Turnout/Signal operation by Model
Aircraft Servo
This
page updated 10/5/07.
Programmable servo
driver type ‘Servo4’
For
point and signal operation.
The use or
misuse for the sale, design, supply, process, installation, delivery,
test, repair, servicing, and alteration of the designs, instructions,
computer code, photographs, circuits, and any other information that
appear on this web site is entirely at the risk of the user or visitor
to the web site. The information is made available in good faith
without warranty of any kind strictly on the basis that no liability
will attach to the Model Electronic Railway Group (MERG), its officers
and members.
Introduction
Servo4 is a microprocessor controlled
driver board designed to operate four separate conventional servo motors. The board has
four standard servo connectors and provides the power supply and variable
width pulses to operate the servos. The power to the board can be either a DC
supply of 10 to 15v, either smoothed or unsmoothed, or an AC supply of between 9
and 12v RMS. The supply to the servos is a stabilised 5v and variations in the
input voltage will not affect the servo operation.
Each servo has two presettable
positions controlled by simple on / off switches or by a logic level of 0 or 5v. Not only can the
positions be individually set but the rate of travel is adjustable with the choice of a
different rate in each direction for each servo.
The unique feature of the Servo4
board is the way the adjustment is achieved. The board has a standard serial interface
(9600B) via a 9 way D type connector. The board can be set, adjusted and tested
through this serial interface using either a PC (a laptop is preferred for portability) or
a separate purpose built ‘setting’ box. The serial interface sends information to the
board without handshake and will operate equally well with a USB connection via a
USB to serial converter.
Once the required settings have been
established, they are saved in memory on the board. This gives very high reliability as
there is no possibility of accidental misadjustment, failure of variable
controls or interference from external sources. The
settings can always be changed though
using the PC interface or the setting box.
Servos are ideal for point mechanisms
as they have a high torque, a variable speed and endstop adjustment and consume
virtually no current when stationary.
Instructions for
use.
Mount the board using screws and
spacers in a convenient location to the servos, ensuring that the serial connector is
accessible. Connect the DC or AC supply to the large, two way connector ensuring correct
polarity if using DC. (It is protected against incorrect polarity but will only work with
it the right way round). Plug in the servos with the ‘black’ wire to the edge
of the board. A reversed connection here may damage either the servo or the driver. Connect
the control wires to the smaller terminal block as per the diagram.
Adjustment using the
‘setting’ box.
If starting ‘from
scratch’ it is best to connect one servo at a time. With the
power on, plug the setting
box into the 9 way connector with the cable supplied. Set the servo selector switch to 1, the function switch
to Off, the position control to its central position and press the Run button. (Green
light on). Plug a servo into connector 1. The servo should drive to its mid
position. Now rotate the position control and the servo should follow it. Set the position
you want for the condition where the control input is ‘off’ if using a
switch or a logic 5v if using electronic control. Use the step+ and step- buttons for fine adjustment.
Each press will move the servo by the smallest possible increment. When satisfied with
the setting, press the Save button.
Now turn the function switch to the
On position (logic 0) and repeat the setting for the opposite end of the travel. ( Run, adjust
and Save).
To set the travel speed, turn
Function to S1. Set the Position and Speed knob to a number. 0 is no speed control and the
servo runs at its maximum. 1 is the slowest speed gradually increasing to speed step
7. Press Run then Save. Repeat for S2 which is the speed in the other direction.
Now set the other servos. If you set
the box to the correct servo number, the position to centre and the run to on, when you plug
in the servo it will automatically centre itself. This is useful both for setting
the mechanism and preventing strain if the servo runs to one end.
Provided the run light is off, the
servos will be controlled by the input states, even though the box is connected. This way you
can test the settings and then readjust if necessary. You can also test settings with
the Run on without saving them. Just press the Reset button and the previous
settings will be restored.
When the servos are set correctly,
unplug the setting box. There is no need to remove the power from the driver boards.
Also the setting box can be plugged in with the power on.
Setting using a laptop.
To install the Servoset programme unzip the software file putting all three files into a common directory then run 'setup.exe'.
The process is identical to that for
the setting box. Start the Servoset program. The correct serial COM port must entered in
the prompt box. Instead of physical controls there are buttons and a slider on the
screen. Plug the serial cable into the Servo4 board. Select the servo number and
function on the button array. Set the slider to the mid position. Click on Run. Move the
slider to set the servo position. Click on the slider ends for fine adjustment. The
position number range is displayed. The range is 0 to 255 with 127 being the centre.
Note: (5/8/06)
Paul King had flagged a minor problem with the existing Servo4
code. During the setting process, other servos, if fitted, also moved
although their actual settings were not changed. I had not noticed this
myself as I never had more than one servo. I have identified why
and the correction to the code was very simple. To correct this use the
revised asm and hex code, now at Rev E.
For speed setting, select a slider
number from 0 to 7. Settings above 7 will be full speed.
Downloads available .zip file contains this description plus drawings,
schematics, board layout and PIC code.
zip file contains the software for setting parameters from PC
(NB, Software zip file
updated 05/08/2006, PIC code updated 16/03/2006)
Mike Bolton
20/12/04
Tel. 0161 969 6630