A common requirement in pick-place equipment is to flip small, thin parts about the horizontal before placing in a tray or other container. A further requirement is to have the flipping element configurable. That is, flip or non-flip, under program control.
When the parts are placed into trays with tightly toleranced vertical walls, it is desirable to make a small, purely vertical move after the flip to avoid any horizontal "scrubbing" motion at the end of the placement. Thus, we have a need for a compact, two-axis, subsystem that may be inserted into pick-place equipment with minimum impact over the base machine. Such a subsystem is shown in the sketch below. The pick/place part is carried on the flipper by a vacuum pick-up with pressure part-presence detection.
Solution
There has also been considerable progress with small stepmotors. The motor selected is a short, lightweight 400-step (or 0.90-degree) NEMA 17. This has excellent stall torque when hold current reduction is disabled. With 1.5 percent accuracy at full step it meets the accuracy required. Note that this accuracy might not be available with microstepping, but due do the fixed 180-degree move, is not necessary. High resolution is not a requirement, and avoiding microstepping increases torque and eliminates "cyclic" error. Mid-band stability or antiresonance reduces the step vibration that often accompanies full or half step control.
The outer axis is a much simpler selection. A NEMA17 sized servo motor with 4,000-step/revolution rotary encoder offers the required resolution and accuracy when paired with a 6-mm leadscrew. The servo feedback solves the problem of lost motion if the parts contact the trays, and servo tuning easily provides the required move time. The motor mass and size are easily accommodated since the motor is stationary. The Z axis motor chosen is a Telnic 1726. The flip motor is a Lin 417-15-03.
AXES REQUIREMENTS
Specification
Z (vertical) axis
r(rotational) axis
1. Range of motion
6 mm, variable, both at top of stroke (receive) and bottom of stroke (place)
180-degree, fixed
2. Accuracy
±2 µm vertical
±2 µm horizontal (X-Y) or 0.2 mrad=0.011-degree ±4 µm vertical at edge of 10 mm part or 0.16 mrad=0.01-degree
3. Move time
20 msec, 2 mm (typical) move
75 msec, full stroke
4. Motor mass
Motor is stationary, So mass is not a constraint
Motor moves with Z, thus minimum mass required
5. Motor volume
Mechanism must be compact
Mechanism must be compact. Horizontal offset between flipper payload and Z-bearings contribute to "Abbe" Error (errors caused by Z-bearing).
6. Crash protection/detection
If the part contacts the tray during placement, the axis shall not lose position
If the part contacts the tray during placement, the axis shall not lose position
7. Motion profile
Trapezoid
Soft start/stop to allow speed without shifting part
8. Static torque
Leadscrew drive multiplies the torque available from the motor
The direct drive and the lever arm of the flipper (19-mm) require high static hold torque
The chart labeled Axes Requirements drives the motor selection requirements. The inner axis, r, might be implemented as a rotary pneumatic actuator. Some of the newer rack & pinion types offer compactness, adjustable end stops, and convenient EOT sensors. However, they do not allow a soft start/stop feature to allow high speed and gentle end of motion. If the part is not absolutely flat, it can lose vacuum and rotate slightly about the pickup tip axis and degrade placement accuracy. Specification 7 eliminates this option. The flip motor could be small servo motor with rotary encoder. Several new examples have small area and decent line counts. However, they are long (# 5), offer inadequate resolution and accuracy (#2), and depend on high integral feedback to provide the static torque (#8) required of the lever arm of the flipper. This high integral feedback upsets the PID control required for acceptable movetime (#3) with the small servo motors. Thus, a small stepmotor is the favored candidate.
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