于仁颇黎@机器人

Anumber of inexpensive microhelicopters are now available to RC modelhobbyists: Ikarus Piccolo, MS Hornet, Carboon, Dragonfly, Hummingbird,Tiny, Aerohawk. These are essentially scaled-down versions of regularmodel helicopters, made possible by advances in battery technology.Some models have a rotor head with fixed collective pitch (FP), whileothers have both cyclic and collective pitch (CP). Most have adedicated tail motor rather than a variable-pitch tail rotor.

Arecent radical innovation is the "ProxFlyer" self-stabilizingdeformable rotor design. Unfortunately, current commercialimplementations are too small to be used as robotic platforms. Besides,stability is obtained by sacrificing maneuverability.

For this project we use a commercial ARF microelicopter kit containing:

The aircraft weighs 270 g and can lift at least 50 g of payload.

Theelectronics package is a "black box" which connects all componentstogether. This is in contrast with larger model helicopters, where theconnections between the receiver, gyro, BEC and motor drivers areexposed and documented. Integrating all these components reduces size,weight and cost, but makes modifications harder.

Fortunately,in some commercial microhelicopters, the "black box" can be tinkeredwith fairly easily. It actually consists of two boards connectedback-to-back with a 2×3-pin connector (see Figure 1):

Figure 1. Contents of the integrated controller in a typical commercial microhelicopter kit.

Table 1 lists the PWM outputs, two of which are routed internally between the two boards.

Any similar helicopter can be used. The main requirements are:

We use a Gumstix single-board computer with the following features:

Some features are not used:

The power/gyro board has two +5 V voltage regulators labelled CX1117-5.0 and mounted in parallel. Idle current is 50 mA.

Sincethis +5 V rail is exposed on each servo connector (including the unusedones), the Gumstix can be conveniently powered from it. In order toaccomodate the extra current (100-200 mA total), it might be necessaryto replace the two voltage regulators with a larger, externally-mounted7805.

In order control the helicopter, we modify the RC receiver board so that the Gumstix can inject its own PWM signal into the PWM demultiplexer.

Why not get rid of the receiver board entirely and generate five PWM signals with the Gumstix ? The proposed approach has several advantages:

Figure 2 shows details of a typical RC receiver board and the modification. Simply cut the correct PCB trace(s) and connect both ends to a 2-pin connector outside of the plastic enclosure. Chronograms should help locate the signals.

Figure 2. RC receiver board details and modifications (in red).

The original functionality can be restored by disconnecting the Gumstix and inserting a jumper.

Xbox controllers are widely available, cheap, and have well-defined functionality (unlike PC joysticks). They can be connected to a PC by replacing the proprietary connector with a USB plug.

Alternatively, any USB joystick (a.k.a. joypad or gamepad) with two dual-axis analog sticks can be used. In this case the mixer configuration file must be adjusted to match the layout of the axes (see Section 4.3).

pxa_mpwm.ko is a timer-based multi-channel PWM signal generator for the PXA255, implemented as a loadable kernel module for linux-2.6.10gum.

It can be configured to a generate a single multi-channel PWM signal and/or multiple single-channel PWM signals on GPIO pins. GPIO pins, channels and timings are configured with module parameters as illustrated in Table 2.

User-mode programs drive the PWM generator through a device interface. See Table 3. PWM generation begins when a program opens the device and sets values for all channels. GPIO pins are reverted to a logical 1 when the program closes the device.

MPWM uses OSMR1 (OS Match Register 1), which is normally available on the Gumstix. (OSMR0 is used by the Linux system timer and OSMR3 is used by the watchdog.)

The PXA255 timer period is 270 ns, which provides 11 bits of resolution for a typical PWM signal with 1 ms range. Although MPWM uses a high-priority PXA255 FIQ rather than a regular IRQ, jitter can be as high as several microseconds. This is still good enough for flying a helicopter. See also Section 9.2.

bluerc_rxis a user-mode program designed to run on the Gumstix. It receives RCdata as UDP packets over a Bluetooth BNEP connection and forwards themto the PWM generator.

The program will terminate if it does notreceive any UDP packet during a configurable interval. This willdisable the PWM outputs and (hopefully) cause the power/gyro board toshutdown the motors.

Usage: bluerc_rx [-p ] [-t ] [-c ]

Example: bluerc_rx -p 9000 -t 2000 -c /dev/mpwm0

bluerc_txis a user-mode program designed to run on a Linux host with a Bluetoothdongle and a USB joystick. It reads the positions of the analog sticksand triggers, mixes them linearly into 6 RC channels, and sends thechannel values to the helicopter as 6-byte UDP packets over a BluetoothBNEP link.

Usage: bluerc_tx [-c ] [-r ] [-m ] [-d ] [-p ]

Example: bluerc_tx -c /dev/js0 -r 50 -m xpad_ccpm120_mode2.mix -d 192.168.10.1 -p 9000

The mapping from joystick axes to RC channels is defined by a matrix in the configuration file. Figure 3 shows the default setup.

Figure 3. Mixer configuration xpad_ccpm120_mode2.mix.

Figure 4. Coefficients for 120° CCPM mixing.

In this scenario (Figure 7) we simply emulate a regular remote control. There are no sensors on the helicopter, no complex embedded software, and no feedback from the helicopter to the ground station.

Figure 7. Bluetooth remote control overview.

For a quick start, try make tx and make rx in the src directory.

Goal: Reduce the jitter of the timer-based PWM generator.

Lock the FIQ handler in the I/D cache ?

Goal: Reduce datalink latency.

Use a more appropriate protocol than BNEP.

Goal: Improve range.

Goal: Get rid of the computer.

Thanks to the next generation of videogame consoles, cheap Bluetooth joysticks should be available in 2006.