Machinist's Tachometer based on Philips P87LPC764
This project is a sophisticated optical tachometer that uses a Philips P87LPC764 as the only integrated circuit. Since tachometer signals are relatively low frequency, it uses a reciprocal counting scheme whereby the frequency (and thus rotational speed) is calculated from the time that it takes to receive an integral number of input pulse edges. This allows an update rate that usually is in the 2Hz range (1Hz worst case) with 1 RPM resolution. A 1Hz resolution would require a 60 second gate time using conventional methods, and would thus have a very slow update speed. The tachometer collects pulses for a minimum time of about 0.5 second and then waits for the next pulse. The RPM is then calculated :
Rotational Speed (RPM) = Tick frequency (Hz) * 60 * (Number of pulses)/(Total ticks)
In practice, a somewhat more elaborate scheme is used, since there may be a number of pulses per revolution. The tachometer pickup disk may have from 1 to 16 black and white bands on it. If possible, the tachometer times integral rotations of the disk, however if the frequency is so low that the update rate would be compromised, it will automatically use individual counts within a rotation. Rotational speeds of only a few RPM up to 10,000 RPM can be measured in this way.
For a given combination of material and cutter type, there is an optimal cutting speed, tabulated in SFM (surface feet per minute) or SMM (surface meters per minute) in metric. For example, when milling a 390.0-alloy aluminum die-cast part with a HSS (High-Speed Steel) cutter, the optimum speed is 60 fpm. The Machinist's Tachometer can directly display the working SFM, calculated from the diameter of the cutter of a milling machine (or work on a lathe) as:
Speed (SFM) = Rotational speed (RPM) * Diameter of Cutter (inches) * p * 12 (feet/inch)
Or
Speed (SMM) = Rotational speed (RPM) * Diameter of Cutter (mm) * 0.001 * p
User interface is with a rotary encoder switch with push switch (the knob is pressed to actuate it), a 4-position DIP switch to set the number of pulses per revolution and a switch to select Imperial or Metric units. A 16.00 MHz quartz crystal provides a stable timebase, and the processor has plenty of processing power to do the required calculations at that clock speed. Since the P87LPC764 has a Limited Pin Count, the display scanning pins are also used to read the switch inputs. A few low-cost resistor networks isolate the two functions and also provide immunity from ESD. A high efficiency 4-digit multiplexed LED display is used to reduce current consumption and allow direct drive from the P87LPC764, while still providing a bright display. Since processor can source only a 5mA total, a pair of dual "digital transistors" work as the digit source drivers. The analog signal from the reflective optical pickup is processed by a Schmitt trigger circuit made from a dual transistor and 3 resistors. The internal comparator could be used instead.
Three discrete LED's indicate what the digital display is currently showing:
Pushing the knob cycles the display between the three display modes. Turning the knob sets the diameter, in inches or mm.
A prototype of this unit has been installed on the author's small milling machine, using a pickup disk made from artwork included in the documentation.
DIAGRAM