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Eaton, Aaron Lillie, Keith Sears, Barbie Tudor, John Christensen, Eric
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Group 5 Eaton, Aaron Lillie, Keith Sears, Barbie Tudor, John Christensen, Eric
Note: There are a handful of other position sensors such as encoders and resolvers, but the variable resistor is common. The next few pages describe encoders.
Position sensor Position sensors use digital encoders to send and receive information. An encoder is a sensor of mechanical motion that generates digital signals in response to motion. As an electro-mechanical device, an encoder is able to provide motion control system users with information concerning position, velocity and direction. There are two different types of encoders: linear and rotary. A linear encoder responds to motion along a path, while a rotary encoder responds to rotational motion. An encoder is generally categorized by the means of its output. An incremental encoder generates a train of pulses which can be used to determine position and speed. An absolute encoder generates unique bit configurations to track positions directly. The two basic types are : Linear and Rotary.
Linear A linear encoder is a sensor, transducer or reading-head linked to a scale that encodes position. The sensor reads the scale and converts position into an analog or digital signal that is transformed into a digital readout. Movement is determined from changes in position with time.
Incremental Rotary An Incremental rotary encoder is also referred to as a quadrature encoder. This type of encoder utilizes sensors that use optical, mechanical or magnetic index counting for angular measurement.
Encoders have become a vital source for many applications requiring feedback information. Whether an application is concerned with speed, direction or distance, an encoders vast capability allow users to utilize this information for precise control. With the emergence of higher resolutions, ruggedness, and lower costs, encoders have become the preferred technology in more and more areas. Today, encoder applications are all around us. They are utilized in printers, automation, medical scanners, and scientific equipment.
A great example of this is @http://www.youtube.com/watch?v=7VjL9zkjQog
This is the feedback (closed) loop that is always checking itself.
Where are servos used? Remote-controlled or radio-controlled toy cars, robots and airplanes, industrial applications, robotics, in-line manufacturing, pharmaceutics, and food services
Advantages among different servo motors High output power relative to motor size and weight. Closed loop system-It gives itself feedback as to its position or speed High efficiency. It can approach 90% at light loads. High torque to inertia ratio. It can rapidly accelerate loads. Has "reserve" power. 2-3 times continuous power for short periods. Has "reserve" torque. 5-10 times rated torque for short periods. Audibly quiet at high speeds. Resonance and vibration free operation.
Disadvantages among different servo motors Requires "tuning" to stabilize feedback loop. Motor "runs away" when something breaks. Safety circuits are required. Peak torque is limited to a 1% duty cycle. Motor can be damaged by sustained overload. Bewildering choice of motors, encoders, and servodrives. Motor develops peak power at higher speeds. Gearing often required. RC Servo motors have a limitation of movement unless hacked. Usually cost more than a stepper motor
Split-Phase Motors All single-phase motors work basically the same. It is the way the are started that differs. In order for a single-phase induction motor to work the rotor has to be subjected to a rotating magnetic field. This is produced in the stator coils of the split-phase motor by having start windings positioned with their axes 90° electrical in relation to the run windings.
How they work The split-phase machine has two windings from a single phase arranged in the stator. One is the main winding and other is starting winding, which is used only for starting purpose. The run winding has characteristics of low resistance but high reactance. The starting winding has high resistance but low reactance. Accordingly, the start windings are first to become magnetizedwhen the power is applied. The current flow through the start winding begins after power is applied to the motor by 20 degrees or so. The current in the run windings begins even later by about 50 degrees. This timing creates a phase differential which creates the necessary rotating magnetic field. When the run windings become strongly magnetized, the start windings begin to fade out. As a result the magnetic field shifts to a new position creating a rotating magnetic field.
The single phase induction motor begins to work like a two phase motor!
After the starter and main winding has developed a rotating magnetic field, the rotor starts to rotate and for further rotation of the rotor, the starter winding is not required. So a switch is provided that can be opened to isolate the starter winding from the motor circuit. Usually the switch may be of centrifugal type and thus after the motor attaining 70 to 80% of the full load speed, the centrifugal switch opens up, isolating the starter winding. The centrifugal switch is connected in series with the starter winding and it is located inside the motor.
Simply constructed split phase motors are among the least expensive.They're widely used on easy starting loads of 1/3 horsepower or less.Washing machines, tool grinders and small fans and blowers are among the applications that use these motors.
