Electronic Lead Screw Drive ( Part 1 of 2)

Fritz.Linck@t-online.de (Germany)

Implemented in 1999 and 2000

Documentation completed March 1, 2001


Drawing 1 Features / Advantages:

Large variety of saddle speeds (i.e. travel per revolution of the main spindle)

Changing speed without changing gears, just setting of two selector switches

This is no speed regulation, but angular position ( = phase) regulation, i.e. lead screw moves like coupled with gears.

Principle of Operation:

Phase lock loop system between main spindle and lead screw. (Take it easy, it’s less complicated than it sounds, see below).

See drawing no. 1. The lead screw is driven by a D.C. gear motor , so its speed can be controlled by the operating voltage applied to the rotor. A timing disk on the main spindle generates pulses that count an electronic counter (phase counter) upwards. A timing disk on the gear motor generates pulses that count the same phase counter downwards. An electronic digital-to-analog converter (ADC) creates a voltage level depending on the count, i.e. full voltage level when the phase counter is at its maximum, or half of that value when the phase counter is at half of its maximum etc. A transistorized power driver converts the (variable) voltage level from the ADC to a (variable) voltage with enough power to drive the gear motor.

Assume both, the main spindle and the lead screw drive, are running. Regarding their speed relation, there are 3 possibilities:

A:) Both happen to run at the same speed:

The phase counter will receive, per unit of time, as many up counting pulses (from the main spindle) as down counting pulses (from the gear motor). Therefore the phase counter will not change its content. ADC output voltage and gear motor drive voltage will not change. Main spindle and lead screw will remain at equal speed.

B:) Main spindle happens to run faster:

The phase counter receives more up counts than down counts, increasing its content. This increases the ADC output and in turn the gear motor drive voltage. The gear motor picks up speed and catches up with the main spindle until case A is reached.

C:) Main spindle happens to run slower:

The phase counter receives more down pulses than up pulses, reducing its content, and gear motor slows down until case A is reached.

The lead screw will always run at a speed that keeps the phase counter in balance. The regulation process as per above takes place within milliseconds and reacts at count differences of single pulses, thus ensuring precise synchronism. Since this is not a speed regulation, but a regulation of angular position, there is no accumulation of speed deviations. The gear motor will always settle to a speed that keeps the phase counter in balance. This keeps both drives in synchronism as if they were coupled by gears.

Above will ensure a 1 : 1 coupling of main spindle and gear motor. We want, however, selectable speed rations. This is done by two additional counters, one each between main screw pulses and phase counter (main spindle / divider counter) and between gear motor pulses and phase counter (lead screw / multiplier counter). See drawing no. 2. Both counters can be preset to various values and work as frequency dividers, i.e. they pass on each nth pulse (n = preset value) to the phase counter.

Let’s look at the divider counter first: If it is set to a value of, say, 3, it will only pass on every third pulse from the main spindle to the phase counter. To keep the phase counter in balance, the gear motor will now have to run only at a third of the speed. In other words, through setting the divider counter, the gear motor can move at any full-digit fraction of the main spindle speed, i.e. a half, third, quarter, hundreth etc.

Just dividing, however, does not give us all the gearing ratios required. Therefore we also need a multiplier counter: It works the same way and is arranged the same way between lead screw pulses and phase counter. When set at the value of n, it will pass on every nth gear motor pulse to the phase counter. To keep the phase counter in balance, the gear motor will now have to run at n times the speed, thus performing a speed multiplication.

Through clever setting of multiplier and divider presets, virtually any speed ratio can be generated.

The system also provides for directional sensitivity, so if the main spindle is being reversed, the lead screw will follow in the reverse direction. This will ensure the system will never get out of synch as long as power is on and the saddle is not decoupled from the lead screw. As with mechanical lead screw drives, due to mechanical backlash, you can only cut in one direction without losing precision necessary for thread cutting. For normal machining, where absolute synchronism is not necessary, reversing during operation is of course possible. Also another way of reversing is possible, i.e. reversing the lead screw while maintaining main spindle direction. This is done by flipping toggle switch S1 at the front panel.

In my (metric) implementation, these saddle movements (in millimeters) per main spindle revolution are possible through setting switches A (multiplier) and B (divider):
table 1In my implementation, with 1.5 mm saddle movement per lead screw revolution and 40 / 120 holes in the main spindle / lead screw disk, the main spindle preset (divider) values are 1, 2, 5, 10, 25, 50, the lead screw (multiplier) values are 5, 6, 7, 8, 9, 10. The multiplier and divider presets are defined by diodes on the electronics board and are selected by selector switches A and B above.

This is the formula for saddle travel per revolution of the main spindle: