Essential Components: Servos in Model Building
Servo stand as fundamental elements utilized across various branches of model building. Their appeal lies in their small size, light weight, affordability, and ease of control. In model building applications, servos directly interface with an RF receiver unit, requiring just three connections: positive supply (+5 V), ground (GND), and a control (Pulse) lead responsible for delivering the control signal to move the servo arm.
Control Mechanism and Signal Processing
The control signal transmitted to the servo arm is in the form of pulse width modulation (PWM). These pulses, supplied by the receiver, determine the movement of the servo arm. A positive pulse lasting 1 ms directs the servo arm to one end of its range. While 2 ms pulses shift the arm entirely in the opposite direction. Pulse widths falling between these extremes position the arm at intermediate points, proportionate to the pulse width. A pulse width of 1.5 ms centers the arm. Notably, the pulse repetition rate stands at approximately 20 ms, equating to 50 Hz. While this rate plays a role, it isn’t overly critical in the functioning of the servo system.
If you have concerns about the model’s performance, potential issues might stem from the remote control transmitter, receiver, or a servo motor. This convenient tool enables you to swiftly assess the servo’s functionality and determine if it’s a factor in your investigation. The pulse generator design depicted in Figure 1 represents a fundamental circuit familiar to nearly all engineers.
A Two timing circuit
The pulse generator consists of a NE556 dual timer chip, where the output pulse width is adjusted using a potentiometer. The repetition rate of the pulse is determined by the combination of resistor R1 and capacitor C2 in timer 1 of the NE556. The output signal at pin 5 of this timer has an almost symmetrical mark-space ratio. The descending edge of this signal triggers the second timer via C3, generating a positive output pulse at pin 9. The width of this pulse is controlled by capacitor C4 and the combined resistance of R3 and P1. Potentiometer P1 allows precise adjustment of the pulse width. Through experimentation, it was observed that the circuit, with the specified component values, produced a pulse width ranging from 0.5 to 2.6 ms, effectively covering the standard pulse width range used by these types of servomotors.
Servo Safety Precautions: Setting the Control Knob
To prevent potential damage to the connected servo, it’s crucial not to turn P1 fully to either end of its travel. Doing so might cause the servo to overshoot its intended end position and collide with mechanical stops. Before powering up the circuit, it’s advisable to ensure that the control knob P1 is roughly in the middle position, minimizing the risk of such issues.
Optimizing Pulse Repetition Rate and Power Supply
The circuit’s pulse repetition rate has been determined to be 18 ms. Most servos operate within a voltage range of 4.8 to 6 V. In this setup, the operating voltage falls between 5 to 6 V. A range easily supplied by four AA primary cells or rechargeable batteries. Maintaining this voltage range is essential for proper servo operation and longevity.
Streamlining Construction: Neat Design with a Dedicated PCB
To simplify construction and ensure a tidy result, a printed circuit board (PCB) has been designed for this setup (refer to Figure 2). The PCB, available from the Elektor Shop , features standard (non-Surface Mount Device) component outlines. This design choice facilitates easy component fitting, eliminating potential challenges during assembly.