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Mechatronics and Control for Production Plants > 11 Gripping Technology > Flashcards

11 Gripping Technology Flashcards

1
Q

Definition of a gripping system

A

o Subsystem of an industrial robot
o To maintain a fixed position and orientation of a set of workpieces relative to a tool or gripper coordinate system

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2
Q

Main Functions of a gripping system

A

 Preparation of a contact e.g. pre-positioning of parts via a described motion to achieve a well-known object position and orientation

 Establishing a contact between the object and the gripper fingers via force or form locking

 Manipulation of the object within or via the gripper (e.g translocation, rotation, assembly)

 Depositing of gripping object by releasing the contact

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3
Q

Special Functions of a gripping system

A

 Information retrieval through sensors
 Supply of parts for assembly
 Cleaning operations during production

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4
Q

Task of a Gripper

A
  • The task of a gripper for an industrial robot is to hold a geometrically defined handling object for a defined time in a defined spatial configuration.
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5
Q

Sequence of the handling process

A

o Approach
(Collision free motion to the gripper’s gripping position)

o Gripping
of an object by force (friction), adhesion or form locking

o Transfer Motion
of the gripped object

o Releasing
the object by removing contact

o Departure
(collision free motion from the gripping position)

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6
Q

Requirements to Gripping Systems

A

o Gripping force
o Gripping accuracy
o Gripper weight
o Size of gripper and handling object
o Gripping force safety
o Gripping force control
o Monitoring and quality control
o Duration of gripping and releasing
o Connectivity to handling device

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7
Q

Complementary Gripper Requirements

A
  • Every application needs to be analyzed individually in regard to choose the best fitting gripper
  • Flexibility, performance and the reduction of investment costs are contrary target quantities
  • Most of the times simple and inflexible grippers are sufficient
  • Customized solutions are only chosen, if no gripper change is possible -> Two simple grippers are preferred
  • Complex high-performance grippers are bounded to application in research areas, due to cost reasons
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8
Q

Gripper Types

A

o Mechanical grippers
o Vacuum grippers
o Enveloping grippers
o Magnetic grippers
o Adhesive grippers

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9
Q

Grip Variants for Finger Grippers

A

 Outside diameter gripping
–> Force applied from outer surface

 Intermediate gripping
–> Hollow part, one gripper on outside, one on inside

 Inside diameter gripping
–> Part gripped at a hole or opening from the inside

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10
Q

Mechanical Grippers - Retention Force

A

Generated via
- Force closure (friction)
- Form locking
- Combination of both

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11
Q

Mechanical Grippers - Drive units

A

 Electric
 Pneumatic/hydraulic
 Piezoelectric

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12
Q

Finger gripper with linear movements - Technical Details and applications

A
  • Force closure
  • Combination with form locking possible
  • Driven by pneumatic or electrical actuators
  • With three fingers the part is centered
  • Adv. With cuboidal or cylindrical parts
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13
Q

Advantages of Finger Grippers

A
  • All gripping variants possible
  • High gripping force through wedge hook kinematic possible
  • Robust and inexpensive
  • Good guidance through multi-tooth guide
  • Flexible through customizing of the finger geometry
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14
Q

Disadvantages of Finger Grippers

A
  • Every gripper itself has a limited flexibility (size, forces, …)
  • There might be a large design effort to realize a large variety of gripping fingers
  • Set-up times to change the fingers
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15
Q

Wedge Hook Kinematic

A

Force closure to handle cuboidal and cylindrical parts

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16
Q

Ways to realize the gripping and releasing moment in electrical parallel grippers

A
  • Ball screw spindle
  • Pinion gear rack
  • Toothed belt drive
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17
Q

Finger Gripper with angular movements - Technical Details and Applications

A
  • 180°-angular gripper
  • Force closure
  • Combination with form locking possible
  • Driven by pneumatic or electrical actuators
  • Mainly used for cylindrical parts
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18
Q

