---Robotics
UNIT-2

ROBOT DRIVE SYSTEMS AND END
EFFECTORS
 Pneumatic Drives-Hydraulic Drives-Mechanical Drives-Electrical
Drives-D.C. Servo Motors, Stepper Motors, A.C. Servo Motors-
Salient Features, Applications and Comparison of all these
Drives, End Effectors-Grippers-Mechanical Grippers, Pneumatic
and Hydraulic- Grippers, Magnetic Grippers, Vacuum Grippers;
Two Fingered and Three Fingered Grippers; Internal Grippers
and External Grippers; Selection and Design Considerations.

ROBOT DRIVE SYSTEMS
 Drive means operate the robot.
 The drive system is to provide a means to control the speed and
also torque (or) power.
 Drive system is used for converting hydraulic, pneumatic, and
electrical energy into useful mechanical energy. It is used to
motion transfer and drive the robot.

Types of drive system
 Hydraulic drive systems
 Pneumatic drive systems
 Electric drive systems

Hydraulic drive systems
 Pressurised fluid is used to transmit and control power.
 The hydraulic drive is mostly suitable for heavy load robot
applications
 The term hydraulic refers to the transfer of energy from
pressure difference not from the kinetic energy of flow

Features of Hydraulic drive system
 High force capability
 High dynamic response
 Assisted braking torque
 Good mechanical stiffness
 High power per unit weight and volume

Consists of
1.Actuators
2.Control valve
3.Power supply

Advantages
 No reduction gear is needed
 High accuracy and better response
 Excellent for heavy duty and large robots
 It provide more power than electric drives

Disadvantages
 High maintenance
 More expensive system
 Not suitable for clean air environment
 Requires pump, reservoir, motor and hoses
 More floor space is required

Pneumatic drive systems
 Pressurised air is used to control power.
 With pneumatic valves control the flow of energy from
pressurized gas
Features of Pneumatic drive system
 Compressed air has most of the desired properties and
characteristics of a gas for pneumatic system.
 It is not poisonous and non flammable.

Consists of
1.Actuators
2.Control valve
3.Power supply

Pneumatic System in Packaging

Advantages
 Good accuracy
 Excellent speed
 Easy installation
 Small leakage can be tolerated
 Economical than hydraulic drives

Disadvantages
 Difficult to control
 Air needs preconditioning
 Precision is less than electric drives
 More vibration is generated
 More noise is produced

Electric drive systems
 An electric drive system is defines as a form of machine
equipment designed to convert electric energy into mechanical
energy and provide electric control of the process.
Features of Pneumatic drive system
 Electric drive offers energy transformation
 The control components are used to limit the
amplitude(armature current of DC motor)

There are three major types of electric drives
1. AC servo motor
2. DC servo motor
3. Stepper motor
Parts of electric drive
1. Power modulator
2. Motor
3. Control unit
4. Sensing unit

Advantages
 Simple construction
 Clean air environment
 Requires less floor space
 Electric drive robot is quiet operation
Disadvantages
 Poor dynamic response
 A large and heavier motor must be used. It is most costly
 Electric drive system do not provide as much speed and power
compared to hydraulic system.

Actuators
 Actuators are the device used for converting hydraulic, pneumatic and
electrical energy into mechanical energy. The mechanical energy used to
get the work done.
 Types of Actuators
1. Hydraulic actuators
2. Pneumatic actuators
3. Electrical actuators
a) Servomotor
b) Stepper motor
c) AC (or) DC motor

Hydraulic Actuators
 Hydraulic actuators transform the hydraulic energy stored in a
reservoir into mechanical energy by means of suitable pumps.
 Hydraulic actuators are also fluid power device for industrial
robots which utilise high pressure fluid such as oil to transmit
forces to the point of application desired

Characteristics of hydraulic actuating systems
The designer should know the basics of
 Operating cycle
 Operating pressures
 Types pf pumps
 Maximum and minimum operating and ambient temperatures
 Loads encountered by various components
Features of the hydraulic actuating systems
 High force capability
 Good mechanical stiffness
 High power per unit weight and volume

