ChRoMicro - Cheap Robotic Microhelicopter HOWTO

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Revision History
Revision 1.0	2005-06-02	PB
First public release.
Revision 0	2005-05-27	PB
Document created.

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

A recent radical innovation is the "ProxFlyer" self-stabilizing deformable rotor design. Unfortunately, current commercial implementations 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.

The electronics package is a "black box" which connects all components together. This is in contrast with larger model helicopters, where the connections between the receiver, gyro, BEC and motor drivers are exposed 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 tinkered with fairly easily. It actually consists of two boards connected back-to-back with a 2x3-pin connector (see Figure 1):

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.

Since this +5 V rail is exposed on each servo connector (including the unused ones), the Gumstix can be conveniently powered from it. In order to accomodate the extra current (100-200 mA total), it might be necessary to replace the two voltage regulators with a larger, externally-mounted 7805.

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.

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_rx is a user-mode program designed to run on the Gumstix. It receives RC data as UDP packets over a Bluetooth BNEP connection and forwards them to the PWM generator.

The program will terminate if it does not receive any UDP packet during a configurable interval. This will disable the PWM outputs and (hopefully) cause the power/gyro board to shutdown the motors.

Usage: bluerc_rx [-p <port>] [-t <timeout>] [-c <PWM device>]

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

bluerc_tx is a user-mode program designed to run on a Linux host with a Bluetooth dongle and a USB joystick. It reads the positions of the analog sticks and triggers, mixes them linearly into 6 RC channels, and sends the channel values to the helicopter as 6-byte UDP packets over a Bluetooth BNEP link.

Usage: bluerc_tx [-c <joystick>] [-r <refresh rate>] [-m <matrix>] [-d <dest IP>] [-p <dest port>]

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 5. Helicopter + ARM + Linux.

Figure 6. Rear view with connector on I/O daughterboard.

Table 4. Gumstix daughterboard I/O pin assignments.

Pin	Signal	Usage
10	NACRESET (disconnected)	+5 V CC in
16	GND	GND
18	GPIO61	6-channel PWM out

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.

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.

[pxa255_dev] Intel PXA255 Processor. Developer's manual.

[pxa255_user] Intel XScale Microarchitecture for the PXA255 Processor. User's Manual.

[pxa255_elec] Intel PXA255 Processor. Electrical, Mechanical, and Thermal Specification.