UNIT I

ME 6702 -Mechatronics

UNIT-1 Syllabus

Introduction to Mechatronics-Systems- concepts of

Mechatronics approach – Need for Mechatronics – Emerging

areas of Mechatronics - Classification of Mechatronics. Sensors

and Transducers: Static and dynamic characteristics of Sensor,

Potentiometers- LVDT – Capacitance sensors – Strain gauges –

Eddy current sensor – Hall effect sensor – Temperature sensors

– Light sensors.

What is Mechatronics

Mechatronics is the synergistic combination of mechanical

engineering (“mecha” for mechanisms), electronic engineering

(“tronics” for electronics), and software engineering.

The word “mechatronics” was first coined by Mr. Tetsuro

Moria, a senior engineer of a Japanese company, Yaskawa, in

1969.

Mechatronics System

Elements of Mechatronics

Why Mechatronics ?

Advantages & limitations of mechanical systems

Advantages & limitations of electronic systems

Role of computers

Systems

A system can be thought of as a box or block diagram which

has an input and an output where we are concerned not with

what goes on inside the box but with only the relationship

between the output and the input.

Classification of Mechatronics

Sensors and Transducers

The term sensor is used for an element which produces a

signal relating to the quantity being measured.

Example- electrical resistance temperature element, the

quantity being measured is temperature and the sensor

transforms an input of temperature into a change in

resistance.

Sensors and Transducers

The term transducer is often used in place of the term

sensor.

Transducers are defined as elements that when subject to

some physical change experience related change.

Measurement Characteristics (Performance of

terminology)

Range: Difference between the maximum and minimum value of the sensed parameter

Resolution: The smallest change the sensor can differentiate

Accuracy: Difference between the measured value and the true value

Precision: Ability to reproduce the results repeatedly with a given accuracy

Sensitivity: Ratio of change in output to a unit change of the input

Zero offset: A nonzero value output for no input

Measurement Characteristics

Linearity: Percentage of deviation from the best-fit linear

calibration curve

Zero Drift: The departure of output from zero value over a

period of time for no input

Response time: The time lag between the input and output

Operating temperature: The range in which the sensor

performs as specified

Deadband: The range of input for which there is no output

Range & Resolution

Range: The range (or span) of a sensor is the difference between the

minimum (or most negative) and maximum inputs that will give a

valid output. Range is typically specified by the manufacturer of the

sensor.

For example, a common type K thermocouple has a range of

800°C (from −50°C to 750°C).

Resolution: The resolution of a sensor is the smallest increment of

input that can be reliably detected. Resolution is also frequently

known as the least count of the sensor.

The resolution of analog sensors is usually limited only by low-

level electrical noise and is often much better than equivalent

digital sensors.

Sensitivity

Sensor sensitivity is defined

as the change in output per

unit change in input.

The sensitivity of digital

sensors is closely related to

the resolution.

The sensitivity of an analog

sensor is the slope of the

output versus input line.

Linear & nonlinear behavior

Error

Error is the difference between a measured value and the true input

value.(measured value – true value)

Two types of errors:

Bias (or systematic) errors and

Precision (or random) errors.

Bias errors can be further subdivided into

Calibration errors (a zero or null point error is a common type of

bias error created by a nonzero output value when the input is

zero),

Loading errors (adding the sensor to the measured system changes

the system),

errors due to sensor sensitivity to variables other than the desired

one (e.g., temperature effects on strain gages).

Repeatability & Reproducibility

A measurement system must first be accurate, precise &

repeatable before it can be reproducible.

Repeatability refers to a sensor‟s ability to give identical outputs

for the same input

Precision (or random) errors cause a lack of repeatability

Accuracy, Precision & Repeatability

Saturation, Dead-Band

Saturation: All real actuators have some maximum output

capability, regardless of the input.

Deadband: The dead band is typically a region of input close to

zero at which the output remains zero. Once the input travels

outside the dead band, then the output varies with input.

0 1 2 3 4 5 6 7 8 9 10-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Time in Seconds

Forc

e in N

ew

ton

Comparison between Un-saturated & Saturdated Signal

Desired Output

Saturated Output

Basic Principle of Sensor / Transduction

Measuring

Parameter Useful Signal Conversion Device

Voltage, current,

capacitance

Displacement,

Temperature,

Pressure etc….

