Issue 131 June 2001
Wireless Data Link

Start • Hardware Architecture • Transceiver Chip • Transceiver Chip Operation • Hardware Construction • Antenna Considerations • Radio Software •Software Design Considerations • Software Implementation • Software Enhancements • Compliance Issues • Your Turn • Sources & PDF

This article will describe the hardware and software design and implementation of a low-power, wireless RF data link. We will discuss a robotic application in which the RF link facilitates the command and control functions of a tele-operated miniature robot. The RF Monolithics (RFM) TR-3000 chip is the core of the transceiver design. We use a straightforward interface to a PIC controller, so you should be able to use or adapt much of this application for your needs.

We are part of a team of researchers and developers led by Dr. Nikos Papanikolopoulos at University of Minnesota’s Center for Distributed Robotics. Our mission for this DARPA-sponsored project was to develop a miniature, self-contained robot for distributed robotics applications.

The robot, called Scout, is packed in a 38-mm diameter tube with coaxial-mounted wheels at each end, approximately 110-mm long. The robot is shown in Photo 1. (For additional information, see the "Key Specifications for Scout Robot" sidebar.) Scout carries a miniature video camera and video transmitter, allowing you to tele-operate the robot by sending it steering commands while watching video images sent back from Scout. The video transmitter and data transceiver contained on the robot are separate devices, operating at 915 and 433MHz, respectively. Also contained on Scout are dual-axis magnetometers (for compass functions) and dual-axis accelerometers (for tilt/inclination measurement).

Key Specifications for Scout Robot Here are some key specifications for the scout robot (a) and its internal data radio (b). For more information, visit the University of Minnesota web site listed in the Resource section. Size Approximately 38 × 110 mm Weight 250 grams Propulsion Geared DC servo motors, one per wheel Power Eight Lithium batteries, Kodak K58L Payload CMOS video camera and 915-MHz video transmitter Sensors Dual-axis magnetometer, dual-axis accelerometers Special feature Ability to hop by winding up a spring steel tail and releasing it to slap the floor

Operating frequency 433.92 MHz (fixed) Modulation OOK (on/off keying) Antenna impedance 50 W Data rate 2.4 kbps Transmit power Approximately 1 mW Typical range Up to 50 m Power supply 2.7 to 3.5 VDC

Photo 1—The robot measures a little over 4². Designed for tele-operated remote surveillance, it contains a video camera and transmitter. Scout can hop over obstacles by hoisting its tail spring (shown extended) and quickly releasing it to slap the ground and propel the robot into the air.

One of Scout’s unique features is its ability to hop. Not frequency hop, but physically hop. This allows the robot to drive up to an obstacle and leap over it. Scout accomplishes this feat by using an onboard winch to retract a spring tail and then quickly release it. The tail slaps the ground and the robot flies into the air. By using the wheels to control the prerelease tilt angle of the robot and the accelerometers to measure it, the robot’s flight trajectory can be varied from shallow to nearly vertical.

Packing all of that hardware into a small area was no easy task. In order to get any height to the hop, a lot of energy had to be put into the spring tail. The winch mechanism has to be strong enough to cock the spring, which means it has to be beefy, which requires more energy to lift, which means more energy put into the spring, which requires a stronger winch…. Designing the mechanism is an exercise in optimizing a positive feedback system.

The electronic packaging has to wrap around the space claimed by the electromechanical components and video camera. Scout’s winch and camera occupy the central part of the robot and are mounted to a circuit board containing Scout’s main processor, a PIC16F877. This board is mounted horizontally within the tube with two round end cap boards attached at right angles, one at each end. One of these boards contains the magnetometers and accelerometers and the other contains another PIC serving as a radio controller.

The radio RF board stacks onto the radio processor board, and power supply boards stack onto the radio and magnetometer boards. Finally, end caps containing lithium battery cells and wheel servo motors stack onto the power supplies. A polycarbonate tube holds everything together.

Obviously, we could write an entire article about the robot. Actually, there are research papers that discuss Scout [1, 2] and more are in progress; check the University of Minnesota web site for links to these and other published works. Now, let’s get back to the subject of our article, the wireless data link.

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