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Robotic Sample Transport

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The Robotics Industry Association (RIA) defines a robot as "a manipulator designed to move materials, parts or specialized devices through variable programed motions for the performance of a variety of tasks. There are a number of common  robotic configurations available today.  This article will focus on the type that are most likely to be used in a laboratory environment. All are electrically driven, vs. hydraulic or pneumatic.  One unifying aspect of all robotic configurations is that the accessible workspace is limited, and each configuration has a different workspace geometry.  Therefore it is essential to understand the configurations and choose a type that is best suited to the application needs. 


Contents

Terminology

The following terms are common to all robotic configurations.

  • Robotic arm: An interconnected set of links and powered joints comprising a manipulator that supports or moves a wrist or end-effector.
  • End effector: Any device mounted to the end of a robot arm that is used to manipulate its environment.
  • Prismatic joint: A robotic joint with one degree of tranlation, i.e. non-rotating. 
  • Revolute joint: A robotic joint capable of rotation. 
  • Monomast: A robotic arm component that is of a rigid, non-collapsing construction. 
  • Accuracy (in Robotics): The degree to which actual position corresponds to desired or commanded position; the degree of freedom from error. Accuracy involves the capability to hit a mark, or reach the point in space, or get the correct answer; repeatability is the ability to duplicate an action or a result every time. Accuracy of a robot is determined by three elements of the system: the resolution of the control system, the inaccuracies or imprecision of the mechanical linkages and gears and beam deflections under different load conditions, and the minimum error that must be tolerated to operate the arm under closed servoloop operation. Accuracy refers to the degree of closeness to a "correct" value; precision refers to the degree ofpreciseness of a measurement. Accuracy is frequently confused with precision.
  • Axis (in Robotics): A direction used to specify the robot motion in a linear or rotary mode.
  • Articulated robot: A type of robotic arm whose joints are all revolute.
  • Articulated structure (in Robotics): Set of links and joints that constitute the arm and the wrist.
  • Articulation (in Robotics): The manner and actions of jointing in a robot. The greater the number, the easier it is for a robot to move and attain any position. Types of articulations are fixed beam, linear joint, ball joint, round joint revolute or pin joint, and other. They vary in the number of degrees of freedom.
  • Relative location: A programmed location or position in a robotic work envelope that is relative to some other robot position, frequently an
    absolute location.
  • Absolute location: A programmed location or position in the robot's work envelope defined by specific coordinates.

A comprehensive list of robotic terms can be found in the List of Robotic Terms article and also in the paper on IUPAC NOMENCLATURE IN LABORATORY ROBOTICS AND AUTOMATION .

Robotic Configurations

Cartesian

Cartesian robot

Cartesian robotic devices axes are at right angles to each other, forming cartesian, or X, Y and Z axes of movement.  Cartesian robotics prismatic joints employ the simplest form of kinematics equations, because of this simple, linear configuration.  The primary advantage of the cartesian geometry is the ability to move in multiple linear directions.  Cartesian structure can also be the most rigid, since each axes can be supported at both ends.  This offers the potential of manipulating heavy loads and operating with very high repeatability across the entire range of movement.  Cartesians have a tighter repeatability over a large work area than scara or articulated arms.  The X and Y axis movement of cartesians tends to be slower than the rotary motion of other configurations. They have the largest surface area requirement of all robotic configurations for a given workspace. Exposed guiding surfaces require covering in corrosive or dusty environments.  The simple geometry limits the ability to reach spaces not directly accessible to the Z axis. 

In the laboratory environment, cartesian robots are very commonly used at the basis for automated liquid handling workstations




Gantry

Gantry robot
A gantry robot is simply a cartesian robot whose X and Y axes have been elevated, usually with the goal of creating a workspace below the XY plane.  Gantry robots can straddle and access multiple devices or workstations.  They embody all the characteristics of cartesian robots, but the elevated mode may place special requirements on the vertical, or Z-axis.  A monomast Z element will offer the best strength and repeatability, but overhead clearance must be provided for the upward extension of the monomast.  A telescoping Z element eliminates the need for overhead clearance, at the sacrifice of strength and repeatability. 

Gantry robots are the cartesian form most often used in the laboratory, especially for automated liquid handling workstations.  Because gantry robots can be made very large, they are often used as Automated Storage and Retrieval Systems (ASRS).  Such large systems are often used in conjunction with compound storage collections.




Cylindrical

Cylindrical robot
A cylindrical robot has a two orthogonal prismatic axes of movement (horizontal and vertical) and one revolute axis, forming a cylindrical coordinate system.  It is capable of higher horizontal plane speeds vs. cartesian systems due the revolute base.  However horizontal, straight line motion is more complex to calculate and tends to be slower.  The resolution of the positioning of the end effector is not constant, but depends on the degree of extension along the horizontal axis.  If a monomast contruction is used for the horizontal element, clearance behind the robot must be accounted for when retracted. 

