This page outlines some of the design information relating to the first tracker I built.
The stepper motor has 48 steps/rev. This is coupled to 250:1 gearbox, which is connected via a flexible coupling to a worm gear with 55 teeth. The resolution of the drivetrain is calculated as follows:
Click image to enlarge
The motor step rates are calculated based on the period of the earths rotation, taken as 23 hr 56 min 4 sec. (The Moon's period is taken as 27.32166 days). For the tracker to rotate at earth's rate of 15.041364 arc second/second, the step frequency for the motor is:
There are four BCD (Binary Coded Decimal) switches inside the case which allows the motor drive frequency to be set. They set the divide factor from the master clock frequency.
The master clock is taken from an 8640AN Programmable Oscillator, the output of which can be programmed by six control pins. This provides a convenient way of introducing faster foward and reverse settings.
8640 CTL
1
2
3
4
5
6
O/P Freq (Hz)
Forward (x1)
1
1
0
0
0
0
100kHz
Reverse (x6)
Forward (x6)0
0
0
0
0
0
600kHz
A set of four BCD switches are used to set a further subdivision using a CMOS 4059 Programmable Divide-by-N Counter. This IC is setup in a mode preset for divide by 8. The BCD values for sidereal rate are calculated as follows:
Following a similar analysis for tracking the Moon gives the BCD switches settings of 1693 REM 5.
The implementation of this subdivision is as follows. For the value 1631 REM 7, the one thousand figure is already preset in the the 4059. The four BCD switches set the remaining digits eg. 631 REM 7.
J4
J13 J14 J15 J16
J9 J10 J11 J12
J5 J6 J7 J8
J1 J2 J3
The accuracy of the crystal clock has been measured on two occasions using a time interval analyser. For the sidereal rate, the theoretical frequency for 1631 REM 7 is;
The measured values are;
This shows excellent accuracy and stability over a time period of 6 years. A repeat measurement, several minutes after the first one in 2001, gave a difference of 0.133 micro Hz.
The main cause of the error is the 100 kHz frequency output from the 8640N. This was measured in 2001 and found to be 100.0007 kHz. Using this figure gives a predicted output frequency after the subdivsion is 7.659954 Hz.Link to sample printout.
Motor Drive IC
The drive circuit for the stepper motor is a Philips SAA1027.
Links to some circuit information from my design notes:
1. Top Board
2. Bottom Board
3. Motor Drive Circuit Diagram
The motor is a four phase unipolar stepper motor, with 48 steps/rev. (Farnell Part No 147-879). This is connected to an ovoid 250:1 gearbox (Farnell Part No. 147-885). The worm gearbox is an AC Delco unit from a Mini Metro windscreen wiper mechanism.
This comprises:
Main Power Switch
Switches the 12V DC from the battery, protection via a 1A fuse.Forward/Reverse Switch
This is a three position switch:
FORWARD Position - Normal operation, x1 speed.
CENTRAL Position - Forward at x6 speed.
REVERSE Position - Reverse at x6 speed.Timer
The START & STOP buttons operate both the timer and the motor drive. Thus the motor can be operated over a set time or indefinitely. If the timer is used and the time counts down to zero, the timer will only 'beep' once before switching the motor off automatically. The timer operates from it's own battery ref. LR44.
Other Functions
A recycled remote control from an old video recorder provides a handy method for starting and stopping the motor. This also switches on a red LED on top of the electronics unit to illuminate an integrated timer. During the design of the electronics I thought it would be a fun to interface a digital timer to the drive. I assumed the timer buttons would provide simple on/off signals which I could easily interface to. However I soon found out that the timer signals are frequency modulated. Each button has a different modulation signal. After some perseverence I managed to fully integrate the timer into the drive circuit. This allows the timer start and stop buttons to start and stop the tracker. The timer also allows the motor to be run for a for a set time. This is useful when the tracker is rewound after a session. The timer automatically stops the motor and the audible alarm only sounds only once before the timer is reset. A remote contol from an old video recorder provides an alternative start and stop method and control for a red LED which illuminates the electronics box. It is worth noting that the timer project was more about the challenge than the practical use for astrophotograhy.
Back To General Tracker Design
