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What is a “Stop Category”?

“Stop Category” is a classification of how robot motion is stopped in a safe way. There are three different types.
See the topic "UR Robots Safety" below.
(See IEC 60204-1 or EN 60204-1 for more details)

What is ”Cat 3” or
”Category 3”?

The term “Category” should not be confused with the term “Stop Category”.

“Category” refers to the type of architecture used as basis for a certain “Performance Level”. A significant property of a “Category 3” architecture is that a single fault cannot lead to loss of the safety function.

 (See ISO 13849-1 for more details)

What is ”PLd” or
”Performance level d”?

A Performance Level (PL) is a discrete level used to specify the ability of safety-related parts of control systems to perform a safety functions under foreseeable conditions. PL=d is the second highest reliability classification, meaning that the safety function is extremely reliable. (See ISO 13849-1 for more details)

What is SIL 2?

SIL (Security Integrity Level), like Performance level, is a measure of how likely it is that a dangerous failure will occur in a system. SIL 2 roughly equivalent to PL d (performance level d).
(See IEC 62061 or EN 62061 for more details)

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What is a risk assessment?

A risk assessment is the overall process of identifying all risks and judging if they are reduced to an appropriate level. It includes both risk analysis and risk evaluation.
A risk assessment must be documented. See UR user manual.
(See ISO 12100 or EN ISO 12100 for more details)

What is a risk analysis?

A risk analysis is a combination of the specification of the limits of the robot installation, hazard identification and risk estimation.
(See ISO 12100 or EN ISO 12100 for more details)

What is a risk estimation?

Risk estimation is defining likely severity of harm and probability of its occurrence.
(See ISO 12100 or EN ISO 12100 for more details)

What is a risk evaluation?

A risk evaluation is the judgment, on the basis of risk analysis, of whether the risk reduction objectives have been achieved.

What is a residual risk?

A residual risk is the risk remaining after protective measures have been implemented.
(See ISO 12100 or EN ISO 12100 for more details)

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Is it required for robots to comply with ISO 10218-1?

It is required to comply with the laws and regulations in the country and/or the state that the robot is installed. Some countries require that robots comply with ISO 10218-1 or the equivalent national standard.  ISO 10218-1 (robot) & ISO 10218-2 (integration) have been adopted in Europe, USA (ANSI RIA R15.06), Canada (CA Z434), and various countries by reference (Brasil, Japan, Korea, China).
See UR user manual.

Do UR robots comply with
ISO 10218-1?

Yes – CB3 and e-Series robots have 3^rd party certification to EN ISO 10218-1, which is identical to ISO 10218-1. A Technical Specification with clarifications on the features described in ISO 10218-1 is published in February 15th 2016 under the name ISO/TS 15066. 

 Are UR robots designed for collaborative operation according to ISO 10218-1?

UR Robots comply with the requirements for collaborative operation defined in ISO 10218-1:2011, clause 5.10.5. (Power and Force Limiting). A risk assessment of the complete robot application is required.
See UR user manual.

What is the difference between ISO 10218-1 and ISO 10218-2?

- ISO 10218-1 is for robots. UR is the manufacturer of robots, which are required to comply with ISO 10218-1. A "naked" robot without an end-effector is considered partly completed machinery. As a result, robots are not allowed to be CE-marked.
- ISO 10218-2 is for robot systems and robot applications. The Robot systems and robot applications are required to be CE-marked.  This can be performed by the integrator, a third party consultant, a notified body or the end-user. As soon as a robot has an end-effector, it  is considered a complete machine.
- ANSI/RIA R15.06 is a national adoption that includes both ISO 10218-1 and -2, published in the United States of America.
- CAN/CSA-Z434 is a national adoption that includes both ISO 10218-1 and -2, published in Canada.

What is ISO/TS 15066, Technical Specification on Collaborative Robots? 

ISO/TS 15066 is NOT a standard, but a Technical Specification with additional guidance for collaborative robot applications published February 2016. It contains guidance on risk assessment for the integration of robots in collaborative applications. It also includes an informative annex with a research study on biomechanical thresholds.

What is ISO 13849?

These standards defines how a safety-related control system must be designed and implemented to achieve a specific performance level.
- ISO 13849-1 is titled Safety of machinery -- Safety-related parts of control systems - Part 1: General principles for design. It provides safety requirements and guidance on the principles for the design and integration of safety-related parts of control systems, including the design of safety-related software.
- ISO 13849-2 is titled Safety of machinery -- Safety-related parts of control systems - Part 2:  Validation. It specifies the procedures and conditions to be followed for the validation by analysis and testing of the specified safety functions, the category achieved, and the performance level achieved by the safety-related parts of a control system designed in accordance with ISO 13849-1.

What is IEC 62061?

IEC 62061 is roughly equivalent to ISO 13849-1. The two standards defines safety-performance-levels using the same principles, but are developed by two different international committees. SIL levels from IEC 62061 can be directly translated to Performance Levels in ISO 13849-1 and vice versa.

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Answer

What is a Stop Category?

“Stop Category” is a classification of  stopping. There are three different types:
  - Stop Category 0 : Stopping by immediate removal of power (to the robot manipulator). It is an uncontrolled stop, where the robot can deviate from the programmed path as each joint brakes as fast as possible. This stop is used if a safety-related limit is exceeded or in case of a fault in the safety-related parts of the control system.
  - Stop Category 1 : Stopping with power available to the robot to achieve the stop and then removal of power (to the robot manipulator) when the stop is achieved. It is a controlled stop, where the robot will continue along the programmed path. Power is removed as soon as the robot stands still.
  - Stop Category 2 : Stopping with power left available to the robot  manipulator. With UR robots, after a stop category 2, the safety-related control system monitors that the robot is in a standstill condition where it stays at the stop position.
(see IEC 60204-1 for descriptions of stop categories and see ISO 13850 for emergency stop details)

Which Stop Category is used for emergency stop in UR robots?

The UR robots emergency stop is designed as a Stop Category 1 stop.
See above.

Which Stop Category is used for safeguard stop in UR robots?

The UR robots safeguard stop is a Stop Category 2 stop. In ISO 10218-1 and ISO 10218-2, the safeguard stop is called a “protective stop”.  In comparison, UR robots use the term “protective stop” for non-safety related robot stops
See above.

What is the difference between emergency stop and safeguard stop?

 - Emergency stop functions are to be used for emergencies only. An emergency stop is  manually actuated by a person pressing the red emergency pushbutton that has a yellow background in accordance with ISO 13850. Use of the emergency stop should be for emergencies and therefore be infrequent and not part of a daily routine.
 - Safeguard stop is used to stop robot movement and then maintain a monitored standstill. Safeguard stop is typically triggered by protective devices (e.g. light curtains, guard interlocking devices, safety scanners, safety PLCs etc. Resuming from a safeguard stop can be automatic or requiring a manual reset.
Both stop functions are performance level d.   
See UR user manual, safety functions for details

What is the Machinery Directive 2006/42/EC?

The European Machinery Directive (MD) is one of many Directives forming laws in the European Union (EU), and its official number is "2006/42/EC". Some countries outside EU also implements these Directives. 

2006/42/EC can be downloaded free of charge and in different languages from the official European homepage:

http://eur-lex.europa.eu/legal-content/EN/TXT/?qid=1426848262724&uri=CELEX:32006L0042

NOTE: Typically it is easy to find Directives by searching the number on Google.com (E.g. "2006/42/EC" ).