Rigel Construction + Operation

Return to Rigel Page. Rigel - OOBOT40 Construction + Operation Info

[Rigel side view]
This page includes some additional information, besides that which already comes with the Rigel - OOBOT40-II Bundles, regards setting up and operating the Rigel Robot Base from Budget Robotics, and controlling it with the OOBOT40-II Controller Board.

The Rigel platform consists of about 200 parts which need mechanical assembly, and the OOBOT40-II controller has about 80 parts, which can be purchased either assembled or in parts kit form (which will require soldering assembly). The Bundles also include additional parts, which makes bringing the robot on-line easier, such as battery holders pre-wired especially for the OOBOT40 controller, and various mounting components. RGL-KIT2 requires no soldering or electrical assembly, rather only mechanical assembly plus mating a few connectors.

Batteries are not included with the Bundles. They require (1 ea) 9v battery and (4 ea) AA alkaline, or else (5 ea) AA NiCad or NiMh. We recommend using NiMh as the best price-performance choice. Alkaline will run down and need replacement, while NiMh can be recharged many times, and have much less "memory effect" compared to NiCad.

<| Sample Program Code

Sample OOPic code for using the OOBOT40-II Controller Board to control the Rigel Platform can be downloaded here:

rigel1.zip - sets up and runs the 4 Rigel drive servos forwards and backwards (1 Kbytes - written 07-Dec-2003).

<| Physical Arrangements

The Rigel Robotic Platform consists of (2 ea) decks, (4 ea) servos pre-modified for continuous rotation, (4 ea) wheels, plus about 150 screws, washers, and nuts. It takes about 1 hour construction time, following the documentation supplied from Budget Robotics. Nominally, the wide end is considered to be the "front" end.

<| Controller + Battery Placement

The Rigel base comes will a number of holes pre-drilled to aid assembly. This includes holes to mount the servos, connect the 2 decks together, plus holes in the decks for running wires. There are no holes specifically drilled to hold the OOBOT40-II controller board, nor for the batteries. This way, customers have maximum flexibility in choosing where to mount these components, as well as sensors and other devices, on the platform. Everyone will have different ideas about configuration.

The diagrams at the right show two possible arrangements. Here the batteries are assumed to be 'AA' cells, although much larger 'C' cells can also be used. Typically, from 4 to 6 batteries will need to be wired in series, as described in the section below.

On the top diagram at right, both controller board and 4 to 6 'AA' cells are mounted on top the lower deck, leaving the upper deck completely free for other devices. This arrangement has space around the periphery of the lower deck to mount light sensors and bumper switches, and other small devices. These can be mounted on both the top and bottom of the lower deck, using existing holes, epoxy or other glue, or by drilling additional drills.

The bottom diagram at right shows the preferred mounting arrangement, consistent with the battery holders supplied with the Rigel - OOBOT40-II Bundles. A holder with (4) 'AA' cells is mounted beneath the lower deck in the space between the deck and the servos, since the servos make a good "ledge" upon which the holder fits. A fifth 'AA' cell, if used, is mounted on top of the lower deck. This arrangement leaves considerable space free on top the lower deck for other devices, plus the upper deck of Rigel is still completely free. In addition, another advantage of this arrangement is that the proto area of the OOBOT40-II board is oriented close to the "front" of the base, where several sensors will no doubt be mounted.

Hold-downs. The Kits provide Velcro for holding the batteries in place, and standoffs plus screws for the controller board. Holes can be drilled for the screws, once final placement is decided upon, or else the screws can be held to the board with glue or epoxy. A pad of reusable putty-like glue is provided for temporarily holding the controller and various sensors in place during initial stages of construction. For "permanent" attachment, we prefer to use Goop, which is a pliable adhesive that holds well but can be removed without undue trouble. It works better than hot glue.

<| Battery Notes

[4-cell]

[1-cell]

AA battery holders

The Rigel Kits come with holders for AA batteries. C cells can also be used, but they are much larger and much more expensive than AA cells. Rechargeable AA NiMh batteries are currently available in the $3/ea price range with 1800-2100 mAh capacity, which will run Rigel for an hour or more.

If alkaline AA batteries are used, then the 4-cell battery holder should be used, which will give nominal 4 * 1.5v = 6 vdc.
If rechargeable NiCad or NiMh batteries are used, then the 1-cell battery holder should be wired in series with the 4-cell holder, to give nominal 5 * 1.2v = 6 vdc. Note that using only 4 rechargeable batteries in series will only provide 4.8 vdc, which is at the low end of usability for the servo motors [see below].
It is possible to operate some R/C servos from 7 vdc or more, but this is outside the normal specification range, and may damage the units.

--- NOTE ---
When inserting batteries, be sure that batteries are inserted to match the polarity diagrams on the holders, otherwise the voltages will not be correct. A voltmeter should be used to measure the battery pack output before connecting to the robot platform.
Always double-check for correct polarity when connecting a battery pack to the controller board.

Servo Power

For motive power, the Rigel base uses 4 standard R/C servos which have been modifed for continuous rotation. Such devices are fairly critical regarding voltage levels used to power them. 4.8 - 6.0 vdc is specified as the typical operating range. With power below 4.8v, servos will show jerky and flakey operation, and voltages above 6 or 7 vdc may damage the servos.

Low-battery indication - with normal servo operation, it's not unusual that the servo arms will suddenly whip over to one side and lock in place after being in operation for some time. This is usually an indication that the battery pack voltage has dropped blelow 4.8 vdc. The batteries still have some power available, but it's so low that the servo feedback-control loops stop operating properly. Likewise, with servos modified for continuous rotation, it's possible that servo operation will go flakey when the battery voltage drops too low.

