Chapter 5-8 study guide

Vaccum symbol

Information about sensors…

It has no moving parts. It is solid state electronic.

*You can’t open it to fix it so when it goes bad, replace it*

What is a Capacitor proximity sensor and its principle

Limit switch with a diamond around it. Its principle is it works from electrostatic capacitances(Like a capacitor.) and it detects everything but gases.

What is an inductive proximity sensor?

It has a diamond around it. And its principle is electromagnetic induction and detects ferrous metals like

How does an Inductive proximity sensor work?

Current flows through inside and when a ferrous metal is detected within range, the coil inside receives a current magnetic field and it robs/absorbs in the magnetic field. And the sensor detects the voltage and then drops/changes state.

Define capacitor=

It is made to store an electric charge(aka filters). It could be accepting current(charging) and could be leaving(discharging) if at least one is receiving(charging/full).

What is the name of the insulator inside of the capacitor and materials?

Dielectric material insulator(aka mica)which is inside. Most common material include ceramic, mica, glass and paper.

1. What does the memory map for a typical PLC processor consist of?

The memory map for a typical PLC processor consists of program files and data files.

2. Compare the function of the PLC program and data files.

Program files are the part of the processor memory that stores the user ladder logic program. The data files store the information needed to carry out the user program.

3. In what manner are data files organized?

Data files are organized by the type of data they contain.

4. List eight different types of data files used by an RSLogix 500 controller.

Output data file, input data file, status data file, binary data file, timer data file, counter data file, control data file, integer data file, floating point data file.

What information is stored in the input image table file?

The input image table file stores the status of the input terminals for the controller. 

What information is stored in the output image table file?

The output image table file stores the status of the output terminals for the controller.

Outline the sequence of events involved in a PLC scan cycle

The PLC scan cycle consists of the input scan, program scan, output scan, and housekeeping.

List four factors that enter into the length of the scan time.

The speed of the processor module, the length of the ladder program, the type of instructions executed and the actual ladder true/false conditions.

Compare the way horizontal and vertical scan patterns examine input and output instructions. 

In the horizontal scan pattern, the processor examines input and output instructions from the first command, top left in the program, horizontally, rung by rung. In the vertical scan pattern, the processor examines input and output instructions from the top left command entered in the ladder diagram, vertically, column by column and page by page.

List the five standard PLC languages as defined by the International Standard for Programmable Controllers, and give a brief description of each.

• Ladder Diagram (LD) –a graphical depiction of a process with rungs of logic, similar to the relay ladder logic schemes that were replaced by PLCs.

• Function Block Diagram (FBD) –a graphical depiction of process flow using simple and complex interconnecting blocks.

• Sequential Function Chart (SFC)–a graphical depiction of interconnecting steps, actions, and transitions.

• Instruction List (IL) –A low-level, text-based language that uses mnemonic instructions.

• Structured Text (ST) –A high-level, text-based language such as BASIC, C, or  PASCAL specifically developed for industrial control applications.

NO contact

NC contact

Coil

A normally closed pushbutton is connected to a PLC discrete input. Does this mean it must be represented by a normally closed contact in the ladder logic program? Explain why or why not.

No. Normally closed pushbutton does not have to be represented by a normally closed contact in the ladder logic program. The PLC uses XIC or XIO instructions based on the desired logic and the status of the input bit in memory, not on whether the physical pushbutton is normally open or normally closed.

What does the address assigned to an instruction indicate?

The address assigned to an instruction indicates what PLC input is connected to what input device and what PLC output will drive what output device

When are input branch instructions used as part of a ladder logic program?

nput branch instructions are used in your application program to allow more than one combination of input conditions.

Identify two matrix limitations that may apply to certain PLCs

Two matrix limitations that may apply to certain PLCs are a maximum of seven parallel lines and ten series contacts per rung are possible.

In what way does an internal output differ from a discrete output?

Unlike a discrete output, an internal output does not directly control an output field device.

A normally open limit switch is to be programmed to control a solenoid. What determines whether an Examine-on or Examine-off contact instruction is used?

If the contacts need to close to energize the output, then the Examine If Closed is used.

Briefly describe each of the following PLC modes of operation: Program

Program mode is used to transfer a new program or update an existing one

Briefly describe each of the following PLC modes of operation:

Test

Test mode is used to operate or monitor the user program without energizing any outputs.

Briefly describe each of the following PLC modes of operation: Run

Run mode is used to execute the user program with input devices being monitored and output devices energized accordingly

Under what condition is a ladder logic rung said to have logic continuity?

Electrical devices and signals can be divided into two categories: analog and digital. What is the major difference between these two categories?

Digital devices operate using, discrete ON or OFF signals that have only two possible values. Analog signals can represent an infinite number of possible values within a given range.

Compare the way memory space is assigned and referenced in rack-based and tag-based PLCs

With rack-based addressing a fixed device with a fixed address is used. Tag-based addressing is more flexible and uses text based addressing.

Explain the basic operating principle of an electromagnetic control relay

An electromagnetic control relay consists of an electromagnet or coil used to switch contacts. When the coil is energized, an armature moves to open or close contacts.

What is the operating difference between a normally open and a normally closed relay contact?

Normally open contacts are open when no current flows through the coil but close as soon as the coil is energized. Normally closed contacts are closed when the coil is de-energized and open when the coil is energized.

In what ways are control relay coils and contacts rated?

 Coils are rated for the type of operating current (DC or AC) and normal operating voltage. Contacts are rated in terms of maximum amount of current the contacts are capable of handling at a specified voltage level and type (AC or DC)

How do contactors differ from relays?

