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Closed-Loop Motion Control
The term closed-loop
refers to a conceptual
circle. The sensors monitor angles, distances, positions, locations, or any variable involved in the action. The outputs of these sensors are sent to a
computing device such as any controller or computer that is capable of executing a stored program. The computing device determines the desired
target positions or locations as the robot moves. The computing device sends control signals to one or more effectors (sources of motion like motors
or pistons). The effectors work to achieve the calculated position or location. Finally, the new positions change what the sensors are detecting. This
completes the conceptual circle called the closed-loop
This is the circle of information transfer that regulates
position. There can be many closed loops in a robot. This means that the computer (according to its program) may make the actual position the same as
the desired position despite environmental influences. The computer continually checks the sensors and adjusts position by means of effectors.
Proportional Speed Control
When a distace is to be closed or reduced, proportional
speed control is often used. It exists within closed-loop control. It associates distances with speeds. To approach a new position, the speed of approach is
reduced as the distance is reduced. This speed is reduced in proportion to the distance remaining. This is why it is called proportional speed control. This
program feature gives rise to two other common features:
Force Feedback and Compliance
If proportional speed control were to proceed unmodified,
eventually, very slow speeds at very small distances would persist indefinitely. The robot must be allowed to reach a location or touch an object without
first achieving a zero speed. This is where compliance comes in. The part of the robot that is likely to collide with objects must not be absolutely hard
and rigid. It must be somewhat compressible. The force with which it is being compressed is typically measured by force sensors to inform the computing
device. This is force feedback and compliance.
Compliance is useful also in another way: When used
together with force sensing, compliance can facilitate searching for objects in the environment by intentionally causing collisions. At appropriately low
forces, these intentional collisions provide an opportunity to sense the presence and location of objects being sought. This method can be used also to
detect the shape of objects in the environment. It is much less expensive than vision systems and, very often, it is all that is need.
Highly Precise Motion from Low-Precision Parts
Closed-loop motion control, proportional speed control,
compliance and force feedback make possible low-cost construction methods. This is because robotic devices designed in this way have a very important
property: The precision with which such devices move does not depend upon the precision with which their mechanical parts are made!
This can be understood from the way people move and
navigate. Suppose a person wakes up one morning with an arm that is one eighth of an inch shorter than it was the previous day. Reaching for a morning
beverage, the hand starts its journey. It comes closer and closer to the handle of a cup. It is unlikely that anything unusual would be noticed. Feedback from
the senses always tell us what to do. A person does not strain to remember how long he or she should press ahead or how fast to move. Anybody would
simply move the hand until it gets there.
Contrast this with the world of machined, ground, and
polished steel parts used in most of today's industrial equipment. A part being an eighth of an inch short would be absolutely unthinkable!
This means that instead of using hardened steel parts
made with great precision, parts may be rough-cut or extruded, or even made from materials having poor shape stability such as wood, rubber, or spring wire.
Here is another example: A piece of lumber is on a table
saw. A robotic arm pushes the board back toward a backstop. This backstop prevents the board from falling off of the far side of the table. One end of the
board happens to reach the backstop first, but soon the board is flush against the backstop.
If the robot arm continues to move past this point, the
board could be dented or crushed. If the accuracy of movement is low and there is no force feedback or compliance, the robot arm may crush the wood or stop short of pressing
the board flush against the backstop.
On the other hand, if force feedback and compliance are
used, we have two fortunate circumstances: 1.) the force sensing provides an indication to the computing device that further movement should cease; and
2.) the bending or compression of the robot arm provides the time that the computing device needs to stop or redirect the effectors.
Without compliance, damage could be done in the time
between sensing the opposition of the backstop and the computer's action to stop the effectors involved.
In the same way, the board can then be moved (let's
say to the left) toward an end stop.
In this case, the dimensions of the table, the backstop,
and the end stop provide the accuracy of positioning that is needed to determine the placement of the cut. Robotic devices using closed loop motion control,
force feedback, and compliance can utilize the same sort of tools, jigs, and fixtures that people do.
Here is an example that does not derive measurement
from an external tool: A wrench is use to torque down a bolt. Turning must stop when the force at the end effector reaches a certain value. The accuracy of
the force sensor in the robotic device determines the accuracy of the result. This accuracy was not determined by the precision with which the mechanical
parts of the robot were made. If the wrench handle can be held at various distances from the bolt, a distance sensor of some kind would be involved; but again, the
sensors and not the structural parts provide the precision. Since the price of sensors and computers are going down faster than the price of precisely made
mechanical parts, this should be the source of the precision.
There is another cost advantage to closed-loop control. The position
sensors and angle sensors need not be linear. This can make their cost 5% or less of the cost of linear devices.
A horse is dragging a 400 pound log. A child steers the
horse but does not need to be strong. The horse can work hard but it does not need to be nimble, dexterous, or brainy.
Small and light machines can attach belts, tie strings,
place pillows, and link hooks to a chain. The separation of robotic arms that are dexterous and nimble from simpler devices that are strong is a very
effective cost-cutting measure. The machines that rescue an unconscious fireman do not each need to have all of the properties used in the rescue. It
is much the same when people use non-robotic tools. It is a combined effect.
Durability and Repairability
Technical innovations are so frequent that it is better
to emphasize repair than endurance. If a robot can survive a task for five years and is easy to repair (easy for another robot to repair), it seems a waste of
money to extend its expected repair-free life too much longer.