If employees have to enter the processing area of a machine, the drive axes must be set to a safe condition. If heavy loads are suspended from the vertical axes, they can fall down due to gravity and thus endanger personnel. To prevent this, the vertical axles are usually secured by brakes. However, dirt or mechanical wear can severely impair their effectiveness. It is therefore important to safely monitor the condition of the brakes and maintain their functionality. Due to the lack of normative requirements, the German Social Accident Insurance (DGUV) has explained the problem in the specialist information sheet no. 005, issue 09/2012. This describes recommendations for securing gravity-loaded vertical axes.

To date, designers and machine builders have generally relied on solutions based on a programmable safety controller. This controls the brakes via contactors and monitors the standstill during the brake test. This places special demands on the motor encoder and its installation. Attached to the motor shaft, it primarily detects its position and sends the actual values to the controller. The connection to the safety controller is usually made via analog 1 Vpp signals. Disadvantage: These solutions require special encoders, special adapters to lead out the analog signals for standstill detection, as well as cables that can transmit the analog signals without interference even over longer distances. In addition, there are standstill and speed monitors - all in all, cost-intensive equipment.

Another negative point is the fault exclusion required for the encoder attachment to the motor, which requires time-consuming repairs in the event of servicing. Another disadvantage is that the suitable encoders do not fit all motor types and do not offer the performance required by a powerful servo system.

The security package

With the SE6 safety module developed jointly by Stöber and Pilz, practicable solutions have been developed and implemented for almost all of the identified weak points. The versatile SD6 stand-alone drive controller from Stöber can be optionally equipped with the safety module. The drive controller is a single-axis controller that is available in four sizes with a rated output current of up to 85 amps and is primarily used in drive-based applications. This includes, for example, the synchronous operation of up to 32 axes in electronic gearboxes or flying saws without a superimposed motion controller. It controls linear and rotary synchronous servo motors as well as asynchronous motors. Thanks to its modular interface concept and extensive range of accessories, it can be configured to fit perfectly.

Design engineers and machine planners are often faced with the challenge of implementing highly automated and flexible production processes in which people, machines and systems must be protected at the same time. The SD6 drive controller also meets these requirements in safety-relevant applications up to SIL 3 in accordance with DIN EN 61800-5-2 or PL e (category 4) in accordance with DIN EN ISO 13849-1.

The SD6 drive controller in combination with the SE6 safety module is suitable for safety-relevant applications up to SIL 3, PL e (category 4).

© Stöber drive technology

A number of safety functions have been implemented so that the drive controller can master these requirements in a practical and reliable manner and intervene immediately in an emergency:

• 'Safe Torque Off' (STO) interrupts the power supply to the motor directly in the drive controller.
• The stop functions 'Safe Stop 1' (SS1) shuts down the drive in a controlled manner and only then interrupts the power supply to the motor.
• 'Safe Stop 2' (SS2) initiates a 'Safe Operating Stop' (SOS) after the controlled shutdown. The control functions of the drive are fully retained.
• The 'Safely limited speed' (SLS) ensures that the drive does not exceed a certain speed, while 'Safe speed range' (SSR) monitors this within a defined corridor.
• A 'Safely limited position' (SLP) ensures that the motor does not exceed predefined limit values here either.
• 'Safe direction of movement' (SDI) only allows a drive to move in one (defined) direction.
• 'Safely limited increment' (SLI) monitors the position within a previously defined range after activation.

The SS1 and SS2 functions can also be used to monitor the brake ramps, for example to bring an axis system to a controlled standstill. If a limit value is violated, the motor is safely stopped by the drive controller.

The module has also been equipped with the 'Safe monitoring' function for almost all safety functions. This feature simply monitors the drive and safely reports limit value violations to the higher-level safety controller instead of forcing a stop. This gives the machine manufacturer a great deal of freedom when reacting to faults - especially with synchronized drives.

In the case of safety functions such as SLS, interference pulses can be specifically suppressed by means of variable tolerance windows. As a result, the user benefits from higher availability: he can make better use of the limit values specified in the standards without false triggering occurring.

Safe brake management

The special feature of the new safety module is the integrated brake management of up to two brakes. This means that the SD6 meets the requirements of the German Social Accident Insurance for gravity-loaded vertical axles. The DGUV is the umbrella organization of the German Social Accident Insurance Institutions for the industrial sector (BG) and the accident insurance institution for the public sector. The development was based on the information sheet 'Gravity-loaded axles' published by the BG. "This describes the requirements for securing vertical axles in a practical manner, as there is currently no harmonized standard that deals with this topic," explains Markus Frei, Product Manager for drive controller accessories at Stöber Antriebstechnik.

