DHK Hobby Optimus Panduan Instruksi Pengguna - Halaman 6
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BLDC motors may be described as stepper motors, with fixed permanent magnets and possibly more
poles on the rotor than the stator, or reluctance motors. The latter may be without permanent magnets,
just poles that are induced on the rotor then pulled into alignment by timed stator windings. However,
the term stepper motor tends to be used for motors that are designed specifically to be operated in a
mode where they are frequently stopped with the rotor in a defined angular position.
RC servos
Servos are hobbyist remote control devices typically employed in radio-controlled models, where they
are used to provide actuation for various mechanical systems such as the steering of a car, the control
surfaces on a plane, or the rudder of a boat.
Due to their affordability, reliability, and simplicity of control by microprocessors, RC servos are often
used in small-scale robotics applications.
RC servos are composed of an electric motor mechanically linked to a potentiometer. A standard RC
receiver sends Pulse-width modulation (PWM) signals to the servo. The electronics inside the servo
translate the width of the pulse into a position. When the servo is commanded to rotate, the motor is
powered until the potentiometer reaches the value corresponding to the commanded position.
RC servos use a three-pin 0.1" spacing jack (female) which mates to standard 0.025" square pins (which
should be gold-plated, incidentally). The most common order is Signal, +voltage, ground. The standard
voltage is 6VDC, however 4.8V and 12V has also been seen for a few servos. The control signal is a
digital PWM signal with a 50Hz frame rate. Within each 20ms timeframe, an active-high digital pulse
controls the position. The pulse nominally ranges from 1.0ms to 2.0ms with 1.5ms always being center
of range. Pulse widths outside this range can be used for "overtravel" -moving the servo beyond its
normal range. This PWM signal is sometimes (incorrectly) called Pulse Position Modulation (PPM).
The servo is controlled by three wires: ground, power, and control. The servo will move based on the
pulses sent over the control wire, which set the angle of the actuator arm. The servo expects a pulse
every 20 ms in order to gain correct information about the angle. The width of the servo pulse dictates
the range of the servo's angular motion.
A servo pulse of 1.5 ms width will typically set the servo to its "neutral" position or 45°, a pulse of 1.25
ms could set it to 0° and a pulse of 1.75 ms to 90°. The physical limits and timings of the servo hardware
varies between brands and models, but a general servo's angular motion will travel somewhere in the
range of 90° - 120° and the neutral position is almost always at 1.5 ms. This is the "standard pulse
servo mode" used by all hobby analog servos.
A hobby digital servo is controlled by the same "standard pulse servo mode" pulses as an analog servo.
Some hobby digital servos can be set to another mode that allows a robot controller to read back the
actual position of the servo shaft. Some hobby digital servos can optionally be set to another mode and
"programmed", so it has the desired PID controller characteristics when it is later driven by a standard
RC receiver.
RC servos are usually powered by the receiver which in turn is powered by battery packs or an
Electronic speed controller (ESC) with an integrated or a separate Battery eliminator circuit (BEC).
Common battery packs are either NiCd, NiMH or Lithium-ion polymer battery (LiPo) type. Voltage
ratings vary, but most receivers are operated at 5V or 6V.
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