Sensor is a device that when exposed to a physical phenomenon

(temperature, displacement, force, etc.) produces a proportional output signal

(electrical, mechanical, magnetic, etc.).

Transducer is a device that converts one form of (energy) signal into another

form of (energy) signal.

Sensors

Position Sensors:

Potentiometer

LVDT

Encoders

Potentiometer

A rotary potentiometer is a variable resistance device that can

be used to measure angular position

Through voltage division the change in resistance can be used

to create an output voltage that is directly proportional to the

input displacement.

Potentiometer

Potentiometer

Applications of Potentiometers

Potentiometer as a Voltage Divider

The potentiometer can be worked as a voltage divider to obtain

a manual adjustable output voltage at the slider from a fixed

input voltage applied across the two ends of the potentiometer.

Now the load voltage across RL can be measured as

VL= R2RL. VS/(R1RL+R2RL+R1R2)

Potentiometer

Audio Control

Potentiometer

Sliding potentiometers, one of the most common uses for

modern low-power potentiometers are as audio control

devices. Both sliding pots (faders) and rotary potentiometers

(knobs) are regularly used to frequency attenuation, adjust

loudness and for different characteristics of audio signals.

Potentiometer

Television

Potentiometers were used to control the picture brightness,

contrast, and colour response.

Transducers

One of the most common application is measuring of

displacement. To measure the displacement of the body,

which is movable, is connected to the sliding element located

on the potentiometer. As the body moves, the position of the

slider also changes accordingly so the resistance between the

fixed point and the slider changes. Due to this the voltage

across these points also changes.

Potentiometer

Potentiometer as a Transducer X

The change in resistance or the voltage is proportional to the

change in the displacement of the body. Thus the voltage

change indicates the displacement of the body. This can be

used for the measurement of translational as well as well

rotational displacement. Since these potentiometers work on

the principle of resistance, they are also called as the resistive

potentiometers. For example, the shaft rotation might

represent an angle, and the voltage division ratio can be made

proportional to the cosine of the angle.

Potentiometer

Linear Variable Differential Transformer

„LVDT‟ is a transducer for measuring linear displacement

It must be excited by an AC signal to induce AC response on

secondary.

The core position can be determined by measuring secondary

response.

Linear Variable Differential Transformer

LVDT

LVDT

The above figure depicts a cross-sectional view of an LVDT.

The core causes the magnetic field generated by the primary

winding to be coupled to the secondaries.

When the core is centered perfectly between both

secondaries and the primary, as shown, the voltage induced in

each secondary is equal in amplitude and 180 deg out of

phase.

Thus the LVDT output (for the series-opposed connection

shown in this case) is zero because the voltages cancel each

other.

LVDT

LVDT

Displacing the core to the left (As per the above figure)

causes the first secondary to be more strongly coupled to the

primary than the second secondary.

The resulting higher voltage of the first secondary in relation

to the second secondary causes an output voltage that is in

phase with the primary voltage.

LVDT

LVDT

Likewise, displacing the core to the right causes the second

secondary to be more strongly coupled to the primary than

the first secondary.

The greater voltage of the second secondary causes an output

voltage to be out of phase with the primary voltage.

LVDT

CAPACITANCE SENSOR

Noncontact capacitive sensors work by measuring changes in

an electrical property called capacitance.

Capacitance describes how two conductive objects with a

space between them respond to a voltage difference applied

to them.

When a voltage is applied to the conductors, an electric field

is created between them causing positive and negative

charges to collect on each object (Fig.). If the polarity of the

voltage is reversed, the charges will also reverse.

CAPACITANCE SENSOR

Figure 1

Applying a voltage to conductive objects

causes positive and negative charges

to collect on each object.

This creates an electric field

in the space between the objects.

CAPACITANCE SENSOR

Capacitive sensors use an alternating voltage which causes

the charges to continually reverse their positions.

The moving of the charges creates an alternating electric

current which is detected by the sensor (Fig. 2). The amount

of current flow is determined by the capacitance, and the

capacitance is determined by the area and proximity of the

conductive objects.

Larger and closer objects cause greater current than smaller

and more distant objects. The capacitance is also affected by

the type of nonconductive material in the gap between the

objects.

CAPACITANCE SENSOR

Figure 2

Applying an alternating voltage causes

the charges to move back and forth

between the objects, creating an

alternating current which is detected by the

sensor.

