4-Rotor Flying Robot

The OU 4-rotor flying robot is based on the Silverlit XUFO. We have added an Atmel Mega8 microcontroller equipped with a Devantech compass. This microcontroller overrides the RF receiver of the XUFO and provides its own pulse-width-modulated control signal. Because we insert this signal immediately after the RF stage, the functionality of the XUFO gyroscope is left intact.

Physical Interface

From the Power Supply

We are using RadioShack 13V ~10A power supplies. These are sitting on the tables next to the helis (as many as two helis will be connected to a given power supply). For each heli, there are two connections:

Power to the craft itself
+5V regulated power for your off-board control circuit. Note that the regulators that we are using can become quite hot to the touch (but not hot enough to burn).

In both cases, ground is black and power is red.

To/From Heli

The heli has a pair of 3m length cables attached to it. The first cable provides power to the craft (as described above).

The second cable provides the bidirectional serial interface as a 2-pin connector. The pin assignments are as follows:

Orange: transmit from heli
Purple: send to heli

Note: this serial interface uses +5 and 0V to encode the serial data. If you are directly connecting the heli to a device that uses the same encoding (such as another Atmel mega8 microcontroller), then the connection can be made directly. However, if you are connecting to a standard RS232 interface (e.g., a DB9 serial port on a desktop/laptop computer), then you will need a level shifting circuit (see an example circuit).

Serial Protocol

Querying the Compass State

The host transmits:
c

The heli responds with:
cDDDD\n\r

Where DDDD is a 4 digit decimal number that gives the orientation of the craft in tenths of a degree. Note that "\n" is the character '\n', not the characters '\' and 'n'.

Commanding the Heli

Roll. The host transmits:
rDDD\n\r

Where DDD is a 1-3 digit decimal number that determines the current roll command. Values outside of the range 1-255 are ignored. A value of 128 corresponds to "no roll"; higher values correspond to positive roll (rotation about X in the figure).

Pitch. The host transmits:
pDDD\n\r

Where numbers larger than 128 correspond to positive pitch (rotation about Y) .

Yaw. The host transmits:
yDDD\n\r

Where numbers larger than 128 correspond to negative yaw (rotation about Z).

Throttle. The host transmits:
tDDD\n\r

Where the rotors will begin to spin around values of 80. Values in the vicinity of 140 are sufficient to hover.

Emergency. The host transmits:
e

This command immediately sets roll, pitch, and yaw to neutral, and throttle to low.

Debugging Commands

These commands are most useful when you have attached the serial interface to a terminal or a computer executing a terminal program (e.g., kermit or hyperterm).

The host transmits:
s

The heli responds with a list of the actual signals being presented to the heli (in units of 1/2 usec). For roll, pitch, and yaw, 2000 is neutral.

The host transmits:
S

The heli responds with a list of the current calibration potentiometer states (in units of 1/2 usec).

Powering On

Step 1: Main Power

Turn on the main power supply.
This will power the Atmel control circuit and your off-board circuit.
- LED 0 (red) will flash for a brief amount of time at power up. This LED will also flash as serial commands are received. However, if this LED is ever on persistently, this is indicative of a system error and should be reported immediately.

Step 2: Craft Power

If you are just starting: check to make sure that the calibration potentiometers (described in step 3 below) are in the center of their range.
Level the craft as best as possible (the current orientation will determine the "neutral orientation" over the next few steps). You can do this either by placing the craft properly on the ground or by holding it.
Turn on the craft power (see image below). This is the switch located on the heli itself.
LED 1 (green or yellow) will flash continuously after this point.
You will hear the mechanical gyroscope spin up. Once this process is completed, you will see the LEDs mounted to each of the rotor motors flash in a circular pattern.
You may then issue commands to the heli. Note that if you issue a throttle command before the gyro is spun up, it will only take effect after the gyro is ready.
If the craft is ever pitched or rolled beyond a relatively narrow window (~ +/- 20 degrees), the gyro will spin down. In this case, or if you ever want to completely recalibrate the neutral orientation, then power down the craft and restart this sequence.

Step 3: Tuning the Neutral Position (Optional)

Once the gyro has been spun up, you have the option of turning the neutral position for the pitch, roll, and yaw dimensions. By "neutral position", we mean zero pitch and roll (and hence, no tendency to translate) and zero yaw (no acceleration about the yaw dimension).

The procedure is as follows:

Send neutral commands via the serial interface for each of pitch, roll, and yaw (128 in all cases).
Holding onto the craft from its base, bring throttle to a level that produces a significant amount of thrust (~120 will work).
Using a small screw driver, you can then adjust the pitch, roll, and yaw potentiometers until all three are truly neutral.

Safety

Safety for the Human

Although the rotor blades are moving at a high rate of rotation, they have very little momentum. As a result, placing one's finger in the path will hurt, but not damage the finger. (Besides huring) since this can damage the craft, however, this is not suggested.

For those who will be holding onto the craft during testing (while the rotors are spinning), safety goggles will be provided.

Safety for the Heli

Although the crafts are rather robust to abuse, it is important to take steps to protect them. In general, it is bad to run the crafts against the ground or the ceiling.

Early in the software testing process, a team member should hold onto the bottom of the craft.
You can stabilize the craft by holding onto the power/serial cables. In particular, this is useful to ensure that the craft never goes too high. However, be ready for sudden altitude drops.
Be conservative with the throttle.
- Throttle commands between 70 and 140 should be sufficient for our purposes.
- Avoid changing the throttle suddenly. In particular, if you are automatically landing the craft, slowly reducing the throttle.
- Because of the power/serial cables, as the craft increases its altitude, it will carry more weight. This means that within a reasonable range of throttle commands, the height is self-regulating. In fact, the equilibrium height of the craft is monotonically related to the throttle command.
For our projects, you will not need to change the roll and pitch commands away from 128 (neutral).
Implement and test incrementally. Convince yourself that each piece is working properly before you go onto the next step.
If something unexpected happens, immediately cut the power at the power supply.
Do not leave the throttle up for more than 2 minutes at a time. Give the craft a break to allow the motors to cool (this will extend the life of our motors).

fagg [[at]] ou.edu

Last modified: Tue Mar 24 00:13:28 2009