chapter 6

identify the components of an electromechanical relay.

An output circuit a coil a contact

Statements about output latch OTL and output unlatch OTU instruction.

Both the latch and the unlatch outputs must have the same address. The OTL and OTU instruction differ from the OTE.

The release coil in a latching relay is momentarily energized to_______________.

Disengage the mechanical latch.

Open loop

Does not have feedback. Stepper motors operate in this mode.

Closed loop

Has feed back Servo motors operate in this mode.

Identify the conditions where the usage of contractors is required.

High powered loads of which overload protection is not required. High powered loads for which overload protection is provided separately. Where a PLC cannot control heavy power loads directly.

The true statements about automatic control.

It involves maintaining a desired set point at an output. it has a feedback.

select the characteristics that define an instrument.

It may include thermocouples it can control any kind of physical process. it an measure a physical process.

Select the correct statement about strain gauges.

Strain gauges are based on the principle that the resistance of a conductor varies with length and cross sectional area. an strain gauge converts a mechanical strain into an electric signal.

identify the true statements about the symbol used to represent a control relay.

The contracts are represented by a pair of short parallel lines. the contracts in the symbol are identified with the coil by means of letters.

Identify the features of ultrasonic sensors.

They can detect solids and fluids. They enable the detection of different objects irrespective of color.

Why is hysteresis need in a proximity sensor?

To keep proximity sensors from chattering when subject to shock. to reduce the effects of electrical noise on sensors. to reduce the effects of temperature drift in sensors.

Identify the purpose of a PLC.

To operate electromagnetic relays with a solid state switching system. to replace small control relays.

The retroreflective scan technique can be used for medium range sensing applications.

True

The velocity of sound in air is relatively constant.

True

instrumentation calibration involves exposure to an actual input stimulus of precise known quantity.

True

identify the factors that affect the sensing of a target by a capacitive proximity sensors.

Type of material being sensed. distance between the target and the sensor. dielectric constant of the target.

A solenoid___________________ is a electromechanical device that works by passing an electrical current through a solenoid, thereby changing its state.

Valve

Statements about output latch

When the instruction is true it is set a bit in the output image file. output latch is an output instruction with a lit level address.

identify the true statements about the ratings of control relay coils and contacts.

control relay coils and contractors have separate rating. control relay coils and contacts can be rated at AC or DC values.

relays are designed to make and break higher powered circuits with ought being damaged.

false

the leakage current flows in ranges of kA

false

encoder

gives feedback about position of the motor.

tachometer

gives feedback about the speed of the motor

an electrical ________________ circuit Is used to prevent multiple electrical motors from operating under certain potentially hazardous or undesirable condition.

interlock

identify the function of a tachometer

it converts rotation speed into an analog voltage signal

identify the features of a contractor.

it handles loads that are beyond the capability of control relays. it is designed to handle heavy loads.

Float switch is a type of ______________ switch.

level

inductive type proximity sensors.

operate under the principal of inductance. are used to detect ferrous and non ferrous metals.

___________________ sensors are pilot devices that detect the precence of ab object without physical contact.

proximity

In case of power loss a mechanical latching relay______________________.

remains in its present state.

Which of the following are manually operated switches?

selector switches toggle switches

motor controller

sends speed or position signal to the amplifier.

select the true statements about the selector switch.

switch position are established by turning the operator knob right or left. selector switch may have two or more selector positions.

identify the true statements about a stepper motor.

the shaft of the stepper motor rotates in discrete increments. every revolution is divided into a number of steps and the motor must be sent a voltage pulse for each step.

identify the true statement about pressure switches.

they can be hydraulic or pneumatic operated. bellows a diaphragm pressed up against a small microswitch and actuates theme.