Some other Servo
Modification Instructions
The HS-300 servo
is the most difficult servo to modify. Its design makes it a challenge
to modify. If you have not yet purchased your servos please look into
using another servo.
!!! Important !!!
The design of
this servo prevents you from easily adjusting the servo's potentiometer.
This means that the software must be modified to calibrate to the servos
neutral position. This goes against the current trend in robotics to
allow you to adjust the servos potentiometer.
One of the simplest bots
you can make is made by converting a couple of servos to free running mode.
You get a small motor, gearbox and speed controller all in one package.
First lets talk a bit
about how a servo works.
Most servos are
controlled by a series of pulses. The width of the high state pulse is
what determines the actual position of the servo.
Generally a pulse of 1ms
will place the servo at one end of its rotation (see figure 1) and 2ms will
place it on the other end (see figure 2)
Figure 1
Figure 2
I say generally because
I found that the range can be any where from .2ms to 3ms depending on the
servo manufacture and model. Again generally when the pulse width is
1.5ms the servo will reach the half way point in its swing.
There is a small
potentiometer inside the servo that rotates with the main shaft. This
is what tells the electronics in the servo where it is at any given time.
When we convert a servo to free running mode we disconnect the potentiometer
from the shaft and set it at its center position. We also remove any
mechanical stops that may keep the servo from turning completely around.
Once the servo can
rotate freely we can find the center spot by slowly adjusting the pulse
width. I call this the neutral position. Now by adjusting the pulse
width greater than the neutral position we can move the shaft in one
direction. By adjusting the pulse width less than the neutral position
we can move the shaft in the opposite direction.
Most servos were
designed so that when it gets closer to its allocated position it slows
down. This will allow us to adjust the speed of the shaft by
varying the amount that deviates from the neutral position.
Modifying the Servo
Note that the following
modifications will void your servo warranty.
There are two ways to
convert this servo. You can open up the servo and disconnect and
remove the potentiometer. You then must solder two 2.2 resistors in
place. Even after all that you need to take the poteniomiter
apart and remove some detents. I feel this is too much work and can be very error prone.
In the following steps
we will work only on the main shaft. You never have to open up the
guts of the servo. Also By purchasing a new gear set you can restore
the servo back to normal operation if you decide to do so later.
| Step 1
Remove the 4 screws on the back of
the servo.
Note that its not necessary to
remove the back of the servo.
Remove any servo wheels or arms
that may be attached to the main shaft. |
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| Step 2
Remove the top of the servo by
lifting.
Note that the two pins shown
may stay attached to the top of the servo. If they do remove
them and insert into position as shown.
|
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| Step 3
A: Remove the intermediate gear by
just lifting.
B: Remove the main shaft by
lifting.
The main shaft also has a nylon
bushing. Remove it from the main shaft. |
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| Step 4
With your fingers or some small
pliers place the potentiometer shaft into the center position.
The position shown seems to be the
center pos on all the servos that I have converted. Just rotate
it to the left detent and right detent to make sure.
Place a very small amount of super
glue around the potentiometer shaft where the shaft comes in contact
with the mounting screw.
Be very careful not to get any
flue on the other gears as this will ruin them. Let the glue
dry.
|
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| Step 5
There is a small stop that must be
removed from the main shaft. I found that using small wire
cutters and then cleaning up with and exact knife works the best. |
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| Step 7
The hole under the main shaft must
be rounded and enlarged. This is what comes in contact with the
potentiometer.
Use a 9/64" drill bit held with
your hand or pliers and insert it into the hole and ream out the
slots. Once you have rounded the hole use a 5/32" bit to
enlarge the hole.
Warning !! do not use a drill.
You will not be able to hold the shaft in place and will cause damage
if you try.
Tip
If you insert the drill bit
into a drill (no power) it make it easier to turn the bit.
|
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| Step 8
Insert the shaft into the top of
the servo case as shown and test that you can rotate the shaft
completely.
Tip
Assemble the servo without the
intermediate gear and rotate the main shaft by hand. It should
rotate freely with out catching. Also examine the potentiometer
shaft and make sure it did not move.
|
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| Step 9
A: Place the main shaft back in
place.
B: Place the intermediate gear
back in place.
C: Place the nylon bushing on
the shaft.
D: Replace the servo top.
E: Reattach the 4 rear screws.
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