digital), and control a small number of outputs. The

system also can be expanded with plug-in modules to receive a large number of

signals, and simultaneously control a large number of actuators, displays or

other types of devices. PLCs are categorized into low-end, midrange, and high-

end, where low-end is from 64 expandable up to 256 I/Os, midrange is

expandable up to 2,048 I/Os, and high-end is expandable up to 8,192 I/Os.

Fig. 2 shows the block diagram of the basic controller. A variety of input

modules are available for interfacing between the digital and analog signals, PID

functions, and the processor’s input bus. Output modules are used for actuator

control, indicators, alarm outputs, and timing functions, and to interface

between the processor’s output bus and peripheral units. The modules are

rack-mounted, so that only the required modules can be used, leaving rack

space for expansion. The memory can be divided into RAM for system operation,

ROM, and EEPROM (nonvolatile memory) for storing set point information.

Fig. 2. Block Diagram of a programmable controller.

Programmable logic controllers, also called programmable controllers or

PLCs, are solid-state members of the computer family, using integrated circuits

instead of electromechanical devices to implement control functions. They are

capable of storing instructions, such as sequencing, timing, counting,

arithmetic, data manipulation, and communication, to control industrial

machines and processes.

Fig. 3 illustrates a conceptual diagram of a PLC application.

Programmable controllers have many definitions. However, PLCs can be

thought of in simple terms as industrial computers with specially designed

architecture in both their central units (the PLC itself) and their interfacing

circuitry to field devices (input/output connections to the real world).

Programmable controllers are now mature control systems offering many

more capabilities than were ever anticipated. They are capable of

communicating with other control systems, providing production reports,

scheduling production, and diagnosing their own failures and those of the

machine or process. These enhancements have made programmable controllers

important contributors in meeting today’s demands for higher quality and

productivity. Despite the fact that programmable controllers have become much

more sophisticated, they still retain the simplicity and ease of operation that

was intended in their original design. (Programmable Controllers, Theory and

Implementation; L.A Bryan, E.A Bryan,1988)

Principles of operation of PLC:

A programmable controller, as illustrated in Figure 4, consists of two basic

sections:

• the central processing unit

• the input/output interface system

Fig. 4. Programmable controller block diagram.

The central processing unit (CPU) governs all PLC activities. The following three

components, shown in Figure 5, form the CPU:

• the processor

• the memory system

• the system power supply

Fig 5. Block diagram of major CPU components.

The operation of a programmable controller is relatively simple. The

input/ output (I/O) system is physically connected to the field devices that are

encountered in the machine or that are used in the control of a process. These

field devices may be discrete or analog input/output devices, such as limit

switches, pressure transducers, push buttons, motor starters, solenoids, etc.

The I/O interfaces provide the connection between the CPU and the information

providers (inputs) and controllable devices (outputs).

During its operation, the CPU completes three processes: (1) it reads, or

accepts, the input data from the field devices via the input interfaces, (2) it

executes, or performs, the control program stored in the memory system, and

(3) it writes, or updates, the output devices via the output interfaces. This

process of sequentially reading the inputs, executing the program in memory,

and updating the outputs is known as scanning. Figure 6 illustrates a graphic

representation of a scan.

Fig. 6. Illustration of a scan.

The input/output system forms the interface by which field devices are

connected to the controller (see Figure 7). The main purpose of the interface is

to condition the various signals received from or sent to external field devices.

Incoming signals from sensors (e.g., push buttons, limit switches, analog

sensors, selector switches, and thumbwheel switches) are wired to terminals on

the input interfaces. Devices that will be controlled, like motor starters, solenoid

valves, pilot lights, and position valves, are connected to the terminals of the

output interfaces. The system power supply provides all the voltages required

for the proper operation of the various central processing unit sections.

Fig. 7. Input/output interface

Although not generally considered a part of the controller, the

programming device, usually a personal computer or a manufacturer’s

miniprogrammer unit, is required to enter the control program into memory

(see Figure 8). The programming device must be connected to the controller

when entering or monitoring the control program.

Fig. 8. (a) Personal computer used as a programming device and (b) a

miniprogrammer unit.

A sensor system is formed with sensors and a controller which are

detachably attached to a network to be connected together. Each sensor stores

an interface program for its own and the controller serves to upload these

interface programs from the individual sensors and use the uploaded interface

programs to access these sensors under optimum conditions.

The invention relates to sensors, as well as controllers and sensor

systems including such sensors and controllers.