John Wenzler
Bosch Rexroth Corp.
Hoffman Estates, Ill.

Motion-based machines are running more axes at higher speeds and faster accelerations. This creates a demand for machines that move safely and that also satisfy regulations and competitive pressures. Once a primary safety mechanism, interlock switches are now obsolete because operators often need access with open guards for setup or clearing jams. When guards are open, motion must be controlled at safe limits to prevent injury to personnel and damage to equipment.

Consider a stacking sequence that is misordered or that has misaligned products. Those products that aren't restacked and reordered create a dangerous situation when a palletizing robot moves several hundred pounds back and forth. To fix this, an operator enters the protective guarding while the machine runs at limited torque and speed. The operator jogs one or two axes at slow speed and holds others at a safe standstill. Setup, inspecting workpieces, and clearing machine jams are addressed this way too.

Drive-integrated safety

When machines operate at high speed, safety devices must react quickly to faults, bringing equipment to a safe speed or complete stop. In the case of an axis accelerating at 1m/sec2 (achieving full speed in a fraction of a second), emergency safety devices must react in milliseconds.

One solution uses discrete, external circuits and controls to limit motion. However, this adds an extra safety layer, increasing design complexity and often resulting in a " nonstandard" solution. Furthermore, these "add-ons" aren't necessarily optimized for best performance.

Another solution to safety involves PLCs that replace conventional hard-wired relays. But, these safety PLCs have relatively slow scan rates and follow a lengthy path from sensor to PLC and back, delaying reaction times.

The best solution is placing the safety responsibility in the drive for autonomous monitoring. Autonomous monitoring allows for fast reaction times — without delays between the drive and controller. Fast reaction times correspond directly to reduced axis movement, while slow reaction times correspond to incorrect positioning moves.

A case in point is contact-based verification: By the time an operator in a protected zone responds to an error, a linear axis with roller ball spindles can move four to eight inches, and linear motors up to 31 in. However, by integrating safety control in the drive, errors are detected within milliseconds, limiting axis moves to one or two millimeters. This is up to 400 times faster than a controller-based solution.

Safety functions that integrate directly into intelligent drives also eliminate PLCs and CNCs and maintain the drive's footprint. Drive-based safety technology operates independently of supervisory control systems, and therefore, does not require contactors on the main power or motor power lines or additional external speed-monitoring devices. And, drives with safety onboard eliminate additional hardware and I/O, cutting wiring and installation costs.

Distributed intelligence is key

In a motion control system, processing power is classified as centralized or distributed — the difference being its location. With centralized intelligence, finite PLC processing power is divided among all axes. The processing power available for each axis diminishes as axes are added. With distributed intelligence, processing power is added each time an axis is added. The processing power available for each axis remains constant (because it resides in the axis itself). Thanks to advances in microelectronic design, intelligence can move outward to sensors, motors, drives, and other components. In simple cases, this intelligence is rudimentary. For example, sensors can consolidate data about temperature trends and send an alarm if thresholds are exceeded. In more complicated cases, an intelligent drive handles complex camming, monitoring, and other processor-intensive functions. Drive processing power provides all control and housekeeping functions of the driven axis and synchronizes other drives. The central PLC becomes a supervisor, and initiates recipes/job parameters, and communicates upward in the factory network.