A LONG TIME COMING: Linear Clock

I guess that there is no point in excusing the pun as I want it to be taken both literally and figuratively. This is a design, built from my simple linear actuator, to tell the time using an articulated slider on a graduated ‘rule’.

A stepper motor mounted in the base is attached to a drive pulley that pulls a belt that runs up and down a long aluminium extrusion. The belt is attached to a pointer that moves up and down a vertical scale graduated from 1:00 to 12:00. An internal real-time clock keeps accurate time and the vertical position of the pointer is updated every minute.

The base unit is cut from a stack of 12.5mm HDPE pieces, while the scale is cut from 1″ x 1/8″ aluminium stock. The numbers 1 through 12 were CNC’ed into the aluminium and the depressions filled with blue paint. (Note: the strange look of the 6, 8, 9 and 10 resulted from my error of removing the inside cuts of the numbers in the CNC file. Whoops!)

One of the inherent issues of using stepper motors is that while they can position relatively, they have no knowledge of absolute position, so when a particular reference has to be established, they have to be homed. While the original design used an inductive proximity detector to accurately position the pointer, I felt that driving the motor to an end-stop would suffice and simplify the design. So now, upon power-up, the pointer is driven to the bottom of the scale where it hits an end-stop, stalling the motor. After a few seconds to ensure the pointer is at its lowest position, the software uses this position as its ‘0’ reference from which it calculates the number of steps needed to drive the pointer to illustrate the time.

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