Definition of safe motion

Safe drive functions have recently made their mark on standards, products and applications and today can be considered as state of the art. They are part of the functional safety of plant and machinery and, as measures that boost productivity, are increasingly gaining ground in the market. The protection of machinery and equipment is also increasing in importance alongside personal protection.

When you examine the application of the failsafe principle within classic safety functions, initiation of the safety function causes the outputs to shut down, and this is called a “safe condition”. If safe drive functions are used, an application may look like this: When a safety gate is opened, the motor is braked safely with a defined ramp and then remains at standstill under active control. The motor will then move in jog mode at safely reduced speed. In other words: if static detection zone monitoring has been violated, production can continue at a reduced number of cycles and with safely monitored movements.

What this simple example illustrates is the transition from static to dynamic safety. Dynamic means something different in the various disciplines. In safety technology, dynamic is understood to be the ability to adapt the safety functions to the changing detection zones. The functional safety requirements for variable speed drives specified in EN/IEC 61800-5-2 open up new horizons on this issue.

The main requirements of safe drive systems in terms of dynamic safety are:

• Safe monitoring of kinematic variables such as acceleration, speed, distance, for example
• Short reaction times to reduce stopping distances
• Variable limit values, which can be adapted to suit the runtime

Drive-integrated safety technology, fast, safe drive buses, high-performance programmable safety systems and safe camera systems are all products suitable for high-end safety solutions. The term “safe motion” is interpreted differently, depending on your perspective. Drive manufacturers generally understand safe motion to be drive-integrated safety, whereas control manufacturers associate it with external solutions. Looking at the issue analytically we can establish that the term “safe motion” only refers in the first instance to the implementation of a safe movement.

Application example of a modular machine design

Plant and machinery are becoming increasingly modular. This means that they are being segregated into mechatronic units with separate functions. In a concept such as this, the electrical engineering follows the mechanical structure of the machine, bringing wide-ranging benefi ts. Once the machine modules have been developed they can be reused in various machines, which ultimately reduces the development effort. Modules can also be manufactured separately and joined together only during final assembly. What's more, modules can be developed in isolation from each other, so tasks can be run in parallel, saving time during development.

This type of engineering follows the building-block principle and enables customized solutions to be implemented at lower cost. Current fieldbus systems prevent this modular approach, as they are mainly based on a centralized master/slave approach. In safety technology in particular, one central instance is usually available: the master.
The publisher/subscriber communication principle applied universally on SafetyNET p does not use a central instance, thereby enabling a modular machine design.