Technology Education Department
Technical Drawing for Mechanical Design
Introduction to Robots
During the 60's and 70's, the manufacturing climate in the
United States was changing as a result of many economic problems
such as increasing inflation, high energy costs, government regulations,
and resistance of workers to perform repetitive and hazardous
jobs. As a result companies invested in capital equipment which
could be paid for quickly and modernization of facilities was
postponed. Therefore, the average age of facilities and equipment
in the U.S. has been increasing relative to that of other industrialized
nations.
The primary concern of the manufacturing industries since the
early 70's was to find ways to increase productivity and reduce
the cost of manufacturing products. Since the early 80's, the
major industries have looked at such technologies as numerical
control (NC) systems, computer aided design (CAD), computer aided
manufacturing (CAM), computer numerical control (CNC), and computer
intergrated manufacturing (CIM). Computer intergrated manufacturing
utilizes CAD, CAM, CNC and robots to create work cells that perform
a series of operations from the design of the part to its complete
creation without the use of human labor. The tasks of selecting
a piece of raw material, placing the material in a machine, selecting
a machine tool, removing the partially completed part, placing
the part in another machine, and eventually placing the finished
part in a storage bin are performed by one or more robots. Robots
are also well suited for doing heavy, dangerous and repetitive
tasks.
The first industrial robot, created by a company called Unimat,
was purchased by Ford Motor Company in 1961. An inventor by the
name of George C. Devol conceived the idea and with the help of
Joe Engelberger, a manufacturing executive, the robot became a
reality. Since the robot replaced human workers, organized labor
resisted the move by major companies to incorporate these devices
on the assemby lines. In addition, the cost of early produced
robots was in the hundreds of thousand of dollars so only the
largest manufacturing concerns could justify their use. Decreasing
productivity and increasing labor costs eventually forced companies
to use robots. With the demand for robots increasing, more companies
began to build newer and better robots and the cost of robots
dropped rapidly. Today, industrial robots start at $5000 and most
robots can be purchased for less than $100,000.
The number one user of robots in the U.S. is the automotive industry
followed by electric machinery, electronic components, plastic
molding products, sheet metal, iron and steel products. Welding,
loading, unloading, machining, moving and painting are the principal
tasks of existing robots with gluing, cleaning, checking, inspecting,
and packaging as the newer jobs facing robots. In the near future,
the fast food industry is hoping to use robots to prepare, cook
and serve food items as well as dispense beverages automatically
in response to customer selections.
As more manufacturers discovered robots, it became apparent that
standardization was needed in robot terminology as well as their
design and manufacture. The United National Bureau of Standards
in Springfield, Virginia, in conjunction with the U.S. Department
of Commerce and the National Technical Informaion Service created
a "Glossary of Terms for Robotics" and the American
National Standards Institute with the assistance of the Society
of Automotive Engineers and the Society of Manufacturing Engineers
developed national standards for robot design and manufacture.
Two journals, "Robotics Today" and "Robots In Industry",
are available to help practicing designers and engineers stay
current on new components for robots, new techniques for the construction
and care of robots, and new applications for the use of robots.
Design of Robots
In order to standardize the design, the robot is divided into
three primary parts: a power supply, a controller, and a manipulator.
Each of the three parts contain many components and the components
may vary to meet the design criteria and parameters for a specific
robot.
The first part is the power supply which provides the energy source
for driving the manipulator. Electricity, pneumatic (compressed
air), or hydraulic (fluid) power supplies can be used to provide
motion for robots. Electric motors are very efficient in changing
electrical energy into mechanical energy and require very little
maintenance. They produce very little noise when running, and
when at rest require no energy. The direct current servo electric
motor is the most common source of power for general purpose robots.
These motors can be manufactured in a wide range of sizes and
outputs which makes them ideal for practically any situation.
Pneumatic powered robots use compressed air and are generally
used in light-duty applications that require fast movements. Air
driven motors are also available in a wide range of sizes and
power ratings and produce very little noise. The main disadvantage
of air power is that another source of energy is needed to provide
the compressed air usually electricity or a fuel such as natural
gas, propane or gasoline. This additional source of energy must
be maintained even though the robot is not moving.
Hydraulically-powered robots use oil under pressure and are generally
used in heavy-duty tasks or in explosive environments. Hydraulic
motors can be made in many sizes and outputs just like electric
and pneumatic motors. This form of power usually generates more
noise, is bulky, and much heavier than similar air-powered equipment.
An additional source of energy must also be used to move the fluids
through components and it must be constant regardless of the robots
actions. Both air-powered and hydraulic-powered robots require
more maintenance on the tubes, hoses and fittings that connect
the components and distribute the energy.
The second part of the robot is the controller. It is usually
contained in a cabinet that is separate from the robot. This is
the brain of the robot and it controls the robot's movements.
Controllers are available in a variety of types and are used to
direct three different types of robot operation. Pneumatic logic
sequencer controllers direct compressed air through air logic
gates. Programmable controllers use electronic gates to change
motion. A microprocessor or a microcomputer can also be used to
direct the movements of a robot. These controllers can also be
classified as non-servos, point-to-point-servos or continuous
path servos.
A non-servo controller is often referred to as a "pick and
place" robot or "bang-bang" robot and is usually
used to move parts from one area to another. This is the simplists
type of robot with little or no horizontal (in or out) movement.