CAPACITANCE SENSOR

Technically speaking, the capacitance is directly proportional

to the surface area of the objects and the dielectric constant

of the material between them, and inversely proportional to

the distance between them (Fig. 3).

Figure 3

Capacitance is determined by Area,

Distance, and Dielectric (the material

between the conductors). Capacitance

increases when Area or Dielectric

increase, and capacitance decreases

when the Distance increases

CAPACITANCE SENSOR

Advantages:

There are some advantages of capacitive transducer which are

given below,

The sensitivity of capacitive transducer is high.

The capacitive transducer is useful for small system.

It has good frequency response.

It requires small power to operate.

The loading effect is less due to high input impedance.

Disadvantages:

CAPACITANCE SENSOR

Disadvantages:

There are some disadvantages of capacitive transducer which

are given below,

The capacitive transducers are temperature sensitive.

It gives non linear behavior.

The output impedance depends upon the frequency used.

The capacitance may get changed by dust particle and

moisture which produce error.

CAPACIVE SENSOR

Applications:

There are some important applications of capacitive

transducer which are given below,

The capacitive transducers are used to measure humidity in

gases.

It is used to measure volume, liquid level, density etc.

It is used for measurement of linear and angular

displacement.

STRAIN GAUGES

The principle of the strain gauge is the Piezoresistive effect,

which means “pressure-sensitive resistance,” or a resistance

that changes value with applied pressure. The strain gauge is a

classic example of a piezoresistive element.

STRAIN GAUGES Working

Strain gauges in their infancy were metal wires supported by

a frame.

Advances in the technology of bonding materials mean that

the wire can adhere directly to the strained surface. Since the

measurement of strain involves the deformation of metal, the

strain material need not be limited to being a wire.

As such, further developments also involve metal foil gauges.

Bonded strain gauges are the more commonly used type.

There is the Wheatstone bridge arrangement where the

change in pressure is detected as a change in the measured

voltage:

STRAIN GAUGES

STRAIN GAUGES

The change in the resistance of the strain gauge breaks the

balance of the Wheatstone’s bridge and change the voltage V.

The voltage V is proportional to the pressure change in the

strain gauge.

Applications:

Residual stress

Vibration measurement

Torque measurement

Strain measurement

Compression and tension measurement

Encoders

Digital Optical Encoders

Absolute Digital Optical Encoders

Incremental Digital Optical Encoders

EDDY CURRENT SENSOR

Eddy-Current sensors operate with magnetic fields. The driver creates

an alternating current in the sensing coil in the end of the probe. This

creates an alternating magnetic field with induces small currents in the

target material; these currents are called eddy currents.

EDDY CURRENT SENSOR

The eddy currents create an opposing magnetic field which

resists the field being generated by the probe coil.

The interaction of the magnetic fields is dependent on the

distance between the probe and the target.

As the distance changes, the electronics sense the change in

the field interaction and produce a voltage output which is

proportional to the change in distance between the probe and

target.

EDDY CURRENT SENSOR

Applications

Eddy-Current sensors are useful in any application requiring

the measurement or monitoring of the position of a

conductive target, especially in a dirty environment.

HALL EFFECT SENSOR

HALL EFFECT SENSOR

In the pictured wheel with two equally spaced magnets, the

voltage from the sensor will peak twice for each revolution.

This arrangement is commonly used to regulate the speed of

disk drives.

HALL EFFECT SENSOR

Hall Effect Sensors consist basically of a thin piece of

rectangular p-type semiconductor material such as gallium

arsenide (GaAs), indium antimonide (InSb) or indium

arsenide (InAs) passing a continuous current through itself.

When the device is placed within a magnetic field, the

magnetic flux lines exert a force on the semiconductor

material which deflects the charge carriers, electrons and

holes, to either side of the semiconductor slab.

This movement of charge carriers is a result of the magnetic

force they experience passing through the semiconductor

material.

HALL EFFECT SENSOR

As these electrons and holes move side wards a potential

difference is produced between the two sides of the

semiconductor material by the build-up of these charge

carriers.

Then the movement of electrons through the semiconductor

material is affected by the presence of an external magnetic

field which is at right angles to it and this effect is greater in a

flat rectangular shaped material.

