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Servos are a critical component in your radio control model. The servos are used to
move the control surfaces and they receive position information from the receiver.
The servos connect to the receiver with a three-wire interface, ground, power,
and the position information. The servos typically run from a 4 cell nicad
battery pack, nominally 4.8 volts. The position information is a pulse stream
with the pulse width determining the servo position. 1.5 mSec is the center
position, 1.0 mS to 2.0 mS define the full range of motion of the servo. The
position pulse is sent to the servo every 25 mS, or at a 40 Hz rate. A timing
diagram for the servo signal is shown below:
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Servos are simple devices and consist of the following key components; a drive motor, a gear box designed to match the motor to the typical load of a control surface, a position feedback potentiometer connected to the servo output shaft, and the control electronics. Analog servos use a analog control integrated circuit to convert the input pulse information into drive signals to control the motor and hold the servo at the desired position. Servo position update information is send each time a input position pulse is detected, at 40Hz.
Digital servos replace the analog control electronics with a microcontroller to perform the servo position control. This greatly improves the servo performance. The most common digital servo trick is to send servo position information at a higher rate than the typical 40 Hz rate used in analog servos. This increases the servo response time. Advanced control algorithms allow digital servos to have incredible centering ability and holding power.
The PositionStar is a digital servo amplifier that you can install in almost any commercial servo to convert it to a modern digital servo. This servo amplifier has all of the features of modern digital servos plus many unique features never seen before. These servo amplifiers will be of special interest to the robotic community or anyone wishing to use servos under computer control.
The PositionStar can be connected to your PC using a very simple interface cable.
This cable can be built for less that $20 in parts. With the interface cable
the servo can be connected to your PC and using a terminal emulator program
like Hyper Terminal you can control and configure your servo. The servo understands
over 30 commands that allow you to "tune it for best performance in your application,
or use the servo under computer control for robotics applications. Each servo
can be given a unique address allowing several servos to be connected to one
computer interface in a multi-drop configuration. Below is an example connection
of two PositionStar servos connected to a
PC interface cable:
The PositionStar is a very simple hardware design and easy to assemble. The circuit consists of less that 20 components than solder to the top and bottom of a small circuit board. The board will fit in most standard and small servos, some of the micro-servos will not accept the PositionStar due to its size.
The PositionStar servo amplifier has four modes of operation. Each mode and its
features will be discussed in the following sections. The mode can be defined
using the MD command discussed in the Command Set section of this document.
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Servo Amplifier Installation
 Amplifier installed in Hitec HS-425BB |
Status
The PositionStar servos are very close to compete, however I don't expect to release this until I finish the receiver. About all that I need to do is documentation and testing. The target cost for the servo amplifier is under $20. Please let me know your interest in this project, your level of interest could change my priorities.
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Copyright
© 2004,2005,2006 GAA
Custom Electronics.
All rights reserved.
Revised:
11/24/07
.