The motion of the non-servo robot is started by the controller
and is stopped by a limit switch hitting a mechanical stop. The
limit switch sends an electrical signal to the controller which
starts the next motion. Light sensing, lazer and fiber optical
limit switches are also used to change motions.
A point-to-point-servo controller is used on a larger robot that
has to move through a greater number of locations in a work cell
such as the spot welding of automotive body sections. The robot
must move to precise points, therefore, only the stops are programmed
and the robot takes the fastest and most direct route to each
point. Extreme care must be taken in the programing of the stops
so the robot does not run into the product as its performs the
required tasks.
A continuous path servo controller is used when the robot must
follow a specified, precisely determined path with a smooth, constant
motion with no acceleration between its end stops. They are used
when a smooth contour is required, such as the joining of two
pieces of metal like fender and body panels. This controller must
be programmed to adjust the movements constantly to allow the
robot to move in all directions.
The third part of a robot is the manipulator, or frame of the
robot. The manipulator is composed of the end effector, wrist,
lower arm, elbow, upper arm, shoulder, body, waist and base. Many
robots may contain only some of these components as they may require
only one, two or three movements. Therefore, more movements suggest
a much more complex and more expensive robot.
The end effector can also be referred to as the hand. It could
be an actuator, like a sensor that sends a signal to a controller,
or some type of gripper, or mechanical devise that is attached
to the wrist of the manipulator. The end effector allows objects
to be grasped or otherwise acted upon. Left to right (x axis)
movement of a gripper is called yaw. Up and down (y axis) movement
of the gripper is called pitch. Rotational movement of the gripper
is called roll.
The wrist is a set of rotary joints between the arm and the end
effector. These joints allows the end effector or hand to be oriented
to the workpiece or object. They must be capable of moving vertically
(pitch), horizontally (yaw) and in a circular (roll) motion.
The arm of the robot is an interconnected set of links and powered
joints composed of the lower arm, elbow and upper arm. The lower
arm or forearm is the part that connects the wrist to the elbow.
This component may be a fixed size or may slide in and out (z
axis) depending on the needs of the robot. The elbow is a joint
between the lower and upper arms. This component usually only
provides up and down (y axis) movement but could be designed to
provide back and forth (x axis) movement as well. The upper arm
is the part that connects the elbow to the shoulder. It may also
be a fixed size or may slide in or out for more flexibility in
movement. In and out movement is provided by hydraulic or air
powered cylinders that contain a piston attached to a rod.
The shoulder is the joint or set of joints that connects the arm
to the body of the robot. This component must be stronger and
more powerful than the elbow or wrist as it must move the weight
of the object as well as the robot arm. These joints usually only
provide lifting, up and down (y axis) movement.
The body is the main support structure that connects the shoulder,
arms, wrist, and effector to a base. The body may be fixed in
place on the base or may be allowed to rotate using a waist about
the y-axis.
The base is the platform or structure to which the shoulder or
body of the robot arm is attached. This component provides the
overall support for the robot and may be fixed in place or may
allow for movement either horizontally or vertically or both directions.
Some robots require devises that help it think. Sensors and limit switches are used to restrict the movements or change the direction of movements of a robot. Sensors can be activated by light, sound, temperature, and video signals.
Design Criteria
The first phase in designing a robot involves the asking of a series of questions. Initially, the questions should address the productivity of the robot in relation to a human; the type of robot needed to meet the job specifications; the robots working environment; the safety and security concerns; and the adaptability of existing robots.
Productivity is based on payback period, return on investment, or net present value formulas. Payback Period compares the cost of the robot to the cost of human labor. Return on Investment is a more complicated comparison that looks at tax credits, depreciation, and nine other factors. Net Present Value relates 19 factors over a period of 1 to 10 years to determine productivity.
Job Specifications identify the characteristics that the robot must be able to demonstrate. Specific operations or tasks, speed, positioning accuracy, repeatability, reliability, mean time before failure (dependability), payload capacity and memory capacity must be defined and addressed when designing a robot.
A robot may be able to work in environments different from humans but temperature, cleanliness and hazardous air conditions may affect robots in different ways.
The Occupational Safety and Health Act (OSHA) regulations must be followed by manufacturers of robots to insure the safety of operators and others the work around robots. Warning signs, labels, shields, guards, fences, floor mats, infared light curtains, horns and buzzers are used to protect workers from robots and robots from workers.
Sample Design Questions
Types of Robots
Robots can be classified according to the direction of their movements along the X, Y and Z axes and the number of movements the robot can perform. The four types are: Cooridinate, Cylindrical, Spherical and Articulated.
The Corridinate or Cartesian Robot moves to the left and the right (X direction), upward and downward (Y direction) and forward and backward (Z direction).
The Cylindrical Robot rotates about a vertical axis (X direction), upward and downward (Y direction) and forward and backward (Z direction).
The Spherical or Polar Robot rotates horizontally (X direction) and vertically (Y direction) about a fixed point and in and out (Z direction).
The Articulated or Revolute Robot rotates horizontally (X direction) on a fixed base, pivots vertically (Y direction) on an arm attached to a fixed point on the base, and moves in and out (Z direction) on a second arm attached to the end of the first arm. This robot usually has more parts such as a wrist that allows for yaw (left or right), pitch (up or down) and rotating motions. The revolute robot has at least six movements and closely matches the movements of a human.
Each of these types of robots will contain some type of gripper that can open and close, move left and right (yaw), up and down (pitch) and rotate 360° (roll).