Advantages of Finger Gripper with angular movements

A
  • All gripping variants possible
  • Suitable for cylindrical parts
  • Quick and simple release movement
  • No stroke movement at the departure of the gripper
  • Flexible through customizing of the finger geometry
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19
Q

Disadvantages of Finger Gripper with angular movements

A
  • Every gripper itself has a limited flexibility (size, forces, …)
  • There might be a large design effort to realize a large variety of gripping fingers
  • Set-up times to change the fingers
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20
Q

Needle Gripper - Technical Details and Applications

A
  • Force and form closure
  • Different designs for special application
  • Electric or pneumatic driven
  • Textiles, fiber composite material, plastics
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21
Q

Advantages of Needle Grippers

A
  • Gripping of air-permeable and/or thin parts (e.g. textiles)
  • Separation of stacked parts
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22
Q

Disadvantages of Needle Grippers

A
  • Parts are damaged through the penetration of the needles
  • No centering of the parts
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23
Q

Anthropomorphic Gripper

A

Every joint of the gripper can be tuned independently, by what the gripping pose results (e.g. hand-like grippers)

Technical details and applications
* Large flexibility with up to 24 degrees of freedom
* Pressure, force and location resolving sensorics fields
* Application in research and service robots
* AI based applications

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24
Q

Advantages of Anthropomorphic Grippers

A
  • High gripping precision
  • Adjustable gripping force
  • Adjustable gripping pose
  • Suitable for highly complex handling operations
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25
Q

Disadvantages of Anthropomorphic Grippers

A
  • Very complex design
  • Complex gripping setup
  • Complex process control
  • Very expensive
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26
Q

Vacuum Grippers

A

Gripping force is created through a negative pressure between gripper and part

Technical details and applications
* Gripping force is defined through the diameter and the quantity of the suction cups
* Pick and place applications
* Plastics, glass, metal sheets, wood and textiles

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27
Q

Advantages of Vacuum Grippers

A
  • Quick process control
  • Large handling forces possible
  • Low wear (no moving parts)
  • Working fluid most times by default at the machine available
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28
Q

Disadvantages of Vacuum Grippers

A
  • Only suitable for air impermeable parts
  • Susceptible to dirt
  • Failures in the vacuum system lead to loss of handling force
  • Deformation of the handled part is possible
  • Time to create the vacuum
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29
Q

Venturi Nozzle

A

 Used to create negative pressures
 Smooth-walled pipe with a cross-section narrowing
 Take-off tube at the smallest cross section

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30
Q

Flow of a fluid

A
  • Smallest cross-section: maximum dynamic pressure; minimum static pressure
  • The fluid’s velocity increases by the ratio of the cross-sections when entering the constricted area (constant volume)
  • Static pressure decrease in the outlet pipe
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31
Q

Advantages to the use of vacuum pumps

A
  • Simple and robust design
  • Low generation of noise
  • Energy supply via air pressure, which is usually available, thus no need for additional energy supply
  • Lower need for maintenance
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32
Q

Form Closure Gripper

A

The gripper is pressed against the part. Through a negative pressure, the gripper gets stiff and lays itself around the part

Technical details and applications
* Combination of form and force closure
* Large variety of objects, while the part can be enveloped partly
* Soft and deformable parts with low surface pressure

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33
Q

Advantages of Form Closure Grippers

A
  • Possible to handle multiple parts at the same time
  • Large flexibility regarding possible parts geometries
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34
Q

Disadvantages of Form Closure Grippers

A
  • No defined handling pose
  • Just suitable for small, light parts
  • Susceptible to dirt
  • Failures in the vacuum systems leads to loss of handling forces
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35
Q

Bernoulli Gripper - Technical Details and Applications

A
  • A negative pressure is created through a positive pressure at the gripper itself
  • Equilibrium between force of the negative pressure and the weight force of the part
  • Used to handle very sensitive parts, e.g. solar cells
36
Q

Advantages of Bernoulli Grippers

A
  • Contact less handling of the part
  • No residue on the parts
  • Smooth handling of sensitive parts
37
Q

Disadvantages of Bernoulli Grippers

A
  • Precise positioning is difficult
  • Uncontrolled blowoff of dirt particles
  • Handling small parts is difficult
  • Possibility of surface damage
  • Only suitable for flat and plane parts
38
Q

Magnetic Grippers

A

Through the attraction of magnetic components results a magnetic handling force.

