 |
| |
 |
| |
 |
| |
 |
I was chatting with a group of customers the other day and
they were making a platform (a bare tank type chassis) to put
a camera on. This drove me to whip up a quick project RC
tank. It is just a pair of tracks with the parts needed to control
it as a tank. It could be made into a tank model, or used as the
'feet' of a robot. It is RC controlled, but could easily be driven
by a microcontroller using a 'servo' output command.
The Hitec Laser 4 radio we used lets us use channels 1 and 2
mixed so that the right hand joystick controls motion
intuitively (up for fwd, down for rev, left to pivot to left, right
to pivot to right). That leaves two channels to run a pan and
tilt camera, or open/close gripper with lift or rotation. So this
is a versatile platform to build a project on.
they were making a platform (a bare tank type chassis) to put
a camera on. This drove me to whip up a quick project RC
tank. It is just a pair of tracks with the parts needed to control
it as a tank. It could be made into a tank model, or used as the
'feet' of a robot. It is RC controlled, but could easily be driven
by a microcontroller using a 'servo' output command.
The Hitec Laser 4 radio we used lets us use channels 1 and 2
mixed so that the right hand joystick controls motion
intuitively (up for fwd, down for rev, left to pivot to left, right
to pivot to right). That leaves two channels to run a pan and
tilt camera, or open/close gripper with lift or rotation. So this
is a versatile platform to build a project on.
Improving on the Tamiya 'Tracked Vehicle Chassis'...
Introduction:
Tamiya makes a kit called the 'Tracked Vehicle Chassis' kit. It has a pair of rubber tank tracks, two sizes of
drive cogs, a pair of large idlers, several pairs of two smaller sizes of idlers, a base plate and a single
motor drive kit. Many table top robot hobbyists use this kit, taking the parts they use and putting the rest in
the junk drawer.
Although it is an interesting model, the limitation is that it only has a single drive motor, so it can go forward
and rev, but there is no way to steer. Our plan is to give it two drive motors, allowing differential drive.
So how to do that? After short consideration I decided to make it RC controlled, and the Hitec Laser 4 RC
control kit has two servos included. So using the two servos to drive the tank made perfect sense.
Construction:
Construction started with assembling the chassis as indicated by the instructions. We didn't bother with the
gear box and battery because we weren't going to use it. In the example photographed we used a Sozbots
expanded PVC replacement chassis, but the supplied wood one is fine.
Next we took the Hitec HS322 servos (included with the Laser 4 RC control) and modified them for
continuous rotation. This is a common robotics hack, and you can get instructions all over the net for
additional or alternate info.
To summarize, servos all work by comparing the input signal to a voltage ladder formed from a
potentiometer attached to the output shaft of the servo. To hack you need to disassemble the servo, and
change the way the servo board detects the shaft position. In one example, if the servo has a 5k pot you
can install two 2.2k, 2.4k, or 2.7k resistors. Electrically the servo control board will think the pot is always in
the center position. When you move the radio control joystick off center the servo will keep turning trying
to move the 'pot' to the new position. Alternately, depending on the physical facts of the servo you can
relocate the pot or cut off its shaft so that the rotation of the output shaft doesn't affect the pot.
For this project using the Hitec HS322 servos we chose the first case. To actually make the modification,
there are 3 steps.
1) Disassemble the servo (4 long screws). Desolder the servo control PCB from the motor and lift. Locate
where the pot connects to the control pcb and identify the end points and center point. Measure the pot
total resistance and choose your resistors to replace it (we used 2.7kohm resistors). Desolder the pot from
the pcb and install the resistors.
2) To make sure the output shaft could rotate 360 deg freely required trimming some plastic off of a gear
that is used to stop the rotation. In other cases (different model servos) one of the gears does not have
teeth all around the circumference. This would be a poor choice for modification unless you can get a
replacement gear from the manufacturer.
3) In the HS322 the pot is held in place by one screw. We removed the pot from the servo since it was not
needed to hold the output shaft in place. In other cases the potentiometer is actually part of the output
shaft or provides support for the output shaft. If so you will need to open the pot and trim a similar plastic
end stop that keeps the pot from rotating freely.
Next is getting the track drive cogs on the servos. In this case we trimmed away the center shaft of the
cog, and used two #2 screws to mount it to the round servo horn that comes with the HS322.
The servos on our example are held in place by a small 'strap' thermo-shaped out of a piece of ABS. This is
needed to keep the drive cogs/ servos from shifting, which would release the tension on the tracks.
The yellow ABS on top is like-wise thermo-shaped out of ABS. It is hinged on one end and double screwed
to the servo strap on the other. This give as secure, flat base for the next part of the project, to mount
something on top.
