Industrial Robots: Seventh-Axis and Vertical Axis Position

The seventh axis of an industrial robot is closed-looped by a servo motor and encoder. The place for an external linear displacement sensor is on external axes and auxiliary mechanisms: seventh-axis floor tracks, lift columns, gantry beams, and pneumatically or hydraulically driven fixture and tooling axes. These mechanisms share a long transmission chain (rack-and-pinion, chain, ballscrew), an encoder that only sees the motor end, and a need to home after power loss. Fitting a magnetostrictive displacement sensor reads the absolute position at the actuator end directly, eliminating transmission error and removing the homing sequence.

Why an external axis needs independent displacement feedback

On a rack-and-pinion seventh axis, error sources include rack-joint error, backlash, carriage guide clearance and thermally induced rail elongation. None of these is visible to the motor encoder: the robot believes it has travelled 3000 mm when the actual deviation may be millimetre-scale. With displacement feedback that measures the actuator end directly, the controller can apply position correction or use the channel as an independent check. Because the output is absolute position, the carriage location can be confirmed on power-up without homing (see absolute vs. incremental position). This matters particularly on lines where several robots share a track, or where there are interference zones at both ends of the floor track.

External-axis measuring-point layout

Matching cycle time and refresh rate

Robot lines run to a tight cycle; external axes often move at relatively high speed. If the displacement-feedback refresh rate is slower than the control cycle, the controller uses stale data between updates, which is equivalent to introducing pure lag. In the high-speed section this appears as positioning overshoot or a small rebound at stop. The refresh rate of a magnetostrictive sensor lengthens as measuring range increases (about 0.5 ms to 5 ms depending on model and range), so a long-stroke floor track in particular must be checked on this point. The matching method is given in matching response time / refresh rate to the control cycle. For multi-axis synchronous sampling, an EtherCAT model with distributed clocks is recommended (Series 197 EtherCAT); configuration is covered in the practical fieldbus guide.

Installation: three details on a long-span floor track

First, rod support: a rigid rod several metres long must be supported in sections. Support seats should constrain only the radial direction and allow slight longitudinal movement, otherwise thermal expansion will bow the rod. Second, magnet-bracket rigidity: the bracket travels with the carriage; insufficient rigidity produces elastic swing during acceleration and deceleration, and the reading fluctuates with acceleration. Third, dust and impact protection: the floor track sits near ground level, so it collects dust and is exposed to forklift and trolley knocks; a guard is required. For profile-type products see Series 18 profile type and Series 19P profile type. General installation practice is covered in installing and commissioning in practice.

Typical coordinated scenes

Coordination between a robot and peripheral equipment often needs position interlocking: before a take-out arm enters the mould area the platen position must be confirmed (see positioning injection molding take-out robots); at a welding station the positioner and fixture must be confirmed in position (see positioning for laser cutting and welding platforms). The selection framework for hydraulically driven auxiliary axes is given in why hydraulic cylinder position control uses magnetostrictive sensing.