Elements of hydraulic actuation system
 Hydraulic reservoir
 Filters
 Hydraulic pump
 Cylinders
 Motors
 Hydraulic valve
 Accumulators
 Hydraulic hose
 Hydraulic seal
 piping

Types of Hydraulic Actuators
 Single acting spring return type
 Double acting cylinder
 Ram type

Advantages
 Self lubrication
 No mechanical linkage is required
 It can also provide precise control at low speeds
 Capable of with standing shock loads
 Greater load carrying capacity
Disadvantages
 Expensive
 Noisy operation
 High maintenance
 Not energy sufficient
 Not suitable for clean environment
 Leakage can occur causing a loss in performance

Pneumatic Actuators
 Pneumatic actuators utilise pneumatic energy provided by a
compressor and transforms it into mechanical energy by means
of pistons (or) turbines. Pressurised air is used to transmit and
control power.
Features of pneumatic actuators
 Limit cycling
 Tend to have performance inconsistency
 Pneumatic actuators are capable of modulating process control
 It can handle high torque loads

Common parts of a pneumatic system
Compressor
Check valves
Regulators
Gauges
Accumulator
Feed lines

Advantages
 Control is simple
 It is cheapest form of all actuators
 No mechanical transmission is usually required
 Individual components can be easily interconnected
 Very quick response time and faster cycles
Disadvantages
 More noise and vibration
 Not suitable for heavy loads
 Air compressor is required

Electric Actuators
 An actuator obtaining electrical energy from mechanical system
is called electric actuator
Features of the electric actuators
 High band with provide accurate and fast control
 High maximum force allows high acceleration

Electrical actuators comprise the following
 Drive system
1. AC motor 2. DC motor 3. Stepper motor
 Switching device
a) Mechanical switch 1. Solenoids 2. Relays
b) Solid state switch 1. Diodes 2. Thyristor 3. Transistors

Advantages
 High power conversion efficiency
 They are easily maintained and repaired
 Structural components can be light weight
 No pollution of working environment
Disadvantages
 Compliance and wear problems are causing inaccuracies
 A larger and heavier motor must be used which is costly
 They cannot be used in explosive atmospheres
Applications
 Wide range of industries where positioning is needed
 Automation applications

Various kind of Motors
1. Stepper motor
2. Servo motor
i) AC Servo motor
ii) DC Servo motor

Stepper Motor
 A stepper motor is an electromechanical device. Stepper motor
is a device which transforms electrical pulses into equal
increments of rotary shaft motion called steps. It converts
electrical power into mechanical power.

Working of Stepper Motor

Permanent Magnet type stepper
Motor
 The permanent magnet type stepper motor has a stator. That is
of electromagnet. A rotor that is of permanent magnet.
Therefore this motor is called permanent magnet type stepper
motor.

Permanent Magnet type stepper Motor

Advantages
 A simpler and more reliable
 Field flux is less affected by temperature rise
 Higher efficiency due to the absence of field losses
 Less heating making it is possible to totally enclose the motor
Disadvantages
 Permanent magnets stepper motor cannot produce a high flex density
 Extra ampere-turns cannot be added to reduce the armature reaction
Applications
 Automobiles and air conditioners
 Computer drives
 Electric tooth brushes, portable vacuum cleaners and food mixers

Variable reluctance type stepper
motor
 The principle of variable reluctance stepper motor is based on
the principle of the flux lines which capture the low reluctance
path. The stator and the rotor of the motor are aligned in such as
way that the magnetic reluctance is minimum.

Variable reluctance type stepper motor

Advantages
 High rate of acceleration
 High torque to inertia ratio
 Simple and low cost machine
Disadvantages
 Generally noisy
 There is no winding on rotor
 Lower torque capability

Hybrid type stepper motor
 The hybrid type stepper motor as the name recommends is a
blending of both permanent magnet type stepper motor (PM)
and variable reluctance motor (VR)

Hybrid type stepper motor

Advantages
 It has more torque
 Lower stepping rate
 More efficient at low speeds
 The length of step is smaller
Disadvantages
 Higher inertia
 If the magnetic strength is varied, the performance of the motor is
affected.
 The weight of the motor is more because of the presence of the rotor
magnet.
 Cost is more

