Toy chopper teardown reveals some basic but unusual design guidelines - Embedded.com

Toy chopper teardown reveals some basic but unusual design guidelines

Embedded Systems Conference, Boston, Mass. —At a live teardown at the Embedded Systems Conference here, Rich Nass, Editorial Director of Techinsights' Embedded Systems Design and John Day, Microchip Technology Field Applications Engineer, took apart an Estes Micro-Tiger remote control helicopter (www.estesrockets.com). Unsophisticated, you think? Not at all: in fact, it's a marvel of clever engineering and safety circuitry, selling for under $30. Day noted “products like this have a very high ratio of function to electronics.”

Day pointed out that the $30 price means there is only $2 to $4 dollars for the electronics, so two design rules come into play: do it in firmware, not hardware; and make the printed circuit board (PCB) an integral part of the design, not just a platform the components.

Let's first look at the underlying specifics. The remote control link is not RF, it's done via infrared, and has a range of about 25 feet (7 to 8 meters). A trio of IR light emitters in the handheld controller disperses a 2 kHz, PWM, non-return to zero (NRZ) data stream, with one 20 msec packet every 150 msec. A checksum is also added so the chopper can verify, to a first level, that the data received is OK. This data includes throttle position, tilt position, and left/right trim information. All data formatting and protocol is done in firmware, without any dedicated hardware.

The controller also includes a charger for the helicopter's lithium-ion battery pack, and works with the pack to manage charging and provide critical safety oversight. The CPU power of the handset is minuscule, just a Microchip PIC16F505 microcontroller with 1Kcode space, 72 bytes of RAM, and no interrupts; the code runs a continuous service loop. When the CPU senses that the controller is connected to the battery pack, it switches itself from “fly and control” mode” to “charge and manage” mode.

The PCB is the start of the “out of the box” design thinking. When the user squeezes the throttle, he or she gets the sense of a continuous speed control, such as via a potentiometer. But the actually throttle position is developed using a 16-step grey code set of tracks on the PCB, and the throttle just shorts out successive tracks as it is pressed. The microcontroller determines which tracks are shorted out and thus knows the throttle position. Similarly, the controller includes a left/right trim adjust, to allow the user to adjust for power imbalance between the main rotor and the tail rotor. The user turns a small knob to make the trim adjustment, but the internal reality is very different. Turning the “knob” activates a pair of opposing pairs of metal fingers on the PCB, which provide simple switch closures. Again, the microcontroller looks for and counts these closures, and so gets what the user is indicating.

Finally, there's the sensor that indicates the tilt of the controller, which provides the user with a realistic sense of piloting the craft. This tilt is sensed by a pendulum in the handset, which has a semipermeable magnetic material at its end. A custom bobbin, wound in an arc shape, surrounds the pendulum and generates a current as the pendulum moves.

Even at the helicopter, which flies for about 4 minutes on a full charge,, there's a lot of engineering in this so-called toy, pushing the limits of simplicity and cleverness. The main rotor is powered by a cell-phone vibrator motor, while the tail rotor has a smaller version of the same. Each is controlled by a FET sized to the respective motor, but since the motors are unidirectional, they only need one FET each, rather than an H-bridge configuration with four FETs per motor.

Before you say “but it's just a toy”, think again! And when you hear someone say it, well, it's probably best you don't try to correct their misconception–they won't appreciate it.

Bill Schweber is the site editor of Planet Analog. You may contact Bill at .

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