Pen Lift Mechanism

Design objectives for the pen lift mechanism for the new four-color plotters included reliable operation and lifetime at least an order of magnitude longer than earlier designs. The life target was set initially at 40 million cycles of operation without any maintenance or service.

When the mechanism lowers the pen it must not slam the pen into the hard platen and smash the pen tip. Once the pen is down and slewing across the paper, the mechanism must hold the pen rigidly or it will chatter and skip, leaving a dotted line.

Before the plotter can begin to draw a line segment, it must pause long enough to lower the pen. The speed with which the plotter can draw such figures as characters or dashed lines depends heavily upon the length of this delay. The new mechanism had to minimize this pen-down delay.

In addition to these objectives, several requirements were imposed by new features designed into the 9872A. With the introduction of the pen stable, the pen holder had to be able to reach off the drawing surface to drop off or pick up a new pen quickly and

Solenoid Drawing Mechanism

Fig. 5. Pen lift mechanism of the four-color plotter. Pneumatic dashpot controls impact velocity so the pen tip is not deformed, there is no ink blob when the pen is dropped, and the pen does not bounce.

with 100% reliability. Since the prime movers of the plotter are open-loop stepper motors and have no position feedback, it is possible to overpower the mechanism and displace the arm and pen carriage in such a way that the pen holder crashes into the front panel at full slew speed and with full motor power when reaching for a new pen. The pen lifting mechanism had to be designed to withstand this kind of crash. In the worst-case collision the impact is to the slender lever that clutches the pen.

The pen lift mechanism of the 9872A and 7221A Plotters is shown in Fig. 5. Underneath the pen lift cover is a pneumatic dashpot that helps achieve several of the objectives. The damping rate of the dashpot can be adjusted in the plotter so the pen impacts the table at a rate of about two inches per second. At this velocity the impact with the table top can be easily absorbed by elastic deformation of the pen tip. This prevents mushrooming of the pen tip after successive pen drops so effectively that now the only reason for a pen to become unserviceable is that it runs out of ink. At low impact velocity there is no ink blob where the pen is dropped and the tendency for the pen to bounce upon impact is reduced, permitting a shorter pen-down delay and a faster character plotting speed.

The two major structural pieces of the pen lift assembly are aluminum die castings. They provide compactness, the rigidity to keep the pen from chattering while drawing high-speed vectors, and the strength to withstand impacts into the front panel and pen stable.

Several different high-strength materials were tested for the lever that grips the pen. Most broke upon impact with the corner of the front panel when reaching for a new pen after the arm had been forced out of position. It turned out that a common acetal resin produced an injection-molded lever that was durable enough to withstand repeated worst-case impacts and had the creep resistance to stand up to the stresses produced by the pen gripping spring.

Precision bearings are used in the hinge of the lifting mechanism for smooth operation over a long service life. Life tests were discontinued after 40 million lift-drop cycles with little noticeable wear to critical parts.

Putting the pen-lift solenoid close to the pen reduced the audible noise to a tolerable level and improved overall reliability by minimizing wear and the number of moving parts. It did, however, necessitate development of a trailing cable inside the slender arm. A thin, twin-lead flexible circuit met the flex-life requirements, but pushing that circuit in the return direction into the slender arm was like trying to push a chain by one end; the tangles that resulted were disastrous. Supporting the flexible circuit on each side with a crowned, stainless steel strip provided the stiffness necessary to control it. The effect is similar to bending a steel tape measure: the straight portion is stiff while bending stresses are kept low in the looped portion. Fig. 6 shows the trailing cable assembly inside the arm.

The pen lift assembly is easy to assemble and service. The three adjustments (pen height, pen force, and damping rate) are accessible from the top after removing one screw and the pen lift cover (Fig. 5). The interior of the arm, shown in Fig. 6, can be exposed by removing one more screw and snapping off the arm cover.

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