Electric Actuator Control Systems

Why Controls are Needed

The motors of simple DC electric actuators would spill full speed if simply connected to power. More complex electric actuators such as those powered by stepper motors which require specific inputs would not work. Additionally, computers and micro-controllers cannot supply the power necessary to power the electric actuators on their own. [2]

Electric Actuator Control

Pulse width modulation () is a method of motor speed control used extensively in DC motors. The basic principle behind is that brief pulses of power are supplied to the motor causing the motor to spin. The inertia of the motor causes it to spin, albeit at a slower rate, when the power is not supplied. The exact speed of the motor can be controlled by controlling pulse width, the amount of time the load is connected to power. Note that the relationship between average voltage across the motor and speed is proportional. This is the same method used in TRIAC lightbulb dimmers. [4] The name comes from the fact that the pulse width is translated from the signal's amplitude. With this method the voltage across the load switches between zero volts and V_CC. works well with the on/off signals of a computer, and can be used to approximate any continuous signal while delivering the same average power. The figure below demonstrates how a sinusoidal signal may be translated to a signal, with pulse widths representing the amplitude of the sinusoid. [1]

H-Bridge

Use of an H-bridge can also be used to control the direction of brushed DC and stepper motors along with their speed. Driven by a source, the H-bridge consists of four MOSFET switches which can change the direction of current flow depending on their states. Figures 2 and 3 below show a simplified H-bridge along with how it can redirect current flow. Inputs A and B in Figure 4 below are signals which control the speed and direction of the motor. Pulses of varying duty cycle and constant amplitude applied to point A will turn on MOSFETs 1 and 4, turning the motor. Connecting the same signal to point B will turn on MOSFETs 2 and 3, turning the motor in the opposite direction. The duty cycle of the signal defines the speed of the motor by dictating the average current. Additional circuitry is required to brake the motor by shorting the terminals to either ground of V_CC. Care must be taken to ensure that only correct MOSFETs connect else the power supply will short and the subsequent large current will instantly damage the components. [1]

H-bridge controllers can range from small microprocessors to dedicated logic circuits. Motor driver boards such as the hobby level L298N can be purchased which include an H-bridge and all associated logic. The L298N can control two DC motors at the same time; instructions and sample code for use of the L298N with an Arduino board can be found here. [3]

Brushless DC Motor Control

Brushless DC motors (BLDCs) require electronic speed controllers (ESCs) for programable speed control. The ESC turn on and off the internal electromagnets of the BLDC in the correct sequence and timing to control the motors. They require a digital signal such as a servo signal or a PWM control signal which they then convert to an AC current, where the switching frequency us controlled by onboard FETs. It is important to consider the max current that the ESC is rated for such that it is higher than required drive current per motor and low enough that the FETs do not overheat. It is also important to consider whether  use of an ESC with a battery eliminator circuit (BEC) to regulate the batter voltage down to power other avionics is necessary. If it is important to save the energy and heat produced by the BECs it may be better to se an optically isolated ESC (OPTO). [5]

References