11 Gripping Technology Flashcards
Definition of a gripping system
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
Main Functions of a gripping system
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
Special Functions of a gripping system
Information retrieval through sensors
Supply of parts for assembly
Cleaning operations during production
Task of a Gripper
- 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.
Sequence of the handling process
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)
Requirements to Gripping Systems
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
Complementary Gripper Requirements
- 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
Gripper Types
o Mechanical grippers
o Vacuum grippers
o Enveloping grippers
o Magnetic grippers
o Adhesive grippers
Grip Variants for Finger Grippers
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
Mechanical Grippers - Retention Force
Generated via
- Force closure (friction)
- Form locking
- Combination of both
Mechanical Grippers - Drive units
Electric
Pneumatic/hydraulic
Piezoelectric
Finger gripper with linear movements - Technical Details and applications
- 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
Advantages of Finger Grippers
- 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
Disadvantages of Finger Grippers
- 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
Wedge Hook Kinematic
Force closure to handle cuboidal and cylindrical parts
Ways to realize the gripping and releasing moment in electrical parallel grippers
- Ball screw spindle
- Pinion gear rack
- Toothed belt drive
Finger Gripper with angular movements - Technical Details and Applications
- 180°-angular gripper
- Force closure
- Combination with form locking possible
- Driven by pneumatic or electrical actuators
- Mainly used for cylindrical parts
Advantages of Finger Gripper with angular movements
- 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
Disadvantages of Finger Gripper with angular movements
- 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
Needle Gripper - Technical Details and Applications
- Force and form closure
- Different designs for special application
- Electric or pneumatic driven
- Textiles, fiber composite material, plastics
Advantages of Needle Grippers
- Gripping of air-permeable and/or thin parts (e.g. textiles)
- Separation of stacked parts
Disadvantages of Needle Grippers
- Parts are damaged through the penetration of the needles
- No centering of the parts
Anthropomorphic Gripper
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
Advantages of Anthropomorphic Grippers
- High gripping precision
- Adjustable gripping force
- Adjustable gripping pose
- Suitable for highly complex handling operations
Disadvantages of Anthropomorphic Grippers
- Very complex design
- Complex gripping setup
- Complex process control
- Very expensive
Vacuum Grippers
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
Advantages of Vacuum Grippers
- Quick process control
- Large handling forces possible
- Low wear (no moving parts)
- Working fluid most times by default at the machine available
Disadvantages of Vacuum Grippers
- 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
Venturi Nozzle
Used to create negative pressures
Smooth-walled pipe with a cross-section narrowing
Take-off tube at the smallest cross section
Flow of a fluid
- 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
Advantages to the use of vacuum pumps
- 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
Form Closure Gripper
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
Advantages of Form Closure Grippers
- Possible to handle multiple parts at the same time
- Large flexibility regarding possible parts geometries
Disadvantages of Form Closure Grippers
- No defined handling pose
- Just suitable for small, light parts
- Susceptible to dirt
- Failures in the vacuum systems leads to loss of handling forces
Bernoulli Gripper - Technical Details and Applications
- 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
Advantages of Bernoulli Grippers
- Contact less handling of the part
- No residue on the parts
- Smooth handling of sensitive parts
Disadvantages of Bernoulli Grippers
- 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
Magnetic Grippers
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
Advantages of Magnetic Grippers
- Simple design
- No additional actuator needed
Disadvantages of Magnetic Grippers
- 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
Adhesion Gripper (Freeze/Cyro gripper) - Technical Details and Applications
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
Advantages of Adhesion Grippers
- No damage to the surface
- Relatively small handling forces
Disadvantages of Adhesion Grippers
- Long process times due to the inertia of the semiconductors
- Parts surface must be able to absorb water
Peltier Elements - Functional Principle
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
Adhesion Gripper: Van der Vaals Gripper “Gecko Gripper”
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
Advantages of the Van der Waals Gripper
- Also suitable for porous materials
- No additional medium/power needed for the handling force
Disadvantages of the Van der Waals Gripper
- More elaborative release strategies as the gripper can’t be turned on and off
- Needs clean and smooth surface
- Limited handling forces
Soft Gripper: Envelope Gripper
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
Advantages of Soft Grippers
- Parts do not need a defined pose to be grabbed
- Gripper adapts to the form of the part
- Handling of sensible and soft parts
Disadvantages of Soft Grippers
- No defined pose of the parts
- Small gripping forces
- Parts must be able to be enveloped
Additional Features of Grippers - Gripping Force Retention
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)
Additional Features of Grippers - Rotary Transmission
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
Advantages of a Rotary Transmission
Multiple turnings of the robot axis possible, without hoses wrapping around the axis
Less interference contours due to hoses or cables
Additional Features of Grippers - Automated Tool Change Systems
Exchange systems for grippers and tools are used in systems to execute multiple tasks with different tools per machine cycle.
Advantages of Automated Tool Change Systems
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
Disadvantages of Automated Tool Change Systems
Increase of auxiliary process time for tool change
Technical effort for the implementation of the exchange system
Additional Features of Grippers - Collision Protection
o Pneumatic bearded adapter plates between robot and tool
o In case of a collision, the adapter plate yields and the robot is stopped
Additional Features of Grippers - Joining Mechanisms
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
Sensors for robotic grippers - Types of sensors
o Proximity switches
o Distance sensor
o Force/torque sensors
o Optical sensors
Different phases of gripping need different sensors
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
Magnetic Switch - Function
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
Magnetic Switch - Application
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)
Capacitive Proximity Switch - Function
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
Capacitive Proximity Switch - Detection
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
Inductive Proximity Switch - Function
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
Inductive Proximity Switch - Function
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
Inductive Proximity Switch - Advantages
Compact design
High repetition accuracy
Non-sensitive towards influence of light, sound, temperature change or humidity
Inductive Proximity Switch - Disadvantages
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
Optical Sensors - Function (gripping)
Sensor emits light by optical fiber
Position detection of the supervised gripper by evaluation of the reflected light or time
Optical Sensors - Advantages
Inexpensive
Non-sensitive towards influence of sound, temperature change, humidity and magnetic fields
Optical Sensors - Disadvantages
Sensitive towards daylight, dirt, fog or particles in the air
Optical Proximity Switches
Light source sends light ray to the receiver (direct or through a reflector)
If the ray is interrupted the sensor switches
Optical Distance Sensor
The light of the source is reflected by the object and detected by the receiver
Time between sending and receiving is measured
Flexible Position sensors
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
Force Measurement Sensors
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
HRC capable grippers
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
Computer Vision
- 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
Vision Based Gripping - Order States
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)
Vision based gripping - Parts’ shape
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
Process description of vision-based gripping
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:
- Recognizing the object
-> Object must be recognized, localized and separated from the environment - Determine the orientation
-> Pose relative to the robot must be determined - Identification of gripping positions
-> Gripping positions are evaluated on the abse of previous gripping tasks and one position is chosen - Path planning
-> kinematic restrictions and objects in the direct way must be included
Automated Workpiece feed of tooling machines: 3 Types of Handling
- External Handling
- Integrated Handling
- Semi integrated Handling
external handling
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
Integrated Handling
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
Semi Integrated Handling
Handling partly integrated into machine tool
Exemplary handling systems
* Handling as shuttle (telescopic system)
* Tool storage
* Removal robot
Pallets Handling
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
Main Time parallel postprocessing
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
