Tuesday, February 21, 2012

Servo Motor

What are Servo Motors?
Servo refers to an error sensing feedback control which is used to correct the performance of a system. Servo or RC Servo Motors are DC motors equipped with a servo mechanism for precise control of angular position. The RC servo motors usually have a rotation limit from 90° to 180°. Some servos also have rotation limit of 360° or more. But servos do not rotate continually. Their rotation is restricted in between the fixed angles.

Where are Servos used?
The Servos are used for precision positioning. They are used in robotic arms and legs, sensor scanners and in RC toys like RC helicopter, airplanes and cars.

Servo Motor manufacturers:
There are four major manufacturers of servo motors: Futaba, Hitec, Airtronics and JR radios. Futaba and Hitec servos have nowadays dominated the market. Their servos are same except some interfacing differences like the wire colors, connector type, spline etc.

Servo Motor wiring:
The Servo Motors come with three wires or leads. Two of these wires are to provide ground and positive supply to the servo DC motor. The third wire is for the control signal. These wires of a servo motor are color coded. The red wire is the DC supply lead and must be connected to a DC voltage supply in the range of 4.8 V to 6V. The black wire is to provide ground. The color for the third wire (to provide control signal) varies for different manufacturers. It can be yellow (in case of Hitec), white (in case of Futaba), brown etc

Servo Control
The servo motor can be moved to a desired angular position by sending PWM (pulse width modulated) signals on the control wire. The servo understands the language of pulse position modulation. A pulse of width varying from 1 millisecond to 2 milliseconds in a repeated time frame is sent to the servo for around 50 times in a second. The width of the pulse determines the angular position.
For example, a pulse of 1 millisecond moves the servo towards 0°, while a 2 milliseconds wide pulse would take it to 180°. The pulse width for in between angular positions can be interpolated accordingly. Thus a pulse of width 1.5 milliseconds will shift the servo to 90°.
It must be noted that these values are only the approximations. The actual behavior of the servos differs based on their manufacturer.
A sequence of such pulses (50 in one second) is required to be passed to the servo to sustain a particular angular position. When the servo receives a pulse, it can retain the corresponding angular position for next 20 milliseconds. So a pulse in every 20 millisecond time frame must be fed to the servo.

Power supply for Servo
The servo requires a DC supply of 4.8 V to 6 V. For a specific servo, its voltage rating is given as one of its specification by the manufacturer. The DC supply can be given through a battery or a regulator. The battery voltage must be closer to the operating voltage of the servo. This will reduce the wastage of power as thermal radiation. A switched regulator can be used as the supply for better power efficiency.

DX Fun Web Cluster

DX Fun Web Cluster is in essence a "chatroom" or node into which amateur radio DXers can post information about DX either worked or heard. Physically, it is a central computer that collects, stores and disseminates information that ham radio operators send to it. There are thousands of nodes around the world, connected together via the internet or radio. Connection to a cluster is by either telnet or packet radio. The networked nature of DX clusters is perhaps its most powerful feature as it gives amateurs almost instant access to information about rare and unusual DX conditions such as meteor scatter or VHF ducting.

Cluster users will use computer software (for example, many logging programs have this capability) that is capable of communicating with the cluster. On initial login, users will be asked to provide station information. This allows other ham radio operators to compare DX openings to their physical position. DX is one of the most fascinating aspects of the hobby.