Welcome to Adobe GoLive 5

EZPEC is a tracking controller for "old-school" telescopes that use a 120V-60 Hz synchronous motor for tracking on the right-ascension axis and a reversible DC motor on the declination axis. The EZPEC controller demonstrates the ease with which peripherals can be added to the Pyxis camera/telescope controller. The EZPEC controller communicates with the PC through the Pyxis auxiliary port using a simple 3-wire serial interface. The EZPEC AVR and PC interface routines are a good starting point to refer to when designing "distributed control" peripherals for the camera system.

At the heart of the EZPEC controller is an ATMega32 micro controller operating at 10 MHz. This MCU generates the variable 60 Hz rate (modified sine-wave output) to 0.001 Hz (average) accuracy. The ATMega32 MCU incorporates a 1kb EEPROM memory on which the periodically modulated rate is programmed and the operation configuration is stored. Aside from the MCU, there are few other components on board; 4 transistors for the output amplifier, a 5V regulator, a DC line filter, some pull-up/down resistors and a monolithic CMOS H-bridge to provide reversible control of DC motors with wide-ranging voltages (5-24V).

Every effort was made to make EZPEC easy to use and fully operational in standalone mode. In fact, one should never have to use the Pyxis interface if the default global configuration settings are modified as required in the EZPEC firmware. However, best programming of the PEC modes requires connection to a computer so that the programmed PEC pattern can be analyzed and smoothed. The current default settings assume an 8-minute worm period.

The controller uses a single input line to determine when the worm has reached its start position. When the worm reaches the start position, the controller returns to the start of its frequency modulation cycle. The operation frequency is programmed in the EEPROM on the ATMega32 MCU. The EEPROM address is incremented every second (to every few seconds, depending on the worm period) and the new operation frequency is read out and applied to the ~60 Hz output timing loop.

Three lines are provided to implement the worm position sensor; the TTL/5V-CMOS digital input worm reset signal line (a high-going signal is interpreted as a worm reset), a ground line and a +5V power line. The easiest and most reliable way to generate the worm reset is to use a reflective proximity sensor. A reflective bar can be glued to the spur gear on the worm to activate the proximity sensor once per worm cycle. Note that approximately 1-2 seconds of "hysteresis" have been added in the software on the worm reset polling code to avoid multiple triggering of the reset during the relatively slow transition of the proximity sensor output.

An electromechanical switch can also be used due to the slow worm period, but this approach is more easily prone to failure.

To record a periodic error correction pattern, the user places EZPEC in "Record" mode (pressing the "Mode" button cycles through the operation modes). Recording begins when the worm returns to its home position and continues until the worm has gone through an entire cycle. During recording the user should be maintaining tracking of the telescope using the joystick. A high-power illuminated cross-hair eyepiece should be used on a moderately bright star located near the local meridian and celestial equator. The telescope should be aligned moderately well, an alignment on Polaris being sufficient. The contents of the EEPROM can be then used as-is or read into a PC using the Pyxis auxiliary port. This second option allows several correction cycles of the worm to be recorded and averaged on a PC so that the applied correction can be smoothed out. The smoothed list of operation frequencies (actually line-cycle periods) can then be reprogrammed into the EZPEC EEPROM for improved tracking.

Design package including Gerber board fabrication files, firmware, bill-of materials, schematics and board layout:

The three wire interface should be simple to emulate using a standard parallel port so that the EZPEC controller can be accessed from a PC directly without using the Pyxis controller.