Set up is a little confusing. On the Hitec Laser 4 we used channels 1 and 2 for directional control. In order
to get the classic 'tank' control we used the 'ELEVON' mixing feature. Because the way we placed our
servos the input was set to 'REV' for both channels. If your tank isn't moving as expected, try switching the
servos between channels and switching the REV settings. Also, the tanks tended to move a little when
'stopped' which was fixed by adjusting the trim controls next to the joystick.
In this case we chose to put a pan and tilt camera system in a turret. This made use of the other two
channels of the radio system. A heavy duty servo was used as a pan-pivot for the turret which was formed
out of ABS like the chassis. A featherweight servo was as a tilt pivot. A 2.4GHz wireless security camera was
mounted to the tilt servo. This was an easy way to allow video and audio to be transmitted back to the
operator. It makes for a fun project that is all the better for being built from scratch.
Have fun!
Introduction:
Tamiya makes a kit called the 'Tracked Vehicle Chassis' kit. It has a pair of rubber tank tracks, two sizes of
drive cogs, a pair of large idlers, several pairs of two smaller sizes of idlers, a base plate and a single
motor drive kit. Many table top robot hobbyists use this kit, taking the parts they use and putting the rest in
the junk drawer.
Although it is an interesting model, the limitation is that it only has a single drive motor, so it can go forward
and rev, but there is no way to steer. Our plan is to give it two drive motors, allowing differential drive.
So how to do that? After short consideration I decided to make it RC controlled, and the Hitec Laser 4 RC
control kit has two servos included. So using the two servos to drive the tank made perfect sense.
Construction:
Construction started with assembling the chassis as indicated by the instructions. We didn't bother with the
gear box and battery because we weren't going to use it. In the example photographed we used a Sozbots
expanded PVC replacement chassis, but the supplied wood one is fine.
Next we took the Hitec HS322 servos (included with the Laser 4 RC control) and modified them for
continuous rotation. This is a common robotics hack, and you can get instructions all over the net for
additional or alternate info.
To summarize, servos all work by comparing the input signal to a voltage ladder formed from a
potentiometer attached to the output shaft of the servo. To hack you need to disassemble the servo, and
change the way the servo board detects the shaft position. In one example, if the servo has a 5k pot you
can install two 2.2k, 2.4k, or 2.7k resistors. Electrically the servo control board will think the pot is always in
the center position. When you move the radio control joystick off center the servo will keep turning trying
to move the 'pot' to the new position. Alternately, depending on the physical facts of the servo you can
relocate the pot or cut off its shaft so that the rotation of the output shaft doesn't affect the pot.
For this project using the Hitec HS322 servos we chose the first case. To actually make the modification,
there are 3 steps.
1) Disassemble the servo (4 long screws). Desolder the servo control PCB from the motor and lift. Locate
where the pot connects to the control pcb and identify the end points and center point. Measure the pot
total resistance and choose your resistors to replace it (we used 2.7kohm resistors). Desolder the pot from
the pcb and install the resistors.
2) To make sure the output shaft could rotate 360 deg freely required trimming some plastic off of a gear
that is used to stop the rotation. In other cases (different model servos) one of the gears does not have
teeth all around the circumference. This would be a poor choice for modification unless you can get a
replacement gear from the manufacturer.
3) In the HS322 the pot is held in place by one screw. We removed the pot from the servo since it was not
needed to hold the output shaft in place. In other cases the potentiometer is actually part of the output
shaft or provides support for the output shaft. If so you will need to open the pot and trim a similar plastic
end stop that keeps the pot from rotating freely.
Next is getting the track drive cogs on the servos. In this case we trimmed away the center shaft of the
cog, and used two #2 screws to mount it to the round servo horn that comes with the HS322.
The servos on our example are held in place by a small 'strap' thermo-shaped out of a piece of ABS. This is
needed to keep the drive cogs/ servos from shifting, which would release the tension on the tracks.
The yellow ABS on top is like-wise thermo-shaped out of ABS. It is hinged on one end and double screwed
to the servo strap on the other. This give as secure, flat base for the next part of the project, to mount
something on top.
Set up is a little confusing. On the Hitec Laser 4 we used channels 1 and 2 for directional control. In order
to get the classic 'tank' control we used the 'ELEVON' mixing feature. Because the way we placed our
servos the input was set to 'REV' for both channels. If your tank isn't moving as expected, try switching the
servos between channels and switching the REV settings. Also, the tanks tended to move a little when
'stopped' which was fixed by adjusting the trim controls next to the joystick.
In this case we chose to put a pan and tilt camera system in a turret. This made use of the other two
channels of the radio system. A heavy duty servo was used as a pan-pivot for the turret which was formed
out of ABS like the chassis. A featherweight servo was as a tilt pivot. A 2.4GHz wireless security camera was
mounted to the tilt servo. This was an easy way to allow video and audio to be transmitted back to the
operator. It makes for a fun project that is all the better for being built from scratch.
Have fun!
