Servo - May 2010

Go XBee-PRO With Your Robot Control

Many of you are monthly readers of both SERVO and Nuts & Volts magazines. For those of you that are SERVO-only readers, there are things going on over in Nuts & Volts that may be of interest to you. We’ve been working with a 16-bit PIC24F general-purpose design in the NV Design Cycle column that is perfectly suited for robotic and machine control work. This month, we’re going to apply that 16-bit Design Cycle technology to SERVO-oriented robotic control applications.

The Nuts & Volts Hardware

The PIC24FJ128GA006-based hardware shown in Photo 1 speaks for itself. A dedicated ZeroG ZG2100M Wi-Fi module is hard-wired into the PIC along with a 2x16 LCD and a serial port. The 10-pin header is the RS-232 port physical attachment point while the 10K potentiometer to its right is the contrast adjustment for the LCD.

The PIC24FJ128GA006 clocks at 32 MHz using an 8 MHz crystal and the internal 4X PLL (Phase Locked Loop). A five-volt power supply feeds the Microchip TC1262-3.3 LDO (Low Drop Out) voltage regulator. I’ve pinned the printed circuit board’s header pads with standard 0.1 inch pitch header pins. The breadboard is socketed to accept the header pins. Both five volt and 3. 3 volt power are available at the breadboard level.

SERVO Modifications

The NV implementation works well and is based on the free Microchip TCP/IP stack which drives the ZeroG ZG2100M Wi-Fi module. In our SERVO implementation, we’re going to replace the Wi-Fi module with an XBee-PRO 802.15.4 radio. The XBee-PRO RF modules are designed to be part of low-cost, low-power wireless sensor networks.

54 SERVO 05.2010

By Fred Eady

PHOTO 1. This is a shot of the ZeroG - PIC24FJ128GA006 Trainer loaded onto a Twin Industries 8100-45-LF prototyping board. A LCD and serial port were recently added to the system over in Nuts & Volts.

Drawing a peak transmit current of 295 mA, the XBee-PRO can attain a range of up to 300 feet indoors and up to one mile outdoors. To give you a perspective of the XBee-PRO’s output power, remember the walkie-talkies you had as a kid? Well, they transmitted voice at 100 m W. The XBee-PRO transmits data at 50 m W. The XBee-PRO RF module requires 45 mA receiving and less than 10 µA when its eyes are closed. We’re going to use the RF module’s out-of-the-box peer-to-peer communications method. However, these modules can be configured for point-to-multipoint and point-to-point operation. A mesh network of XBee-PRO RF modules is self-routing, self-healing, and fault tolerant. These tiny networks are capable of delivering RF data rates of 250 Kbps. We’re also going to upgrade the project’s microcontroller motor with the new 32-bit PIC32MX795F512H. This PIC is pin-compatible with the PIC24FJ128GA006 with the exception of the PIC32MX795F512H’s USB I/O interface. If desired, we could actually run the 32-bit SERVO configuration with the Wi-Fi module since the Microchip TCP/IP stack supports the 32-bit PIC32MX795F512H and ZeroG Wi-Fi combination.