Torque vs speed characteristics

Salient features of stepper motor
 Stepper motor is small step angle
 They are high stepping rate and accuracy
 Stepper motor has high positioning accuracy
 A stepper motor is used to achieve precise positioning is a digital
control
 They are ideal for applications requiring quick positioning over a shot
distance
Capabilities of stepper motor
 Precise positioning control
 Finer positioning capabilities
 Low speed with high precision
 Easy control with pulse signals
 Generating high torque with a compact size

Advantages
 Low cost
 High reliability
 High torque at low speeds
 Operates in almost any environment
 Stepper motor is simple to operate
Disadvantages
 lower efficiency
 Low torque to inertia ratio
 Lower power output for their weight and size
 Smooth movement often requires micro stepping
Applications
 Gaming machines
 Textile machinery
 Welding equipment

Servo Motor
 The motors that are used in automatic control systems are called
servo motor. The servo motors are used to convert electrical signal
applied to them into an angular displacement of the shaft.
Features of the servo motor
 Fast response
 Steady state
 stability
 Wide range of speed control
 Low mechanical and electrical inertia

Controlling of Servomotors

Advantages
 Servo motor is small and efficient
 High speed operation is possible
Disadvantages
 Higher cost
 It is not suitable for precision control of rotation
 It is not suitable if we need to prevent vibration
Uses of the servo motor
 The servo motor is built into the camera to correct a lens of the camera to
improve out of focus image
 The servo motor is used in textile to control spinning and weaving
machines
 Used in metal forming and cutting machines to provide specific motion
control for milling machines.

Applications of the servo motor
 Toys
 Cars
 Robotics
 Aeroplanes
 Computers
 Home electronics
 CD/DVD player

AC Servo Motor
 Servo motor is basically consists of stator and rotor

Rotor
 The rotor is generally of two types.
 1. squirrel cage rotor 2. Drag cup type rotor

Types of AC Servo motor

2 phase AC Servomotor
 The stator of the two phase AC servo motor has the two
distributed winding which are displaced from each other by 90
degrees electrical. One winding is known as a reference phase
and other one is known as control phase

3 phase AC Servomotor
 Three sets of winding are placed 120 electrical degrees apart
with each set connected to one phase of the three phase power
supply.
 When three phase current passes through the stator windings, a
rotating magnetic field effect is produced that travels around the
inside of the stator core.

Advantages
 Less maintenance
 High efficiency
 High speed operation is possible
 Resonance and vibration free operation
Disadvantages
 Complex
 Poor motor cooling
 Motor can be damaged by sustained over load
 Most difficult to control in position application
Application
 Robotics
 Machine tools
 Suited for lower power application

DC Servo Motor
 DC servo motor is more (or) less similarly to the normal DC
motor. DC motors are separately excited DC motor (or)
permanent magnet DC motor. They are controlled by armature
voltage. The armature is designed to have large resistance, so
that the torque speed characteristics are linear.
 There fore a step change in the armature voltage results in quick
change in position (or) speed of the motor.

Types of the DC servo Motor

1. Series Motor
 The series motor have a high starting torque and draws large
current
 The speed regulation of this kind of motor is poor

2. Split series Motor
 Split series motor with split field rate with some fractional
kilowatts
 Split series motor has a typical torque speed curve

3. Shunt Control Motor
 It has two separate winding
1. Field winding is on the stator
2. Armature winding is on the rotor of the machine

4. Permanent Magnet shunt motor
 Permanent magnet shunt motor is a fixed excitation motor
where the field is actually supply by a permanent magnet

Brushless DC servo motor

Pulse coded modulation

Torque speed characteristics

Advantages
 Free of vibration and resonance
 High torque to inertia ratio
 High output than from a 50Hz motor of same size
 High efficiency
 Easier speed control from zero speed to full speed in both direction
Disadvantages
 Overload can damage motor
 Has complex architecture and requires encode
 The brush turnout in limited life of 2000 hrs, then service is required
 Motor does not work when something breaks, hence safety circuits are
needed