Encoders Integrate Safety Technology

New functional safety incremental encoders from Pepperl+Fuchs use a sine/cosine output interface to measure speed, rather than traditional square waves, to create a decentralized safety solution for use with safety speed monitoring PLCs and relays. 

The RVS58S encoder is the first and only ISO13849 certified SinCos encoder specifically designed for use in safety aligned systems up to SIL3 (IEC 61508, IEC62061), performance level "e" (IEC 13849). The encoders are also suitable as a motor feedback system for safe drives in accordance with IEC 61800-5-2. Functional safety requirements like Safe Limited Speed (SLS), Safe Torque Off (STO), Safe Operating Stop (SOS), Safe Stop (SS1, SS2), Safe Direction (SDI) and further safety functionality can be realized without redundant sensors.

Because the SinCos interface is compatible with existing safety PLC products, safety controllers that implement cable break detection technology and verification checks on the input signals can achieve the highest degree of safety. The rotary encoders also provide internal self diagnostic features to verify the calculated output, along with additional temperature and voltage checking safe diagnostic features.

According to David Rubinski, a product manager for Pepperl+Fuchs, sinusoidal feedback provides continuous wave forms and the technology offers a mechanism to detect wire breaks, a key issue and limitation with incremental encoders.

"When an encoder is functioning correctly and there is no wire break, the safety device connected to the encoder can look at the sine-cosine waves and calculate whether or not the signal is genuine, uninterrupted and reliable," says Rubinski. "Since sine squared plus cosine squared equals one, the connected device can calculate whether or not the signal is functionally safe." 

Since the connected system is a safe device which can calculate and verify that the two signals are proper, if there is anything wrong with the signal, the controller can determine that condition and take appropriate action. The big problem with using standard incremental encoders is that there are no check mechanisms because at zero speed the square wave signal could be either high or low. But with a SinCos encoder, the outputs can still be verified to determine if the signals are proper at zero speed.

The big benefit for customers is a system that offers cost and time-savings by allowing for higher machine availability and productivity. The encoders can also be directly attached to a motor due to temperature resistance up to 115C.

The RVS58S encoders offer 1024 or 2048 signal periods and are thermally stabilized for high resolution interpolation. They operate at 5V dc (±5 percent) and deliver a 1 Vpp output signal up to a maximum output frequency of 200 kHz in operating temperatures ranging from -20 to 80C.  They are rated to handle axial shaft loads of 40N at a maximum of 6,000 RPM or 10N at a maximum of 12,000 RPM. Maximum radial shaft loads are 60N at 6,000 RPM or 20N at 12,000 RPM.

Targeted applications for this new safe encoder technology include motor and drive speed monitoring, elevators, theater stage equipment, cranes, storage robots and printing.