Electronic speed control
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An electronic speed control or ESC is a device mounted on board an electrically powered radio control model in order to vary its drive motor's speed, its direction and even to act as a dynamic brake in certain controllers.
An ESC can be a stand-alone unit which plugs into the receiver's throttle control channel or incorporated into the receiver itself, as is the case in most toy-grade R/C vehicles. Some R/C manufacturers that install proprietary hobby-grade electronics in their entry-level vehicles, vessels or aircraft use onboard electronics that combine the two on a single circuit board.
Regardless of the type used, an ESC interprets control information not as mechanical motion as would be the case of a servo, but rather in a way that varies the switching rate of a network of field effect transistors, or "FET's."[1] The rapid switching of the transistors is what causes the motor itself to emit its characteristic high-pitched whine, especially noticeable at lower speeds. It also allows much smoother and more precise variation of motor speed in a far more efficient manner than the mechanical type with a resistive coil and moving arm once in common use.
Most modern ESCs incorporate a battery eliminator circuit (or BEC) to regulate voltage for the receiver, removing the need for extra batteries. ESCs are normally rated according to maximum current, for example, 25 amperes or 25A. Generally the higher the rating, the larger and heavier the ESC tends to be which is a factor when calculating mass and balance in airplanes. Many modern ESCs support nickel metal hydride and lithium ion polymer batteries with a range of input and cut-off voltages. The type of battery and number of cells connected is an important consideration when choosing a BEC, whether built into the controller or as a stand-alone unit. A higher number of cells connected will result in a reduced power rating and therefore a lower number of servos supported by an integrated BEC.
DC ESCs in the broader sense are PWM controllers for electric motors. The ESC generally accepts a nominal 50Hz PWM servo input signal whose pulse width varies from 1ms to 2ms. When supplied with a 1ms width pulse at 50Hz the ESC responds by turning off the DC motor attached to its output. A 1.5ms pulse-width input signal results in a 50% duty cycle output signal that drives the motor at approximately half-speed. When presented with a 2.0ms input signal the motor runs at full speed due to the 100% duty cycle (on constantly) output.
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[edit] ESCs for cars
ESCs designed for sport use in cars generally have reversing capability; newer sport controls can have the reversing ability overridden so that it can be used in a race. Controls designed specifically for racing and even some sport controls have the added advantage of dynamic braking capability. Simply put, the ESC forces the motor to act as a generator by placing an electrical load across the armature. This in turn makes the armature harder to turn, thus slowing or stopping the model. Some controllers add the benefit of regenerative braking. This puts the voltage being generated by the motor back to work recharging the vehicle's drive batteries. On full-sized vehicles, regenerative braking is used in electric and hybrid golf cars and hybrid automobiles while dynamic braking is used in diesel-electric locomotives to help slow trains on long downgrades.
[edit] ESCs for helicopters
ESCs designed for radio-control helicopters do not require a braking feature (indeed, turning it on would likely result in the main rotor assembly being severely damaged) nor do they require reverse direction. Many high-end helicopter ESCs do provide a "Governor mode" which fixes the motor RPM to a set speed, greatly aiding CCPM based flight.
[edit] ESCs for boats
ESCs designed for boats are by necessity waterproof. Also, many are water-cooled. Like cars, boats need braking and reverse capability.
[edit] Brushless ESCs
Brushless motors have become very popular with radio controlled airplane hobbyists because of their speed, power, longevity and light weight in comparison to traditional motors. However, brushless DC motor controllers are much more complicated than brushed motor controllers. They have to convert the DC from the battery into phased AC (usually three phase) that the brushless motor can use. The correct phase varies with the motor rotation, which is where the complication lies. Usually, back EMF from the motor is used to detect this rotation, but variations exist that use magnetic (Hall Effect) or optical detectors. Computer-programmable speed controls generally have user-specified options which allows setting low voltage cut-off limits, timing, acceleration, braking and direction of rotation. Reversing the motor's direction may also be accomplished by switching any two of the three leads from the ESC to the motor.
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