DC Servo Motor Basics

A Servo Motor is a small device that has an output shaft which can be positioned to specific angular positions by sending the servo a Pulse Coded Modulation signal. As the coded signal changes, the angular position of the shaft changes. DC servo motors are used in radio controlled airplanes, radio controlled cars, robots and a host of other applications that one can think of. A picture of a servo motor is as shown below.

Though the servo is small in size, it has a printed circuit board with control circuit built in and a standard servo manufactured by Futaba is model S3003. The power consumed is proportional to the mechanical load, thus saving energy when it is used in a varying type of load. The servo motor consist of a motor, gears and its casing. Three wires are used to interface to other control circuitry which are +5V DC, Ground and Control Signal.

It is using a control called proportional control of which the amount of power applied to the motor is proportional to the distance it needs to travel. This means that if the shaft needs to turn a large distance, the motor will run at higher speed. Usually a servo is used to control an angular motion of between 0 and 180 degrees.

The servo expects to see a pulse every 20 milliseconds (.02 seconds). The length of the pulse will determine how far the motor turns. A 1.5 millisecond pulse, for example, will make the motor turn to the 90 degree position (often called the neutral position). If the pulse is shorter than 1.5 ms, then the motor will turn the shaft to closer to 0 degress. If the pulse is longer than 1.5ms, the shaft turns closer to 180 degress.

DC Servo Motor Driver Circuit Description

The input signals are between 0 - 5V delivered by connecting up the 10K potentiometers as voltage dividers. The Microchip PIC 16C71 has an AD converter that changes the voltage signal into the Pulse Code Modulation system used by the servo motors. This signal is a 5V pulse between 1 and 2 msec long repeated 50 times per second. The width of the pulse determines the position of the server. Most servos will move to the center of their travel when they receive a 1.5msec pulse. One extreme of motion generally equates to a pulse width of 1.0msec; the other extreme to 2.0msec with a smooth variation throughout the range, and neutral at 1.5msec.

It will be a good experience to experiment the control of servo motors in this project by doing your own software programming using PIC 16C71 microcontroller.

 

 

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Simple DC Motor Driver

This simple DC motor driver circuit uses a 741 operational amplifier operating as a voltage follower where its non inverting input is connected to the speed and rotation direction of a potentiometer VR1. When VR1 is at mid position, the op-amp output is near zero and both Q1 and Q2 is OFF.

When VR1 is turned towards the positive supply side, the output will go positive voltage and Q1 will supply the current to the motor and Q2 will be OFF. When VR1 is turned to the negative supply side, the op-amp output switches to the negative voltage and Q1 will turn OFF and Q2 ON which reverses the rotation of the motor's direction.

As the potentiometer VR1 is moved toward either end, the speed increases in whichever direction it is turning.

The TIP3055 Q1 NPN power transistor has a collector current specs of 15A and VCE0 of 60V DC.

The MJE34 Q2 PNP power transistor has a collector current specs of 10A and VCE0 of 40V DC.

Parts List

Source : Extracted from Popular Electronics Nov 1997, By Charles D. Rakes

2V to 25V Power Supply Schematic

This project uses a LM338 adjustable 3 terminal regulator to supply a current of up to 5A over a variable output voltage of 2V to 25V DC. It will come in handy to power up many electronic circuits when you are assembling or building any electronic devices. The schematic and parts list are designed for a power supply input of 240VAC. Change the ratings of the components if 110VAC power supply input is required.

As shown in the figure above, the mains input is applied to the circuit through fuse F1. The fuse will blow if a current greater than 8A is applied to the system. Varistor V1 is used to clamp down any surge of voltage from the mains to protect the components from breakdown. Transformer T1 is used to step down the incoming voltage to 24V AC where it is rectified by the four diodes D1, D2, D3 and D4. Electrolytic capacitor E1 is used to smoothen the ripple of the rectified DC voltage.

Diodes D5 and D6 are used as a protection devices to prevent capacitors E2 and E3 from discharging through low current points into the regulator. Capacitor C1 is used to bypass high frequency component from the circuit. Ensure that a large heat sinkis mounted to LM338 to transfer the heat generated to the atmosphere.

Parts List

 

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