Development of a novel mechatronic system for mechanical weed ...

Materials and methods
change its input to the motor to cause the desired motion. If the servo motor in
speed-control mode is trying to achieve a speed of 1000 rpm but is only running
at 900 rpm, the feedback control will detect the problem and inform the
positioning controller to provide more power until the detected speed reaches
1000 rpm (Baldor Electric company 2007). Closed loop systems are required for
applications whose motion profile is complex, when high resolution and
accuracy are necessary, when the rotation speed ranges from very slow to very
high and when high torques are demanded in small package sizes.
The actual feedback device is the last important part of a servo device.
Feedback devices included analog and digital tachometers and resolvers. An
analog tachometer is essentially a small generator, usually hard-wired to the
output shaft of the motor and using the resulting rotational speed to output a
variable voltage which varies linearly with speed. While they are cheap and
simple, analog tachometers always introduce some AC-type ripple into the
output signal because they are not ideal devices and subject to design and
manufacturing tolerances. Digital tachometers operate either optically with
photoelectric detection or with contact through a brush assembly. Two types
exist: absolute encoders which assign a discrete address to each position
through 360° and incremental encoders which simply emit electrical pulses at
defined intervals which must then be counted to obtain position and distance
(Baldor Electric company 2007).
A concept of a servo system is illustrated in the Figure 4.5.
50
External
controller
+
Servo
amplifier
Servo
motor
Feedback loop
Encoder
Figure 4.5 Concept of a servo system in a closed loop
Controlled
system (load)