Brake management in the SE6 includes the 'Safe Brake Control' (SBC) function, which ensures that the brakes are applied on demand. There is also the 'Safe Brake Test' (SBT) feature. This checks the defined braking torque on demand and detects deviations due to dirt or defects in the mechanics before the braking torque reaches a critical state. The prescribed test interval is also monitored. Depending on the application and requirements from the risk analysis, this can either be once in every production cycle or, for example, every eight hours at the start of a shift. If the holding torque of the brake is no longer present, the 'Loop in brakes' function is available in the drive controller, which takes into account the requirements of the brakes of Stöber motors. The system can then check again whether the required test torque can be maintained.

Because the SE6's brake management supports the control of up to two brakes, it covers all applications of the specialist area information sheet on vertical axes. "If the operator's whole body is under the load, the risk is very high. The DGUV therefore recommends redundant fall protection in accordance with DIN EN ISO 13849-1 both in automatic mode and during set-up," explains Frei.

Technical and economic advantages

The complete package SD6 with SE6 is technically and economically very interesting for machine builders. SD6 can be combined with synchronous servo, asynchronous, linear or torque motors. This gives the designer considerably more scope. They have a free choice of encoder. There is no need for coupling contactors, expensive cables, special adapters, standstill and speed monitors. In addition, brake management is independent of the brake type. The user can therefore set up a safe braking and holding system in accordance with DIN EN ISO 13849-1 up to category 4 - with manageable effort.

"Because the motor is monitored by the safety functions within the drive, our SD6 enables a very fast worst-case fault response of less than ten milliseconds," explains Frei. The integrated safety module can intervene directly in the axis movement and stop the drive if a limit value is exceeded or in the event of an emergency stop. "This works much faster than with an external speed monitor," explains Frei. "It can take up to 100 milliseconds for this to record and evaluate the required information and transmit the command to switch off to the controller via the safety controller. In the world of drive technology, that's an eternity." The safety module enables rapid shutdown and allows the designer to keep safety distances to a minimum.

The operator also benefits in the event of servicing: when replacing a device, the SD card with the stored safety configuration simply needs to be exchanged, the new controller started and the change confirmed at the touch of a button. The solution can be commissioned quickly using the DriveControlSuite project planning and commissioning software. The integrated PASmotion software supports the operator in creating the safety configuration with minimal effort.

Author:
Ulla Göransson, responsible for marketing at Stöber Antriebstechnik.

Important component of the systems business

Patrick Stöber: "At SPS IPC Drives 2018, the topic of 'safe brake management' in particular met with great interest."

© Stöber drive technology

What is behind the development of the SE6 safety module and how does Stöber intend to position itself in the future? Patrick Stöber, Managing Director at Stöber Antriebstechnik, provides the answers.

Mr. Stöber, can you explain the advantages of the new safety module in a few words?

Patrick Stöber: There are a whole range of advantages, many of which set it apart from competitor products. These include safe brake management for gravity-loaded vertical axes in accordance with DGUV requirements or encoder independence. Added to this is the achievable safety level SIL 3, PL e (Cat. 4) for all safety functions. As a rule, our competitors only offer this safety level for the 'STO' function. In addition, users are more flexible in the design of the machine thanks to manufacturer-independent interfaces. They can make the optimum choice of motors, encoders and cables. Another advantage: if a device needs to be replaced, users can continue production immediately by replacing the SD card without the hassle of re-parameterization.

What motivated you to develop the SE6 safety module?

Patrick Stöber: Today, machines and systems are becoming increasingly flexible and productive. At the same time, requirements are increasing to ensure that operating personnel are not put at risk. Good safety technology in our sense provides targeted protection in accordance with the normative specifications, offers fast response times, high availability and is simple, fast and uncomplicated to operate. That was our goal. With the SD6 drive controller with integrated SE6 safety module, we provide maximum safety, a wide range of functions at the highest safety level and more flexibility in the design and operation of the machine. This means that the requirements of the standards and the DGUV can be implemented safely, efficiently and economically.

You worked together with Pilz on this. How did this cooperation come about?

Patrick Stöber: We have been working with Pilz in various areas for many years. The name Pilz is synonymous with safety technology. Our expertise lies in drive technology. So it was only logical to tackle this project together. And there are also approaches for further cooperation in the future. If we did everything ourselves, we would no longer be able to survive in today's world. Networking between market participants and their core competencies is becoming more and more necessary. This is a trend that is also being reinforced by digitalization.

How is the new product integrated into your long-term strategy?

Patrick Stöber : Our origins lie in mechanical drive technology and were traditionally more component-oriented. We have also been at home in servo drive technology for many years. Thanks to our in-house expertise - in both mechanical and electrical drive technology - we want to focus even more on machine movement as a whole in the future and significantly expand our systems business. The integrated safety module SE6 is an important building block for this.

What opportunities do you expect this change to bring?

Patrick Stöber : The increased demands on productivity and the much-cited shortage of skilled workers are leading to a vacuum for many machine manufacturers. We are trying to compensate for this with our expertise. We offer users comprehensive advice for their machine movements. They receive a motion solution from us without needing a lot of resources themselves. Another advantage: customers and users have just one point of contact for both the mechanical and electrical drive components.