Positioning Injection Molding Take-Out Robots

The positioning problem of an injection-molding take-out robot is, in essence, a balance between a high cycle rate and a cramped mould space: the faster the motion, the shorter the cycle, but the greater the risk of position error when entering the mould area. If position feedback on the three axes (traverse, strip, and up/down) relies only on the servo-motor encoder, belt elasticity, backlash and frame vibration are all invisible. Fitting a magnetostrictive displacement sensor that measures the actuator end directly yields absolute position, eliminates transmission error, and, on power-up, confirms without homing whether the arm is stopped inside the mould area.

Different requirements on the three axes

The traverse axis has the longest stroke (along the machine); the concerns are absolute error at long range and refresh rate during high-speed motion. The strip axis reaches into the mould area to take the part; position error relates directly to mould-crash risk, so repeatability and the in-position criterion are the most critical. The up/down axis (Z-axis) has a medium stroke; the concern is reading stability during acceleration and deceleration. Of the three axes, at least the strip axis should have independent position feedback as one of the permits to enter the mould area, interlocked with the injection-molding machine’s platen-position signal. Measuring-point schemes on the machine side are given in injection molding: closing the loop on clamping force and shot volume.

Three-axis configuration

Refresh rate and lag at a high cycle rate

Take-out cycles are typically counted in seconds; position data in the high-speed section of the arm motion must be dense enough. Refresh rate lengthens as measuring range increases (about 0.5 ms to 5 ms depending on model and range). The long-stroke traverse axis in particular must be checked on this point, otherwise the judgement of the deceleration point shows an “overshoot then pull-back” behaviour. The matching method is given in response time / refresh rate and the control cycle. On a fieldbus architecture, EtherCAT distributed clocks let the three axes be sampled at the same instant, which is convenient for trajectory checking. See Series 197 EtherCAT and the configuration in the practical fieldbus guide.

Installation and vibration: the most typical site pitfall on take-out robots

Take-out robots are lightweight. At high-speed start and stop the frame and beam exhibit elastic deformation and residual vibration. If the magnet bracket is not rigid enough, the reading fluctuates strongly with acceleration and deceleration, and this is easily misread as a sensor-accuracy problem. The order of treatment should be: first stiffen the bracket and shorten the cantilever, then examine the feedback itself. Mount the rod parallel to the direction of motion and leave allowance at both ends of the full stroke to stay clear of the dead zones (see effective range and dead zones). An injection-molding shop has oil mist and mould-release spray; a protection rating of IP65 or better is recommended (see IP protection ratings). For profile-type products see Series 18 profile type.

Common ground with robot external axes

A take-out robot and an industrial-robot seventh axis are highly consistent in “long-stroke linear positioning + absolute position + high cycle rate”. Installation and synchronisation experience can be borrowed in both directions; see industrial robots: seventh-axis and vertical-axis position. The selection framework for hydraulically driven auxiliary clamp axes is given in why hydraulic cylinder position control uses magnetostrictive sensing.