Technical details and applications
 Handling through magnetic force closure
 Force can be created with:
* Permanent magnets: needs fixture to release the part, not suitable for a precise positioning
* Electromagnets: no additional actuators needed, looses handling force at failure
 Used for mostly smooth and ferromagnetic parts

39
Q

Advantages of Magnetic Grippers

A
  • Simple design
  • No additional actuator needed
40
Q

Disadvantages of Magnetic Grippers

A
  • Remaining magnetism possible
  • Low positioning and repeating accuracy
  • High tare weight of the gripper
  • Just ferromagnetic parts
  • Need of an additional fixture to release parts
41
Q

Adhesion Gripper (Freeze/Cyro gripper) - Technical Details and Applications

A

 Adhesive bond between frozen water and part
 Adhesive force depends on the wettability of the active medium
 Heat extraction through a Peltier element
 Release through a polarity reversal in the Peltier element
 Suitable to handle textiles and fiber composite materials

42
Q

Advantages of Adhesion Grippers

A
  • No damage to the surface
  • Relatively small handling forces
43
Q

Disadvantages of Adhesion Grippers

A
  • Long process times due to the inertia of the semiconductors
  • Parts surface must be able to absorb water
44
Q

Peltier Elements - Functional Principle

A

 Thermo-electric conductor
 Peltier element consists of two materials with different conductivity (mostly semiconductors)
 Semiconductors are connected in alternating order; junctions alternating on the line
 Jean C. A. Peltier: “Emergence of heat and cold on the boundary of two semiconductors of different conductivity when current is applied”
 Current flow through two junctions in a row: thermal absorption (cooling) at one junction, heat dissipation at the other

45
Q

Adhesion Gripper: Van der Vaals Gripper “Gecko Gripper”

A

On the gripper surface are many very thin hairs, which establish a force of attraction with the parts surface

Technical details and applications
 Based on the Van der Waals forces
 No remaining stains on the part
 Suitable to handle light sheets or platins

46
Q

Advantages of the Van der Waals Gripper

A
  • Also suitable for porous materials
  • No additional medium/power needed for the handling force
47
Q

Disadvantages of the Van der Waals Gripper

A
  • More elaborative release strategies as the gripper can’t be turned on and off
  • Needs clean and smooth surface
  • Limited handling forces
48
Q

Soft Gripper: Envelope Gripper

A

The soft fingers change their forms through an applied negative pressure and envelope themselves around the part.

Technical details and applications
 Form closure with low surface pressure
 No defined position, as the fingers are flexible
 Suitable to handle different forms, soft and light parts
 Mostly used in the food industry

49
Q

Advantages of Soft Grippers

A
  • Parts do not need a defined pose to be grabbed
  • Gripper adapts to the form of the part
  • Handling of sensible and soft parts
50
Q

Disadvantages of Soft Grippers

A
  • No defined pose of the parts
  • Small gripping forces
  • Parts must be able to be enveloped
51
Q

Additional Features of Grippers - Gripping Force Retention

A

o Prevents the release of the part at energy loss.
o Different variants of retentions, depending on the type of gripper
o Direction of retention depends on the application (o.d. or i.d. gripping)

52
Q

Additional Features of Grippers - Rotary Transmission

A

o Pneumatic and electric transmission for robotic application
o Enables multiple turns of a gripper
o Integrated slip ring contacts and air transmission supplies the gripper with energy

53
Q

Advantages of a Rotary Transmission

A

 Multiple turnings of the robot axis possible, without hoses wrapping around the axis
 Less interference contours due to hoses or cables

54
Q

Additional Features of Grippers - Automated Tool Change Systems

A

Exchange systems for grippers and tools are used in systems to execute multiple tasks with different tools per machine cycle.