The contactor is built to handle heavy power loads that are beyond the capacity of control relays

What is the main difference between a contactor and a magnetic motor starter?

The magnetic motor starter is a contactor with an overload relay physically and electrically attached.

The current requirement for the control circuit of a magnetic starter is normally much smaller than that required by the power circuit. Why? 

The control circuit requires only a small amount of current to energize the coil, while the power circuit must be capable of handling the much higher motor load current.

Compare the method of operation of each of the following types of switches: 

Manually operated switch

Is controlled by hand

Compare the method of operation of each of the following types of switches: Mechanically operated switch

Is controlled automatically by factors such as pressure, position, or temperature

Compare the method of operation of each of the following types of switches: Proximity switch

Senses the presence or absence of a target without physical contact

What do the abbreviations NO and NC represent when used to describe switch contacts? 

NO stands for a normally open pushbutton switch that completes a circuit when it is pressed and returns to its open position when it is released.

NC stands for a normally closed pushbutton switch that opens a circuit when it is pressed and returns to the close position when it is released

Outline the method used to actuate inductive and capacitive proximity sensors.

Inductive sensors are actuated by a conductive material Capacitive sensors may be actuated by both conductive and nonconductive materials.

How are reed switch sensors actuated?

Reed switch sensors are actuated if the magnetic force between the poles is strong enough to overcome the restoring force of the reeds, the reeds will be drawn together to actuate the switch.

Compare the operation of a photovoltaic solar cell with that of a photoconductive cell. 

The photovoltaic cell operates by converting light energy directly into electric energy. 

The photoconductive cell operates when light energy falls on the cell and causes its resistance to change.

What are the two basic components of a photoelectric sensor?

 The two basic components of a photoelectric sensor are LEDs for the light source and phototransistors for light sensing.

Compare the operation of the reflective-type and through-beam photoelectric sensors. 

The reflective-type detects the light beam reflected from the target. 

The through-beam type measures the change in light caused by the target crossing the optical axis.

Give an explanation of how a scanner and a decoder act in conjunction with each other to read a bar code. 

A light source within the scanner illuminates the bar code symbol; those bars absorb light and spaces reflect light. A photo detector collects this light in the form of an electronic-signal pattern representing the printed symbol.

How does an ultrasonic sensor operate? 

By sending sound waves toward the target and measuring the time it takes for the pulses to bounce back.

Explain the principle of operation of a strain gauge. 

Strain gauges are based on the principle that the resistance of a conductor varies with length and cross-sectional area. The force applied to the gauge causes the gauge to bend and change its resistance.

Explain the principle of operation of a thermocouple. 

The temperature difference between the measuring and reference junction of a thermocouple causes a small DC voltage to be generated

What is the most common approach taken with regard to the measurement of fluid flow? 

The most common approach taken with regards to the measurement of fluid flow is to convert the kinetic energy that the fluid has into some other measurable form. 

Explain how a tachometer is used to measure rotational speed. 

A tachometer generator is a small AC or DC generator that develops an output voltage proportional to its rpm whose phase or polarity depends on the rotor’s direction of rotation.

How does an optical encoder work? 

An optical encoder uses a light source shining on an optical disk with lines or slots that interrupt the beam of light to an optical sensor. An electronic circuit counts the interruptions of the beam and generates the encoder’s digital output pulses.

Explain the function of each of the following actuators: Solenoid 

A solenoid is used to turn an electrical signal into linear mechanical motion.

Explain the function of each of the following actuators: Solenoid valve

A solenoid valve is used to restrict or allow the flow of a fluid.

Explain the function of each of the following actuators: Stepper motor

A stepper motor converts electrical pulses applied to it into discrete rotor movements called steps.

Compare the operation of open-loop and closed-loop control. 

Open-loop is control without feedback. And is repeatable, reliable.

Closed-loop has a self correcting feature and has a CV and sensor

What is a seal-in circuit? 

A seal-in circuit is a method of maintaining current flow after a momentary switch has been pressed and released.

In what way is the construction and operation of an electromechanical latching relay different from a standard relay? 

An electromechanical latching relay differs from a standard relay by it having two coils, one to latch the contacts and one to unlatch the contacts, and a latching relay does not have to constantly energize its coil to keep its contacts closed.

Give a short description of each of the following control processes: Sequential

Sequential controls are required for processes that demand that certain operations be performed in a specific order.

Give a short description of each of the following control processes: Combination

Combination controls require that certain operations be performed without regard to the order in which they are performed.

Give a short description of each of the following control processes: Automatic

Automatic control involves maintaining a desired set point at an output.

Compare the type of sensor signal obtained from a thermocouple with that from an RTD. 

The thermocouple DC voltage varies with temperature while the RTD resistance value varies with temperature. 

Explain how a magnetic reed float switch works. 

A permanent magnet installed within the movable float arm and a magnetic reed switch is installed within the fixed housing. The movement of the float, due to the changing liquid level, will cause the magnet to switch the reed contacts and open or closed a circuit at a preset liquid level. 

What is the function of an electrical interlocking circuit? 

It is used to prevent a piece of equipment from operating under certain potentially hazardous or undesirable conditions.

What is the role of instrumentation in an industrial process? 

To monitor and control a process.

You have been assigned the task of calibrating an instrument. How would you proceed? 

Check readings using a known quantity and if necessary adjust the response of the instrument so that the output accurately corresponds to its input throughout a specified range.

Photoelectric sensor: REFLECTIVE PHOTOELECTRIC SENSOR

Does not have a reflector. Needs a transmitter and receiver. Some are packages and some are not. Light(infrared) bounces off an object it detects back to the receiver. Not good for transparent/opaque, unsymmetrical pieces or dark colors.