The following are several ways to provide operating power for the R/C servos:

4 ea alkaline batteries in series, giving 4 * 1.5v = 6 vdc.
5 ea NiCad or NiMH batteries in series, giving 5 * 1.2v = 6 vdc.
6 ea NiCad or NiMH batteries in series with a regular silicon diode (used to drop the voltage down by 1v), giving (6 * 1.2v) - 1v = 6.2 vdc.
a 5 - 6v high-current regulator powered off a 8.4v or 9.6v rechargeable battery pack used with R/C cars or airplanes.

The OOBOT40-II controller board as shipped is wired to implement options 1 or 2 above. A 1-cell 'AA' holder is provided which can be wired in series with the 4-cell holder.

Option 3, for rechargeable batteries, is mentioned because 4- and 6-cell battery packs are very common, whereas 5-cell packs are not. In this case, a diode is wired in series in the battery (+) lead to "drop" the voltage by 1v - the diode anode is wired towards the battery side and cathode towards the controller side. Option 4 will allow the servos to be run off common 7- or 8-cell R/C rechargeable packs. In this case, voltage regulator REG2 can be installed on the OOBOT40-II controller board, per the instructions included with the Rigel Bundle.

For most economical operation of the Rigel, it is recommended that rechargeable NiMH AA or C cells of a high amp-hr rating be used. NiMH AA cells now come with 1800 - 2100 mAH capacity, which will drive Rigel for over 1 hour. C cells hold more capacity, and will allow the robot to run longer yet.

<| Servo Operation

Standard Servos.

servos are controlled by sending them 1.0 - 2.0 msec long positive-going 5v pulses, typically once every 20 msec or so. This will move the servo arm through nominal 90 degrees of travel. 1.5 msec is considered to be the "center" position.

most servos will respond to pulse signals as low as 0.7 msec and high as 2.3 msec or so, which increases servo arm travel to close to 180 degrees for most servos.

NOTE - the geartrains inside standard servos can be damaged if the control pulses are too short or too long. These pulses will drive the servo over hard into its mechanical stops. Such gear clash is easy to hear.

---- every servo hits its stops at a slightly different control pulsewidth.

Continuous-Rotation Servos.

for servos modifed for continuous rotation, 1.5 msec control pulses will stop or "almost" stop the rotation. However, the "potentiometers" inside modified servos may need to be periodically adjusted to center the servos. The Rigel documentation describes how to do this.

control pulsewidths less than 1.5 msec will cause the modified servo to rotate in one direction, and pulsewidths greater than 1.5 msec will cause opposite direction rotation.

the farther the control pulsewidth is from 1.5 msec, the faster will be the rotation.

typically, modified servos will reach their maximum speed at some pulsewidth near the normal limits, ie 1.0 and 2.0 msec.

[Rigel bottom view]
Mirror-Image Servo Arrangement.

note that the servos on the 2 sides of the base are physically mirror-images of each other. Therefore, when sent the exact same control pulses, eg 1.4 msec, the wheels on one side will rotate in the opposite direction to those on the other side. Therefore, it's necessary to take this into account when writing control software to operate the servos.

---- for example, pulsewidths of 1.4 msec to the servos on one side of the robot, and 1.6 msec to the servos on opposite side, will produce rotation of about the same speed and drive the base in the same direction, forwards or backwards. It may be necessary to "tweak" the pulsewidths to get exactly the same rotation speeds out of all servos.

---- note that "flipping", or reversing, pulsewidths when flipping servos in a mirror-image fashion is a common practice used by R/C model airplane hobbyists - the "reverse" function switch on the R/C transmitter was added for this purpose.

These matters are discussed in the documentation included with the Bundles, and also in the sample software available on this website.

<| Additional Suggestions for Constructing the Rigel Base

When attaching the wheels to the servo horns, it is important that the screws not be over-tightened, else the holes might be stripped. It is fairly easy to know when the screws are properly tight, however. As the screws are turned in, the pressure will be rather constant as the screws proceed, and then will increase significantly as they seat. This can be easily gauged by moving the screws forwards and backwards a couple of times.

Once the servos are installed and the wires connected to the controller board, they can be wrapped in a coil beneath the lower deck and secured with a plastic bag tie. The wires should be secured and not stick out, else they may snag on obstacles being run over during operation of the platform.

The batteries and controller board can be mounted in several different positions and orientations on either lower or top deck. There are purposely no specific mounting holes in the decks for these components. This way the platform can be customized as desired by the customer. It is suggested that various configurations be tried out before permanently attaching the controller and batteries. A supply of sticky putty-like, reusable Elmer's adhesive is provided in the kits to hold components during initial testing.

Wheel positions.

there is no specific front or rear end on the Rigel platform. It will go in either forward or backward direction equally well, so which end is which is at the discretion of the user.

the Rigel documention shows the servos mounted so that both front and rear wheel axles are positioned towards each other, at about 2 5/8" [66 mm] axle spacing. This gives the base steering ability similar to the BobCat loader.

however, either front or back set of servos, or both, can also be flipped over during installation to lengthen the wheelbase. Because the Rigel base uses 4-WD and skid steering, different wheel arrangements will give the platform slightly different steering characteristics, and also different stability. With the wheels farther apart, the turning will be a little more jerkier when traveling over hard surfaces, but climbing stability will better.

with one set of servos flipped, the wheelbase lengthens to 3 3/8" [86 mm]. With both front and rear sets of servos flipped, it lengthens to 4 3/16" [106 mm].

it is also possible to not install the spacers which move the one set of servos farther away from the center of the Rigel base. All in all, there are several possibilities for mounting the servos to the base, which each producing slightly different driving characteristics.

<| TOP