HALL EFFECT SENSOR

he effect of generating a measurable voltage by using a

magnetic field is called the Hall Effect after Edwin Hall

who discovered it back in the 1870’s with the basic physical

principle underlying the Hall effect being Lorentz force.

To generate a potential difference across the device the

magnetic flux lines must be perpendicular, (90o) to the flow

of current and be of the correct polarity, generally a south

pole.

HALL EFFECT SENSOR

The Hall effect provides information regarding the type of

magnetic pole and magnitude of the magnetic field. For

example, a south pole would cause the device to produce a

voltage output while a north pole would have no effect.

Generally, Hall Effect sensors and switches are designed to be

in the “OFF”, (open circuit condition) when there is no

magnetic field present. They only turn “ON”, (closed circuit

condition) when subjected to a magnetic field of sufficient

strength and polarity.

HALL EFFECT SENSOR

Applications

1.Position sensing

Sensing the presence of magnetic objects (connected with the

position sensing) is the most common industrial application

of Hall effect sensors, especially those operating in the switch

mode (on/off mode). The Hall effect sensors are also used in

the brushless DC motor to sense the position of the rotor

and to switch the transistors in the right sequence.

Smart phones use hall sensors to determine if the Flip Cover

accessory is closed. See Galaxy S4 Accessories.

HALL EFFECT SENSOR

2.Automotive fuel level indicator

The Hall sensor is used in some automotive fuel level

indicators. The main principle of operation of such indicator

is position sensing of a floating element.[7] This can either be

done by using a vertical float magnet or a rotating lever

sensor.

In a vertical float system a permanent magnet is mounted on

the surface of a floating object. The current carrying

conductor is fixed on the top of the tank lining up with the

magnet. When the level of fuel rises, an increasing magnetic

field is applied on the current resulting in higher Hall

voltage.

HALL EFFECT SENSOR

As the fuel level decreases, the Hall voltage will also

decrease. The fuel level is indicated and displayed by proper

signal condition of Hall voltage.

In a rotating lever sensor a diametrically magnetized ring

magnet rotates about a linear hall sensor. The sensor only

measures the perpendicular (vertical) component of the

field. The strength of the field measured correlates directly to

the angle of the lever and thus the level of the fuel tank.

HALL EFFECT SENSOR

3.Direct current transformers Hall effect sensors may be utilized for contactless

measurements of DC current in current transformers. In such a

case the Hall effect sensor is mounted in the gap in magnetic

core around the current conductor.[6] As a result, the DC

magnetic flux can be measured, and the DC current in the

conductor can be calculated

Temperature Sensor

What is a Temperature Sensor?

A simple temperature sensor is a device, to measure the

temperature through an electrical signal it requires a

thermocouple or RTD (Resistance Temperature Detectors).

The thermocouple is prepared by two dissimilar metals

which generate the electrical voltage indirectly proportional

to change the temperature.

The RTD is a variable resistor, it will change the electrical

resistance indirectly proportional to changes in the

temperature in a precise, and nearly linear manner.

Temperature Sensor

Thermocouple Sensor

The thermocouple sensor measures the popular thermals,

which are composed of the two different metal alloy wires.

By combining the two different metals will generates the

strong voltage which is the same capacity as a temperature.

In general, the thermocouple gives the vast measurement

ranges and they are worked by using the Seebeck effect.

The Seebeck effect invested for changing the temperature in

the electrical circuit. The sensor reads the temperature by

taking the measurement of voltage output.

Temperature Sensor

Thermocouple Sensor

Temperature Sensor

THERMISTER SENSOR

The thermistor sensor is a type of sensor. This type of sensors

is used mostly in the human thermometers.

If there is a change in the temperature, then the electrical

current or resistance also changes.

The thermistor is prepared by using the semiconductor

materials with a resistivity which is especially sensitive to

temperature.

The resistance of a thermistor decreases with increasing

temperature so that when the temperature changes, the

resistance change is predictable.

