What is safe motion monitoring?

Image credit: Pilz

Machines and equipment placed on the European market must conform with essential health and safety requirements of the Machinery Directive. The Directive specifies that manufacturers of machines and equipment must conduct a risk assessment and implement measures to eliminate or reduce hazards that pose a risk to people, equipment, product, or the environment.

These hazards can be mechanical, electrical, thermal, or from risks posed by falling or by exposure to radiation, among others. And they must be identified for all phases of the machine’s service life, from transport and installation to dismantling and disposal.

According to the Machinery Directive, risk reduction measures should be undertaken by integrating safety into the design and construction of the machine, which is often referred to as “safe design” or “inherently safe design.” If a risk cannot be eliminated through safe design, risk reduction can be achieved by implementing technical protective measures.

Risk reduction is an iterative process that can be achieved through safe design or by the implementation of technical protective measures.
Image credit: SICK

Technical protective measures include devices such as doors and light curtains that are part of a safety function; devices to monitor the machine’s function (speed or position, for example); or devices and measures to reduce emissions. When a technical protective measure’s effectiveness depends on the correct functioning of the control system, functional safety must be implemented.

Functional safety requires that a safety function be defined for each hazard. Some examples of safety functions include:

• preventing access to a hazardous point (either temporarily or permanently)
• preventing parts, substances, or radiation from being ejected or emitted
• initiating a stop
• avoiding an unexpected startup or preventing a start
• monitoring machine parameters (such as speed, position, temperature, or pressure).

Once a safety function has been defined for each hazard, a safety level must be assigned for each safety function. For manufacturing equipment, safety levels can be determined and assigned according to the harmonized standards EN/IEC 62061 or EN/ISO 13849-1.

Safe motion monitoring

The safety function that involves monitoring machine parameters — speed, position, travel, or acceleration — is often referred to as “safe motion monitoring.” Safe motion monitoring also includes the ability to detect potential errors in the encoder/feedback system and initiate the appropriate reaction to those potential errors.

Safe motion monitoring was traditionally achieved through the use of external devices, such as safety sensors and actuators, whose output can be combined to determine whether an operator is in danger. However, modern drives (servo amplifiers and variable frequency drives) and controllers typically include safety functions and software that reduce or eliminate the need for additional sensors and monitoring devices.

Examples of drive-based safety functions that can be invoked with safe motion monitoring include those that control stopping, braking, velocity, acceleration, position, and travel direction.

Safety functions related to motion monitoring often control stopping or braking, but they can also control velocity, acceleration, position, or direction of travel.
Image credit: PLCopen

Safe motion monitoring can also refer to monitoring the machine when the drive is brought to a standstill. In this case, the objective is to ensure the safety of personnel (especially operators and technicians) who may need to enter or work in a hazardous area of the machine. In this scenario, safe motion monitoring can allow an operator to intervene even when the machine is running, which reduces unintended downtime and lost productivity.