Safe stop functions

When considering safety on axes, the main factors are to prevent the axes from starting up unexpectedly and to shut down moving axes safely in the case of danger. The corresponding functions are summarized here under the heading of “Safe stop functions”.

Safe torque off (STO)
The power to the motor is safely removed, so that no further movement is possible. It is not necessary to monitor plant at a standstill. If an external force effect is to be anticipated, additional measures should be provided to safely prevent any potential movement (e.g. mechanical brakes). Classic examples are vertical axes or applications with high inertia. This safety function corresponds to a category 0 stop (uncontrolled stop) in accordance with IEC 60204-1. If the function is triggered during operation, the motor will run down in an uncontrolled manner, which is not desirable in practice. That is why this function is generally used as a
safe reset lock or in conjunction with the safety function SS1.

Modern servo amplifiers include an integrated safe shutdown path, so devices are now available that prevent unexpected start-up and shut down safely in the case of danger.


 

Safe stop 1 (SS1)
With safe stop 1 (SS1), defined motor braking is part of the safety function. When the motor is at standstill, the STO function is triggered. There are various options for implementing these requirements; the key factor is the dovetailing of safety technology and drive technology. This safety function corresponds to a category 1 stop ( controlled stop) in accordance with IEC 60204-1.




In many applications, drives cannot simply be shut down as they would then run down slowly, which could cause a hazard. Also, an uncontrolled run down of this type often takes considerably longer than controlled axis braking. The safe stop 1 function (SS1) monitors controlled braking of the axis directly within the servo amplifier. Once the set braking ramp has run its course, the drive is shut down safely. The reaction times are reduced compared with external monitoring solutions; as a result, in many cases the safety distances to the danger points can also be reduced. This provides a number of benefits, such as improved ergonomics for the plant operator, space savings due to the reduced distance between the guards and the danger points and, last but not least, cost savings.


Safe stop 2 (SS2)
With safe stop 2 (SS2), defined motor braking is again part of the safety function. When the motor is at standstill, a safe operating stop (SOS) is triggered. Unlike safe stop 1 (SS1), the motor at standstill is in closed loop operation. This means that the standstill position is held precisely, due to the active control loop. Again, there are several options for implementing these requirements. This safety function corresponds to a category 2 stop ( controlled stop) in accordance with IEC 60204-1.




So what are the benefits of the safe stop 2 (SS2) function? If the axes no longer need to be shut down at standstill, they will actively hold their current position, so the synchronization between axes and process is no longer lost. As a result, the axes can be restarted immediately at any time, which clearly increases plant availability. Here too, the drive-integrated function leads to shorter reaction times, thereby minimizing the risks. The monitoring functions’ response times have a direct influence on the potential channels available until a safety shutdown occurs. As the reaction times are used in the calculation of the safety distances, the benefits listed for the safe stop 1 function will also apply here.



 

Wind turbine safety increasingly important as industry expands

Wind energy has begun to lead the way in alternative energy solutions.  Manufacturers of wind turbines face challenges such as reducing downtimes and maintenance work.  Some of the demands placed on turbine components include extreme temperature conditions, vibration, oscillation and aggressive offshore air composition.

Keeping wind turbines safely operating is becoming increasingly important as more windmills are built and America begins to use wind as larger source of energy heading into the future.

In the event of a management failure or the failure of individual components, the configurable control system PNOZmulti from Pilz guarantees that the wind turbine is brought to a safe condition for both human and machine.  Some of the safe monitoring functions include: emergency stop pushbutton, master low voltage switch, oil pressure protection and hermetic protection for the medium voltage transformer, and pitch system.  Additional safe monitoring functions include: limit values for true power, vibration, generator speed, rotor speed and line transposition (monitoring of azimuth position) and signals from the fire detection system.

All the safety functions are created directly on the PC with the PNOZmulti Configurator and can be stored on a chip card and downloaded to the base unit.

By differentiating the reaction to various operational circumstances, the PNOZmulti can clearly reduce the load on mechanical wearing parts. A safe condition for both humans and wind turbine can be achieved by reducing the rotor speed and preventing all movements or by switching off hazardous voltages, provided these are not required for the safety function. With PNOZmulti, manufacturers can avoid unnecessary downtimes and thus increase the cost effectiveness of their wind turbines.

PNOZmulti is available in a coated version for use under extreme environmental conditions. This version can be used in extended temperature ranges, permits condensation and is resistant to aggressive air composition.  Also, the configurable control system PNOZmulti can communicate via all relevant fieldbus systems.  With PNOZmulti, applications can achieve up to Performance Level (PL) e of EN ISO 13849-1 and Safety Integrity Level (SIL) CL 3 of IEC 62061. PNOZmulti is certified worldwide.

Another benefit of the PNOZmulti is that the user program is password protected, enabling manipulation protection to be differentiated – with a range of access levels for maintenance staff and installation engineers.

Stanford Linear Accelerator Center- Laser Source Protection and Access Control

Stanford’s Linear Accelerator Center (SLAC) has built a safety and notification system for the variety of laser source equipment that is used at SLAC. The laser source safety configuration is designed with two major levels of protection; Access Control and Laser Source Protection. Pilz’s PNOZ Multi is used to monitor the status of the safety sensors and access control modules that are in place around the laser sources. The laser source itself is contained in a Laser Source area with entrances through various access doors and covers/flaps; these access points are all monitored using the Pilz PSENmag sensors. The PSENmag sensor status are monitored with a PNOZmulti, a Device Net Module is added to the system for annunciation of the safety system’s status. The access to the various laser sources and control areas while monitored by the PNOZmulti are used in conjunction with the Device Net Module, this allows those working in the Laser Source area to manage the status of access points to the laser. As part of the monitoring system, Pilz E-STOP pushbutton PIT is used to shutdown components during maintenance or configuration of the laser source or enclosure areas. The combination of safety products that SLAC has implemented thus far gives the several benefits from a safety design standpoint; the component nature of the system design itself, allows for a high degree of flexibility and reconfiguration allowing for the system to grow/change with minimal downtime. Additionally, the use of PNOZmulti monitors the amount of wiring involved in installation makes the safety system itself more economical. Finally, system status and safety diagnostics can be completed through use of a fieldbus as opposed to manual inspection, giving the system higher level of system integrity and accuracy. SLAC has plans to expand the safety system to create a more robust and efficient system for other areas of the SLAC research tools and equipment.

SLAC National Accelerator Laboratory is home to a two-mile linear accelerator—the longest in the world. Originally a particle physics research center, SLAC is now a multipurpose laboratory for astrophysics, photon science, accelerator and particle physics research. Six scientists have been awarded the Nobel Prize for work carried out at SLAC and the future of the laboratory promises to be just as extraordinary.