Temperature Sensor

THERMISTER SENSOR

Digital Optical Encoders

Schematic Diagram

Typical Construction

Simple Rotary Encoder

Quadrature Encoder

Binary Encoder

Grey Code Encoder

Absolute Encoder

Absolute Encoder (Gray Code)

Incremental Encoder

Proximity sensors

Proximity sensors:

Optical

Inductive

Capacitive

Proximity sensors

Application of Proximity sensors

Inductive Proximity sensors

• Detects metal object

• Uses an electro-magnetic field to detect a conductive target

• Sensing coil in the end of the sensor probe

• When excited creates an alternating magnetic field which induces small

amounts of eddy current in the target object

• Eddy currents create an opposing magnetic field which resists the field

being generated by the sensor probe coil.

• The interaction of the magnetic fields is dependent on the distance

between the sensor probe and the target.

• Comparatively inexpensive but conducting targets sensing

Inductive Proximity sensors

Capacitive Proximity sensors

The sensing surface of the sensor‟s probe is the electrified plate.

The sensor electronics continually changes the voltage on the probe

surface

The amount of current required change this voltage is measured

which indicates the amount of capacitance distance between the

probe and target.

Can be used for nonmetallic materials such as paper, glass, liquids,

and cloth

Capacitive Proximity sensors

• Motion Sensors:

• Variable Reluctance

• Temperature Sensor:

• RTD

• Thermocouples

Variable Reluctance sensor

A magnet in the sensor creates a

magnetic field

As a ferrous object moves by the

sensor, the resulting change in the

magnetic flux induces an emf in

the pickup coil

Variable Reluctance sensor

• Used to measure speed and/or position of a moving metallic

object

• Sense the change of magnetic reluctance (analogous to

electrical resistance) near the sensing element

• Require conditioning circuitry to yield a useful signal (e.g.

LM1815 from National Semi.)

Temperature measurement

• EMF based

• Thermocouple

• Resistance based

• Resistance Temperature Detectors (RTD)

Thermocouples

If two different metals „A‟ and „B‟ are connected as in Figure, with a junction and a voltmeter, then if the junction is heated the meter should show a voltage.

This is known as the Seebeck effect.

Construction of Thermocouples

At the tip of a grounded junction probe, the thermocouple wires are physically attached to the inside of the probe wall. This results in good heat transfer from the outside, through the probe wall to the thermocouple junction.

In an ungrounded probe, the thermocouple junction is detached from the probe wall. Response time is slower than the grounded style, but the ungrounded offers electrical isolation.

The thermocouple in the exposed junction style protrudes out of the tip of the sheath and is exposed to the surrounding environment. This type offers the best response time, but is limited in use to dry, non-corrosive and non-pressurized applications.

Types of thermocouples

Sr.

No

Type Thermocouple Material Sensitivit

y in

(µV/oC)

Useful

temperature

range

1 T Copper-Constantan 20 – 60 -180 to +400

2 J Iron-Constantan 45 – 55 -180 to +850

3 K Chromel-Alumel 40 – 55 -200 to +1300

4 E Chromel-Constantan 55 – 80 -180 to +850

5 S Platinum-Platinum/10% Rhodium 5 – 12 0 to +1400

6 R Platinum-Platinum/13% Rhodium 5 – 12 0 to +1600

7 B Platinum/ 30% Rhodium-Platinum/6% Rhodium 5 – 12 +100 to +1800

8 W5 Tungsten/5% Rhenium-Tungsten/20% Rhenium 5 – 12 0 to +3000

Constantan = copper/nickel; Chromel = nickel/chromium; Alumenl = nickel/aluminium

Selection of Thermocouples

The following criteria are used in selecting a thermocouple:

Temperature range

Chemical resistance of the thermocouple or sheath material

Abrasion and vibration resistance

Installation requirements (may need to be compatible with

existing equipment; existing holes may determine probe

diameter)

Resistance Temperature Detector

(RTD)

Uses the phenomenon that the resistance of a metal changes with

temperature.

Are linear over a wide range and most stable.

Advantages of platinum as RTD

The temperature-resistance characteristics of pure platinum

are stable over a wide range of temperatures.

It has high resistance to chemical attack and contamination

It forms the most easily reproducible type of temperature

transducer with a high degree of accuracy .

It can have accuracy ± 0.01 oC up to 500 oC and ± 0.1 oC

up to 1200 oC.

Limitations of RTD

These are resistive devices, and accordingly they function by

passing a current through a sensor.

Even though only a very small current is generally employed,

it creates a certain amount of heat and thus can throw off the

temperature reading.

This self heating in resistive sensors can be significant when

dealing with a still fluid (i.e., one that is neither flowing nor

agitated), because there is less carry-off of the heat

generated.

