Safe Motion Monitoring

Motion is described through the kinematic variables acceleration, speed and distance. As far as potential hazards are concerned, torques and forces also play a key role. The above variables are covered by the safety functions listed in the standard EN/IEC 61800-5-2. The implementation of safety-related monitoring is heavily dependent on the sensor technology used within the system. The sensor technology used within the drive technology is generally not safety-related and must be monitored for errors. For example, a critical status would occur if the rotary encoder was unable to supply a signal due to a defect, while power is applied to the motor and it is accelerating.

Moved axes in safety-related applications need redundant positional information in order to carry out relevant  safety functions. There are various ways to obtain independent position values: One possibility is to detect the defect through a second encoder. In this case, a safe component would have to monitor both encoders and guarantee that the plant is switched to a safe condition if an error occurs. Sometimes the advantage of this solution is that the two encoder systems detect the movement at different points on the machine and so can  detect defective mechanical transmission elements.

Rotary encoders generally have several signal tracks, enabling them to detect direction or defined positions within a revolution, for example. These signals can also be consulted for feasibility tests, so that a second encoder system is not required. However, this is not a universal dual-channel structure as the movement is recorded from a shaft or lens. Dual encoder systems are also now available on the market. Such systems are suitable for functions such as safe absolute position. With a strict, diverse, dual-channel design it is even possible to achieve SIL 3 in accordance with EN/IEC 61508. In addition to an optical system a magnetic sensing system may also be used, for example. In terms of costs, however, an increase by a factor of two to three is to be expected compared with a non-safety-related encoder system.

Multi-turn encoders offer a more economical solution; they set their separate multi-turn and single-turn tracks in proportion and can therefore detect errors. In this case, safety-related pre-processing takes place within the  encoder system itself. Another option is to use motor signals: by recording voltages and/or currents,  calculations can be used to indicate the mechanical movement of the motor. A comparison with the encoder signals will uncover any dangerous failures.

Safe Interaction Between Man and Machine

Modern safety and control concept leads to greater productivity

Faster, more flexible, safer – with this objective in mind, the level of automation on plant and machinery continues to advance in all industries. Both safety and control technology take these high requirements into account. If necessary, powerful components can be combined to form complete systems, which guarantee safety on the one hand, and can increase productivity on the other. If the user can adapt technology to suit his requirements, then he has a cost-effective, tailor-made solution.

In many industries, packing is still associated with manual work. Where man and machine work together, the risk of injury must be minimized to the greatest possible extent. Safety devices used to be viewed as obstructive and inconvenient, but today the opposite is true: modern safety and control concepts make handling easier, and plant and machinery more productive.

Faster thanks to the new safety concept
The semi-automatic vacuum-packing machine feeds in the bottom foil via a roller; it is then transported via a chain drive. The foil is brought up to temperature in the forming chamber; a special compressed air/forming procedure and the appropriate tool are used to create the desired tray shape for each packaged product. Staff use both hands to insert grilled sausages into the tray-shaped plastic film, six pieces per row and per pack, before the conveyor moves on and another set emerges from the forming station. After the filling station comes the sealing station: The top film seal is fed through a second roller; a vacuum pump extracts the air and thereby the oxygen. This slows down the activity of the micro-organisms, while the inflow of nitrogen prevents the sausage slices from sticking together. Finally, the top and bottom film layers are impulse sealed under pressure in the sealing chamber. The product packaging is now secure and durable. At the machine outfeed, longitudinal and transverse cutters separate the packaged product into individual trays, which are then fed into transport crates via conveyor. Two coils wind up the residual film cut from both sides.

Sprinter is the name of the latest development from Komet, manufacturer of semi and fully automatic vacuum packaging machines. The company is based in Plochingen, Germany, and has its roots in the butchers’ trade. It’s in this field in particular that Komet’s table-top units are used, as they provide an airtight seal for everything that needs to be kept fresh and preserved. Even in the butchers’ industry there appears to be an irresistible trend away from the skilled producer and towards industrial production; customers increasingly ask for ready-packed goods, so Komet is increasingly focusing on semi and fully automatic machinery. Flexible and quick to convert, they can pack items in up to eight parallel trays, depending on the packaged product and the size, with or without automatic infeed. Even non-food goods can be packaged and sealed efficiently using automated packaging machines.
Another reason why the machine is called Sprint is because, in comparison to the previous model Quickvac 2000, it can convey products to their packaging more quickly, using a less complicated procedure. That’s because of a modern safety and control concept, which Pilz developed for Komet.

Designed for maximum compatibility: sensor technology, evaluation device and visualization on the Sprinter, the multifunctional safety system PNOZmulti configures simpler, safer interaction between man and machine. The safety system has a modular structure and can be used flexibly to suit the application. Instead of wiring, the user configures the needed functions simply, in a configuration tool. This saves time, space and money because there is no longer any complex individual wiring and all the safety-related functions are located in one housing. PNOZmulti is cost effective when monitoring four or more safety functions and can also undertake standard control tasks.

By comparison the safety devices on its predecessor were simplistic and mainly based on individually wired switches and contactors. Large covers safeguarded wide-ranging potential danger zones such as the forming and sealing stations via switch contacts. The infeed area in between was narrow and only allowed a one-up arrangement.

On the Sprinter, the cover on the forming station is reduced to a minimum. Instead, an advance security slide ensures that it is impossible to reach into either the forming or the sealing station from the filling station. If the slide switch is not closed, the safety system will prevent the bottom film from being formed and stop pressing and heat-sealing in the sealing station. Propulsion drives and valves are switched off; the central PMI operator terminal (Pilz Machine Interface) receives an error message, stating the cause and the source.

