|Equatorial Platform Dob Tracker Drive Design
Which type of Drive Should I use on My Equatorial Platform?
A DC motor drivescrew can be all that is needed to get your platform tracking the earth's rotation (upper right). Nils Olof Carlin has the best tangent coupling I have seen. If I were to build another tangent drive, I would use his coupling and would also follow his advice of keeping the Center of Gravity close to the rotation axis when using a D.C. motor. See below for link to a picture of Nils' coupling. A really nice drive combination is a simple H.O. train transformer and a D.C. gearmotor out of a savage catalog or eBay. I (like many platform builders) took my first steps by looking at a tangent drive and saying, "I can do that". You can too. A tangent drive is not perfect, but for the casual visual observer, or for the limited photo work, it can be a blast.
A roller driven stepper motor is the ideal motor to run a platform (upper left). It delivers maximum torque from a stopped position. The friction roller must turn very slowly to move the platform in sync with the earth's rotation. The micro stepping controller that I use cost $1000s of dollars new. They can be purchased from scrap dealers on eBay for $40 - 80 dollars. The stepper motors are available for $10 - 25. That is cheaper than a low-cost gear motor. People shy away from stepper motors because of the complex circuits that must be built to drive them. With low-cost packaged controllers, it is Plug and Play.
With a direct drive stepper system, there is no backlash. The motor speed is constant over varying loads. The force is applied at right angles to the rotation (unlike the tangent drive). With a quality indexer, you can set the speed and forget it.
My handheld unit has a 555 timer to generate the step signal. The micro stepping controller takes the signal and increments the motor from 1/200 of a turn to 1 /50,000 of a turn, based on what I set the controller for (one time switch setup). Once the motor is stepped, it is locked in position and cannot move until the next step pulse arrives. I use 1/20,000 steps for smooth motion (that 1/20,000 step is increased to 1/200,000 step with the reduction gear). A controller that only allows 1/200 or 1/400 of a step will cause the scope to jump as it moves from step to step (really shocking the abrupt motion) with a drive wheel the size that I am using. You might be able to use a very small drive wheel and a 1/400-step controller. I never tried it because of concerns over wheel to runner slippage. Many builders use a low cost stepper motor that has a very high gear ration and find a half step controller is usable. See our EQ Platforms site for motor part number etc.