The first commercial laboratory robot, the Zymate, was of cylindrical (monomast) architecture.  Today in the laboratory cylindrical robots are used for a variety of applications, from basic pick and place microplate stackers to complex workcell applications. 





Articulated

Articulated Robot
A robot arm is said to be articulated if all the joints are revolute[1].  The workspace of an articulated arm is complex, often a three-dimensional crescent.  With all joints revolute, this type of robot requires the most complex kinematic calculations.  An articulated configuration can most closely approximate an anthropomorphic, or human-arm motion, and thus offers a high degree of flexibility for accessing objects, devices or workstations within it's work envelope.  Articulated robots may have two or more joints, with highly complex examples having as many as ten joints.  A higher degree of flexibility comes at the cost of higher overall complexity, slower speed and higher cost.  The resolution of the positioning of the end effector is not constant throughout the workspace.  Positional repeatability can be more effected by gravity and load weight than other types because of the joints are oriented orithoginal to gravity.  

Articulated robots are very popular in the laboratory for complex workcell and integrated system applications, due to their high degree of flexibility and adaptability.  Such robot arms can range in size from 50cm to 2m vertical height.   





SCARA

Scara Robot

The SCARA (Selective Compliance Assembly Robot Arm)[2] configuration was devised specifically for assembly work.  It consists of two or more revolute joints and one prismatic, all of which operate parallel to gravity, easing the mechanical burden.  As the name indicates, this configuration has been designed to offer variable compliance in horizontal directions, which can be an advantage in assembly tasks.  The kinematics of this configuration are quite complex and the vertical component of movement is generally rather limited.  Thus, it can reach around objects in the workspace, but not over them.  The resolution of the positioning of the end effector is not constant throughout the workspace, but these robots do have a high degree of positional repeatability.  They are generally faster and more expensive than cartesian systems. 


Scara robots are not very common in laboratory applications, limited to high speed pick and place uses.   








Hybrid types

There are always hybrid configurations that complicate exact categorization.  In the case of laboratory applications, the most common hybrid configuration would be the placement of a cylindrical or articulated robot arm on a one or two-dimensional positioning stage, i.e. a rail or a platen.  This has the effect of increasing the work envelope.  However, since the robot arm being moved has significant mass, acceleration and deacceleration are slow, and top movement speed is modest.  Moment is generally electrically driven, but simple two-position movement can be achieved via pneumatic or hydraulic drive. 

End effectors

As stated above, an end effector[3] is any device mounted to the end of a robot arm that is used to manipulate its environment.  Commonly in the laboratory the "environment manipulation" is to move a sample or sample vessel.  Thus the end effector is often referred to as the "gripper".  However, note in the case of an automated liquidling handling workstation, the robot end effector is a pipette tip, either fixed or disposable.  Grippers come in a wide variety of configurations, usually optimized for the vessel to be carried.  In some cases, it is not possible to design an end effector that is optimal for all the manipulations required by the system.  In these cases, robots that are designed for interchangable end effectors may be a solution, but it should be strongly noted that use of interchangable end efforts generally reduces the positional repeatability of the end effector, limits payload, and requires costly and significant hardware and engineering.  It also impacts throughput because time is taken to attach, detach, and park end effectors.  If such a device is considered, key considerations include the mechanical repeatability and security of the attachment mechanism and the ease, reliability and speed of executing the exchange. 

Robotic safety

This subject cannot be concluded without a discussion of safety around robotic devices.  The devices described above are all capable of moving at high speeds and can inflict harm on people if a collision results.  It is the responsibility of both the developer and user to assure that people in the workplace are not at risk.  The most basic element is an emergency stop button(s).  Secondly, an enclosure or light curtain, with interlocks which cause an emergency stop when the protected envelope is breached.  There are standards:  ANSI/RIA/ISO 10218-1-2007 is the adoption of the International Standard ISO 10218-1:2006 as an American National Standard. This new standard provides safety guidance for the robot, but only the robot. The current American National Standard for robot safety – ANSI/RIA R15.06-1999 covers not only the robot but the complete robot system (installation) and talks to a variety of stakeholders including robot manufacturers, safety equipment manufacturers, integrators, installers, and ultimately the users.  See also the RIA Technical Report, R15.206 , to be released soon and on-going standards work being conducted at NIST.

References

  1. The Electrical Engineering Handbook, Richard C. Dorf, CRC Press, 2006, p14-6
  2. Robot Analysis: The Mechanics of Serial and Parallel Manipulators, Lung-Wen Tsai, Wiley-IEEE, 1999 p26
  3. Robotics and automation handbook, Edited by Thomas R. Kurfess, CRC Press, 2005

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