This problem does not arise with thermocouples, which are

essentially zero-current devices.

Comparison: Thermocouple vs RTD

Force/Pressure Sensor

Stress measurement using strain

Strain is change in length (dl) per unit length (l)

Strain gauge is primary sensing element used in pressure, force

and position sensors

l dl

Strain Gauge

Based on the variation of resistance of a conductor

or semiconductor when subjected to a mechanical

stress.

The electric resistance of a wire having length l,

cross section A, and resistivity ρ is:

When the wire is stressed longitudinally, R

undergoes a change.

Passing small amount of current through such wire

will, thus, help measure voltage change.

The sensing element of the strain gage is made of

copper-nickel alloy foil. The alloy foil has a rate of

resistance change proportional to strain with a

certain constant.

A

lR

Strain Gauge

Strain Gauge Type

Types:

Semiconductor Strain Gauge

Thin Film Strain Gauge

Diffused Semiconductor Strain

Gauge

Bonded Resistance Gauge

Selection Criterion

Operating Temperature, Nature

of Strain, Stability Requirement

Strain Gauge

To measure the strain requires accurate measurement of very

small changes in resistance.

For example, suppose a test specimen undergoes a strain of

500 x10-6.

A strain gauge with a gauge factor of 2 will exhibit a change

in electrical resistance of only 2x(500 x 10-6).

For a 120 Ω gauge, this is a change of only 0.12 Ω.

Strain Gauge Circuit

The Wheatstone bridge is an electric circuit for detection of minute resistance

changes. It is therefore used to measure resistance changes of a strain gauge.

Strain gauge is connected in place of R4 in the circuit. When the gauge bears

strain and initiates a resistance change, ΔR, the bridge outputs a corresponding

voltage.

• With no force applied to the test specimen, both strain gauges have

equal resistance and the bridge circuit is balanced.

• However, when a downward force is applied to the free end of the

specimen, it will bend downward, stretching gauge #1 and

compressing gauge #2

Strain Gauge Circuit

l

lRR

GF

GFV

V

GFV

V

GFV

V

input

output

input

output

input

output

:eqns aboveIn

:Bridge Full

2

1 :Bridge Half

4

1 :BridgeQuarter

Effect of Temperature on Output of Gauge

Ideally, we would like the resistance of the strain gauge to change only in response to applied strain.

However, strain gauge material, as well as the specimen material to which the gauge is applied, will also respond to changes in temperature.

Strain gauge manufacturers attempt to minimize sensitivity to temperature by processing the gauge material to compensate for the thermal expansion of the specimen material; compensated gauges reduce the thermal sensitivity, they do not totally remove it.

Temperature compensation

• By using two gauges

• One gauge is active, and a second gauge is placed transverse to the applied strain.

• The strain has little effect on the second gauge, called the dummy gauge.

• Because the temperature changes are identical in the two gauges, the ratio of their resistance does not change, the voltage does not change, and the effects of the temperature change are minimized.

Electromagnetic Flow sensor

Magnetic flow meters operate based upon Faraday's Law of electromagnetic induction, which states that a voltage will be induced in a conductor moving through a magnetic field.

Faraday's Law: E=kBDV

The magnitude of the induced voltage E is directly proportional to the velocity of the conductor V, conductor width D, and the strength of the magnetic field B.

Magnetic field coils are placed on opposite sides a pipe to generate a magnetic field.

Electromagnetic Flow sensor

As the liquid moves through the field with average velocity V, electrodes sense the induced voltage.

An insulating liner prevents the signal from shorting to the pipe wall.

The output voltage E is directly proportional to liquid velocity, resulting in the linear output of a magnetic flow meter.

Stepper Motor

Discrete Positioning device

Moves one step at a time for each input

Appropriate excitation in winding/s, makes the rotor attract

towards the stator

Stepper Motor

Servo Motor

Electric (DC/AC) motor driven using Pulse Width Modulation (PWM)

Servo mechanism consists of position sensor (potentiometer/encoder),

gear mechanism and intelligent circuitry

Servo Motor

Solenoid

Electromagnetic actuator

Consist of a movable ferrite core that is activated by current flow

When the coil is energized, a magnetic field is established that

provides the force to push or pull the core

Provide large force over a short duration