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Monotype Optical Cycle Sensor

Created by hww3. Last updated by hww3, 4 years ago. Version #4.

Over the past 6 years, I've had the pleasure of supplying computer interfaces for ’Monotype‘ installations at a number of type founders across the U.S. and Europe. Overall, the system works well, and its users are producing sophisticated work that they probably wouldn't have been able to otherwise. However, there is one aspect of the computer interface that's been a consistent problem and concern for me, and that is the cycle sensor.

In order for the computer interface to work properly, it needs to know when the casting machine is ready for a signal. As originally designed, the computer control system for ’Monotype‘ composition casters used a reed switch to detect the machine's cycle position. A reed switch closes, or turns "on", when a magnet moves close. Similarly, the switch opens, or turns "off" when the magnet moves away. In keeping with my somewhat arbitrary rule of "no caster modifications", this reed switch is (quite literally) clamped on the caster's air tower, and activated by a magnet attached to the rod that activates the air bar clamp. When the caster is in good operating condition, and none of the fancy attachments are being used, this system works fairly well.

Students of ’Monotype‘ equipment know that the engineers at Lanston Monotype were incredibly precise, and they left very little room for error when they designed the equipment that we're now effectively hacking. As a result, the extra length of air line that the computer interface introduces to the system is just enough that the machine isn't always able to respond to the air signals in time to act on them. The result is that signals are occasionally missed. This is especially true of machines with 15x17 mat cases or unit shift: the extra valve boxes that allow codes to do double duty cause delays that push things over the edge.

The first attempt to improve the situation involved a modest change. The sensor was repositioned to the cam lever that operates the air bar clamp. This gave a little more wiggle room, because the cam lever operates over a fairly wide arc, allowing the reed switch to operate a little sooner. Ultimately, though, the whole approach was hindered by the fact that the cam only produces an air pulse timed to the use of a paper ribbon. We needed more flexibility in order for the system to work reliably in all situations.

The solution turns out to to be quite elegant: a transparent disc is attached to one of the machine's camshafts. The camshaft, and by extension the transparent disc, rotates once for each cycle of the machine. A section of the disc is made opaque, and an infrared sensor is placed over the outer edge of the disc. The infrared sensor then detects the opaque segment as it rotates past, causing the air valves to send air to the caster.

The optical sensor in place on a caster

There are 2 aspects of the air pulse that are of interest to us: the time the pulse begins, and its length. The exact moment that the air valves are turned on can be controlled by rotating the disc in relationship to the sensor. For example, moving the disc forward in the direction of rotation causes the opaque sector to arrive at the sensor a little earlier. Similarly, the length of the air pulse can be controlled by controlling the width of the disc's opaque sector. The transparent disc is made up of a set of overlapping discs, each with an opaque section. Sliding these overlapping discs relative to each other allows the width of the opaque section to be varied, causing the air pulse to be shorter or longer.

Now, with the new optical cycle sensor, we have complete control and simple control over the timing and duration of the air pulses to the casting machine. A few hardy souls, including Nick Gill at Hand & Eye Letterpress have been testing the new approach out, and the initial reports back are good. An added bonus is that unlike the previous reed switch, the new sensor has no moving parts to wear out, which should improve reliability.

The new Optical Cycle Sensor is available for retrofit on existing computer interfaces, and will ship standard on future interface installations.


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sensor.jpg image/jpeg 40 kb Thursday, 27 February 2014 Bill Welliver

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