Introduction

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.