55
Q

Advantages of Automated Tool Change Systems

A

 Increase the flexibility of an industrial robot
 Open up new fields of application
 Combination of handling and machine tasks
 Consideration of gripper and tool change within a programmed procedure

56
Q

Disadvantages of Automated Tool Change Systems

A

 Increase of auxiliary process time for tool change
 Technical effort for the implementation of the exchange system

57
Q

Additional Features of Grippers - Collision Protection

A

o Pneumatic bearded adapter plates between robot and tool
o In case of a collision, the adapter plate yields and the robot is stopped

58
Q

Additional Features of Grippers - Joining Mechanisms

A

o High precision joining of parts with transition or press fit
o Position and angular compensation
o Anti-twist device is used if torque occurs

59
Q

Sensors for robotic grippers - Types of sensors

A

o Proximity switches
o Distance sensor
o Force/torque sensors
o Optical sensors

60
Q

Different phases of gripping need different sensors

A

o Gripping phase:
Status of the gripper (open/closed/gripped)

o Securing the part:
Applying a defined gripping force

o Recognizing the part:
Camera systems recognize the position and orientation of the part

61
Q

Magnetic Switch - Function

A

 Design usually realized as a “reed switch”
 Magnetic field magnetizes and deflects ferromagnetic contacts
 Magnetic force needs to overcome spring force of the contacts
 Required field force for opening switch much smaller than for closing

62
Q

Magnetic Switch - Application

A

 Query of predefined positions
 Detection of a gripper’s finger position via a permanent magnet mounted on the piston rod or gripper jaws
 Not applicable for areas with magnetic interference fields (welding)

63
Q

Capacitive Proximity Switch - Function

A

 Capacity change affects the oscillator’s frequency
 Disturbance filter removes unwanted switching signals, however it also reduces the possible switching frequency to 1-100Hz
 Compensation electrode to balance out influence of dirt

64
Q

Capacitive Proximity Switch - Detection

A

Electroconductive object:
 Object forms counter electrode to the sensor electrode
 System corresponds to a series circuit of 2 capacitors
 Overall capacity is greater than undamped sensor

Isolating object:
 Object increases capacity as a function of dielectric constant 

65
Q

Inductive Proximity Switch - Function

A

 Query of predefined positions
 Creation of high frequency alternating magnetic field
 If an electrically conductive material enters the field, energy is absorbed so that the amplitude is reduced
 The sensor is switched on, if there is a significant change

66
Q

Inductive Proximity Switch - Function

A

 Query of predefined positions
 Creation of high frequency alternating magnetic field
 If an electrically conductive material enters the field, energy is absorbed so that the amplitude is reduced
 The sensor is switched on, if there is a significant change

67
Q

Inductive Proximity Switch - Advantages

A

 Compact design
 High repetition accuracy
 Non-sensitive towards influence of light, sound, temperature change or humidity

68
Q

Inductive Proximity Switch - Disadvantages

A

 For applications with high magnetic fields (welding), an additional protection is required
 Switching distance depends on the construction of the proximity switch and the material of the supervised object -> Correction factor might be necessary

69
Q

Optical Sensors - Function (gripping)

A

 Sensor emits light by optical fiber
 Position detection of the supervised gripper by evaluation of the reflected light or time

70
Q

Optical Sensors - Advantages

A

 Inexpensive
 Non-sensitive towards influence of sound, temperature change, humidity and magnetic fields

71
Q

Optical Sensors - Disadvantages

A

 Sensitive towards daylight, dirt, fog or particles in the air

72
Q

Optical Proximity Switches

A

 Light source sends light ray to the receiver (direct or through a reflector)
 If the ray is interrupted the sensor switches

73
Q

Optical Distance Sensor

A

 The light of the source is reflected by the object and detected by the receiver
 Time between sending and receiving is measured