Magnetic safety switches, PSENmag, monitor the mobile protective hood over the sealing station, the longitudinal and transverse cutters, and the lower positions of the lift cylinder plus the security slide on the forming/sealing station. The PNOZmulti also monitors the E-STOP and the enablers that control the heaters and valves (pneumatic + hydraulic). The risk of injury from the packaging machine, therefore, is kept to a minimum, both during operation and in setup mode. “With PNOZmulti the machine has a flexible safety concept which is really easy to adapt to our requirements. Now the infeed area has a three-up arrangement, so assembly is quicker and easier, plus the machine is more productive“, says Harald Janke, Technical Manager at Komet.

Solution for safety and standard
A modern control solution also fell within the scope of the cooperation between Komet and Pilz. The standard control technology on the Sprinter also comes in useful for automatic operation and setup mode. “People at Komet were unhappy with the existing control solution and were looking for an efficient alternative”, recalls Ralf Kessler, Applications Engineer at Pilz. Now they are using a PMI operator terminal in conjunction with a Profibus Master. A soft PLC runs under the Windows CE operating system. The standard I/O modules are logged and controlled via the PSSuniversal. The Pilz centralized control platform covers the whole I/O periphery as one system and is universally applicable. Users have three options: pure safety applications, combined safety and standard applications or exclusively conventional/standard control functions. With the grey, standard head modules it is possible to use analog and digital standard inputs and outputs on the fieldbus systems, without a functional connection to the safety technology.
On the Sprinter, the head module of the PSSuniversal is also connected to the PMI via Profibus-DP, just like the PNOZmulti. Due to the system’s modular design, the PSSuniversal can be individually adapted specifically to meet the respective requirement. If any adaptations are required, modules can simply be expanded or exchanged.

Benefitting from expertise
Pilz was also contracted to complete the electrical design, prepare the circuit diagrams and carry out the programming, including commissioning.
“As a small company with around 30 staff we are reliant on competent, reliable partners for our electrical design and programming; we need partners who know what’s happening and know what’s important to us”, says Janke, “Here we found Pilz to be a suitable partner, their technical advice was superb and the collaboration with the Pilz staff ran smoothly.”

The competence of Komet in terms of automation technology, combined with Pilz’s expertise in matters of control technology and safety, jointly produced a new vacuum packing machine that operates more productively and is therefore of greater value.


Pilz is a global company, as such, not all standards and information will be applicable worldwide. Please check with your local Pilz office to ensure your factory and/or machines meet local standards.

Q. What challenges or hurdles still need to be addressed with safe motion.

A. The standards provide the performance criteria for more sophisticated applications, but there are still many opportunities to implement the more complex applications that are now permitted. In robotics, the most challenging applications involves systems that continue to operate in the present of operators. The collaborative operating mode of maintaining a safe distance between the robotic arm and the operator is yet to be realized. Also, yet to be achieved, is the mode of operation in which the robotic arm force, torque and momentum is limited to inherently safe levels.

The advantage of safe motion, is that it allows the user a closer proximity to the actual process. With safe motion (whether it's safe speed, position or torque), if you can guarantee that the robot/drive is doing exactly what it's intended to (ie. working in a restricted space, at a restricted speed or is at stop), you can reduce and even eliminated your "safe distance" requirements. This would allow the operator to essentially work hand-in-hand with the motion component, increasing production time, reducing floor space and giving the operator a more friendly & ergonomic working environment.

Q. How do safe motion systems work? What role does advance motion control technology play?

A. Reliability theory has been applied to adjustable speed drive technology to define the reliability required for safety applications. There is an acceptance in industry for concept of integrity levels. Higher CPU power, reliability of components and a high level of diagnostics to detect dangerous faults before a failure can prevent a safety function from occurring has allowed these functions to be realized. Application of redundancy and diversity to complex control systems provides a means to predictable and acceptable failure rates.

What is the concept of safe motion? How does it apply to robotics?

Safe motion is the capability of providing motion control functions at an integrity level high enough to reliably provide risk reduction in safe dependent applications. An integrity level is the measure of the predicted failure rate of a control function. Risk, the possibility of harm in relationship to the potential degree of severity, must be reduced to a tolerable level in a safety application. A given integrity level ensures the possibility of harm occurring is sufficiently reduced to be tolerable for a safety related function. These integrity levels and design requirements are now defined in IEC 61800.

Safe motion extends the safeguarding of hazardous motion beyond the traditional dropping of power to the motors when personnel are exposed to a hazard. With safe motion, these are some of the new safe functions available by allowing power to continue to flow to the drives and to the motors:

• Safe torque off
• Safe controlled deceleration to a stop
• Safe operational stop (safe stand still)
• Safely limiting speed or torque
• Monitoring a position or maintaining a speed 

Protective measures implemented in traditional control systems such as reduced speed and hold-to-run functions can be further enhanced through this higher integrity.

In robotics, new applications are now available to the user. The international standard ISO 10218, for robotic safeguarding, is defining new applications for safe motion.

Setting safety related axis limits and defining safety zones in three dimensional spaces is allowing easier reduction of the restricted space of a robot to provide maximum clearance while minimizing the work cell.

ISO 10218 allows for collaborative operation between the robot and the human. In this application the robot may be allowed to stay in automatic, with power available to the motors and possibly continue its motion. The robot may safely limit its speed and if necessary top in a safe standstill mode (safe operational stop) while the operator works on a part in the robots gripper. A safe operational stop is characterized by the halting of motion, but keep torque output on the motors to resisting external forces. Coupled with sophisticated presence sensing technology, all the variations of this application have not been thought of yet.