Servos Explained

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Servomechanisms, commonly known as servos, are integral parts of various systems that date back further than most realize. In essence, a servo encompasses any motorized assembly equipped with an embedded feedback component. One can find servos in numerous applications, including robust machinery, vehicle power steering, robotics, and an extensive range of electronic devices.

In this article, we will discuss the three primary sizes of servos and provide a few straightforward examples that showcase their capabilities.

Understanding servo operation

Upon examining a typical hobby servo motor, you'll identify three fundamental elements: a direct current (DC) motor, a controlling circuit, and either a potentiometer or an alternative feedback system. The DC motor is linked to a gearbox, which increases output speed and torque. As the DC motor operates, it drives the output shaft, while the controller circuit decodes the signals received and the potentiometer functions to provide feedback on the output shaft's position. Most hobby servos utilize a standard three-pin, 0.1"-spaced connector for power and control, with pin color coding varying amongst brands. Nevertheless, they generally maintain a consistent arrangement. By consolidating these components, you can manage the servo's output shaft's direction, speed, and position using a mere trio of wires.

An illustration of a standard hobby servo's internals

Servo control methods

To maneuver a servo along its designated motion arc or, in the instance of continuous rotation servos, to regulate the motor's speed and direction, the controller must transmit a precisely timed signal for the servo to decipher. Most hobby servos anticipate a pulse approximately every 20 milliseconds, with the signal's width determining its position. Typically, these widths range between one and two milliseconds. This signal control method is commonly known as Pulse Width Modulation (PWM). A servo controller usually consists of dedicated hardware capable of accepting inputs from other components, such as joysticks, potentiometers, or sensor feedback, in order to establish the control signal for the servo. Alternately, the PWM-compatible pins on a microcontroller can be employed to relay that information directly to the servo.

Example of a servo being manipulated by a SparkFun 9DoF IMU Breakout

Supplying energy to a servo

The input voltage required for your servo varies depending on its size and torque capabilities; however, most hobby servos operate seamlessly on a 5V supply from your selected microcontroller or battery setup. While voltage is important, the current draw from a servo during operation, particularly when a load is present, holds greater significance. For instance, a typical unloaded hobby servo may draw as little as 10mA. Conversely, larger servos under load may consume over an amp. Therefore, it is crucial to review the servo's specifications to ensure that your power supply can deliver the appropriate voltage and sufficient current to effectively move the servo when a load is applied.

Examining a practical example:

Key power specifications extracted from the Hitec HS-422 datasheet

The Hitec HS-422 (Standard Size) servo specifications indicate a Test Voltage or Operating Voltage range of 4.8V to 6V. At 4.8V, the servo achieves a 60-degree movement in 0.21 seconds without any force limitations (load). Operating at the same voltage allows a load of up to 3.3kg/cm (Stall Torque). When running at 6V, the servo moves 60 degrees in 0.16 seconds with no load and demonstrates a higher stall torque limit of 4.1kg/cm. It’s pivotal to note that measuring between no load and stall torque will likely result in a decreased operating speed.

Insights Before Purchasing a Servo Motor Joint

When considering a servo motor joint for your applications, keep in mind the following five vital factors:

1. Understanding of Servo Types and Uses

Servos come in various styles and sizes, from standard hobby servos to more specialized models for industrial and robotic applications. Each type serves different purposes, so getting familiar with their characteristics will influence your selection.

2. Quality of Control Systems

Assess the controller capabilities since precise signal timing and feedback are critical for effective servo operation. Understanding PWM and how controls interface with the motor can help ensure satisfactory performance.

3. Specifications and Power Requirements

Always verify the voltage and current specifications needed for optimal servo function. Matching your power supply to the servo's requirements avoids compatibility issues.

4. Torque and Load Capacity

Understanding the torque output required for the specific loads in your application is essential. Higher torque capabilities may be necessary to accommodate various tasks without overloading the servo.

5. Supplier Reputation and Support

Purchasing from a reputable supplier like ARCSEC DRIVE ensures you have access to quality products and support. Suppliers should provide clear datasheets and have a good record for reliability and performance.

For further details, explore servo motor joint.