74
Q

Flexible Position sensors

A

o Measurement of the gripper’s finger position
o Sensor is penetrated by a permanent magnet mounted to the jaw of the gripper -> Level of penetration depends of the distance between magnet and sensor
o Output if jaw is in 1 of 5 programmable ranges
o Software allows the determination of the exact position

75
Q

Force Measurement Sensors

A

o Assembly of intermediate jaws between basic jaw and gripper finger
o Contact forces induce flux along the intermediate jaws
o Measurement of deformation by clever orientated strain gauges inside the middle jaw
o Deformation is transformed into an analog signal by an electronic device

76
Q

HRC capable grippers

A

o Human robot collaborations is regulated in many guidelines and standards

o Safety principles
-> A gripper never hurts a human while gripping
-> A gripper never looses its part
-> A gripper recognizes contact with a human every time

o Equipped with different sensors, e.g. capacitive sensors, camera systems, force limitation and force measuring

o Information is processed in real-time and the software can, because of the information, distinguish between a human and an object

77
Q

Computer Vision

A
  • Computer vision solutions can recognize undefined objects and can identify robust gripping positions through knowledge of prior gripping operations
  • Vision based systems are especially attractive for processes with small quantities of parts, for which manual gripping programming or arranged part feeding would not be cost-efficient
  • Complexity of the needed system depends on general conditions
    -> Order state of the parts
    -> Knowledge of the parts’ shape
  • AI based systems enable a higher flexibility than standard algorithms, but need a lot more and top-quality data in order to run properly
78
Q

Vision Based Gripping - Order States

A

Unordered
-> Bulk Material, random orientation and overlapping possible
(e.g. Silo)

Partially ordered
-> Parts are not ordered, but all on one plane without overlapping
(e.g. Conveyer Belt)

Ordered
-> Parts are ordered and can be picked up all the time at the exact same spot
(e.g. magazines)

79
Q

Vision based gripping - Parts’ shape

A

Unknown
-> Shape is completely unknown, the system needs to define gripping positions independently

Similar parts known
-> The shape is not completely known, although the system has gripped similar parts before and can use this data

Known
-> The shape is completely defined

80
Q

Process description of vision-based gripping

A

Parts are transferred from an unordered to an ordered condition

o Idea: Parts are delivered unordered, get recognized from a camera system and are gripped depending on their position and orientation

Reference process can be divided in 4 steps:

  1. Recognizing the object
    -> Object must be recognized, localized and separated from the environment
  2. Determine the orientation
    -> Pose relative to the robot must be determined
  3. Identification of gripping positions
    -> Gripping positions are evaluated on the abse of previous gripping tasks and one position is chosen
  4. Path planning
    -> kinematic restrictions and objects in the direct way must be included
81
Q

Automated Workpiece feed of tooling machines: 3 Types of Handling

A
  • External Handling
  • Integrated Handling
  • Semi integrated Handling
82
Q

external handling

A

 Handling system separate from the machine tool (above/next to machine tool)
 Enables workpiece feed of multiple machines with one centralized handling system)
 Connected to machine tool via interface
 Suitable with mobile handling systems

83
Q

Integrated Handling

A

 Handling system is integrated in the machine tool
 Usage as design element in the machine
 No additional safety area necessary
 Machine spaces does not get larger
 Cost reduction at a serial use 3-5 times against an external handling

84
Q

Semi Integrated Handling

A

 Handling partly integrated into machine tool

 Exemplary handling systems
* Handling as shuttle (telescopic system)
* Tool storage
* Removal robot

85
Q

Pallets Handling

A

o Workpieces are prepared on a pallet by a worker
o The complete pallet is loaded into the clamping system in the machine
o Positioning with a reference point system

86
Q

Main Time parallel postprocessing

A

o Parts can be postprocessed with a robot parallel of the machining process

o Cost-efficient in combination with long cycle times

o Mostly classic tasks for robots
-> Brushing and deburring of work pieces
-> Part measurement
-> Part description
-> Part cleaning

o Parts are held by a robot or are placed in separate systems

Mechatronics and Control for Production Plants (12 decks)

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