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Calculating Electrical Requirements for Direct Current Electric Actuators

When designing electro-mechanically actuated systems, there are several electrical design requirements that must be determined.

Servo control systems require accurate control of motion parameters such as acceleration, velocity, and position. This requires a controller that can apply current (torque) to accelerate a motor in a given direction, as well as provide an opposing current to decelerate it. When this application of aiding and opposing torque can be carried out in both directions, it is referred to as four quadrant motor control (Figure 1).

Figure 1. Four quadrant servo control

In four quadrant electric actuation systems, energy changes its form from electrical current flow to mechanical motion and vice versa. This conversion of energy is performed by an electric motor. An electric motor can be modeled electrically as a resistor, an inductor, and a voltage source. The resistor represents the resistance of the windings and internal wiring. The inductance is created from the turns of the wire that make up the windings. The voltage source is a result of the back electromotive force (EMF) created by the rotation of the motor shaft. When an electric motor shaft rotates, it produces an opposing voltage proportional to the motor’s angular velocity.

When the applied voltage exceeds the back EMF voltage, motoring occurs. When the back EMF voltage is greater than the applied voltage, braking occurs and the motor generates energy. In steady state, the difference between the applied voltage and the motor’s back EMF, divided by the circuit’s resistance, gives the current flowing in the motor windings. A motor’s current is directly proportional to its mechanical output torque.

Figure 2. Electrical to mechanical energy conversion

Figure 2 depicts the conversion of energy from electrical input to mechanical output. Electrical energy is input to a power supply. The power supply converts the input energy into a form that can be used by the motor drivers [i.e., alternating current (AC) to direct current (DC)]. The motor driver applies the energy from the supply to the motor as necessary to obtain the intended motion. The electric motor then converts the electrical energy into mechanical energy. The output of the motor is typically mated with some form of mechanical actuator that converts the motor’s output to the intended motion. At each point in the conversion process, some energy is lost due to inefficiencies in the system.

A moving object possesses kinetic energy. When a motor decelerates a moving object, the energy returned to the system has to go somewhere. Similarly, potential energy in the form of gravitational forces, springs, etc., can be returned to the system as objects move. The energy is passed to the motor, which converts the mechanical energy back to electrical energy. The motor driver converts the electrical energy from the motor and returns it to the power bus between it and the power supply. At this point, something must be done with the remaining energy (Figure 3).

Figure 3. Mechanical to electrical energy conversion

Similar to the motoring scenario, the conversion process is not 100% efficient, and a portion of the regenerated energy conversion energy is lost in the system as heat. There are several methods that can be used to handle the remaining energy. In some cases, it can be returned back to the power source (batteries, grid, etc.). If the energy is not removed from the system, the supply voltage will rise as the energy charges the bus capacitance. If the voltage rises too high, it could exceed voltage ratings of components and cause damage.

This work was done by Joshua Stapp for the Army Armament Research, Development and Engineering Center. ARDEC-0006

Topics:
Aerospace Automation Batteries Control systems Electronics Energy Energy Efficiency Energy Harvesting Energy Storage MEMs Mechanical Components Motion Control Motors & Drives Power Management Power Supplies Semiconductors & ICs Sensors Sensors and actuators Vehicle acceleration
This Brief includes a Technical Support Package (TSP).
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Calculating Electrical Requirements for Direct Current Electric Actuators

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    This article first appeared in the February, 2020 issue of Aerospace & Defense Technology Magazine (Vol. 5 No. 1).

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    Overview

    The document titled "Calculating Electrical Requirements for Direct Current Electric Actuators" is a technical report that outlines methods for determining the electrical requirements associated with the use of electric actuators. Authored by Joshua Stapp and published in November 2017, the report is unclassified and approved for public release.

    The report begins with a summary that highlights the key topics covered, including the calculation of power source requirements, current draws, and the management of regenerative energy. It emphasizes the importance of understanding these factors to ensure the efficient operation of electric actuators in various applications.

    The introduction sets the stage for the discussion, explaining the significance of electric actuators in modern systems and the need for precise calculations to optimize their performance. The methods, assumptions, and procedures section details the approaches used in the calculations, providing a foundation for the subsequent analysis.

    Key sections of the report include fundamental equations relevant to the calculations, as well as a discussion of the Euler method, which is a numerical technique used for solving ordinary differential equations. This method is particularly useful in modeling the behavior of electric actuators under different conditions.

    The report also addresses power supply requirements, detailing how to determine the necessary specifications for the power sources that will drive the actuators. Additionally, it discusses approaches to handling regenerative energy, which is energy that can be recovered during the operation of the actuators, thus improving overall system efficiency.

    Conductor selection is another critical aspect covered in the report, as it impacts the performance and reliability of the electrical system. The results and discussions section presents findings from the calculations, illustrating how the equations are applied in practical scenarios.

    The report concludes with a summary of the key findings and recommendations for future work in the field. It emphasizes the need for continued research and development to enhance the efficiency and effectiveness of electric actuators.

    Overall, this document serves as a comprehensive guide for engineers and researchers involved in the design and implementation of electric actuators, providing essential information for optimizing their electrical requirements and improving system performance.

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