Risk Assessment

Note: The information given below is a guide only. ST Robotics takes no responsibility for any decisions made from or accidents which result from or in spite of the advice given.

There are two concepts to consider: hazard, which is the robot or robot fingers or the product etc. and risk, which is the probability of someone being harmed by the hazard.

Because stepping motors raise more torque at low speeds a substantial low speed force can be brought to bear especially on fingers, which may become trapped under or between axes. Robot end effectors typically have sharp edges or fingers made of thin metal, which can cause injury at the low speed high forces or at the higher speeds.

The biggest risk comes from the element of surprise. If the system is active and receives a command from a supervising scheduler or a signal from an associated machine the robot will appear to move unpredictably. A human being caught in the way can receive injury.

Safety measures

  • Where possible a robot system should be guarded. Any gate in the guarding can be fitted with a switch, which is connected to the controller stop circuit.
  • If physical guards cannot be provided then light curtains are easily connected to the robot controller
  • Where guarding is not appropriate and bench-top robots work closely with human workers interlocks should be provided. For example if the user has access to the workspace then he/she should be required to press a switch or keyboard key after clearing the area.
  • A good rule is that the robot should not be allowed to move outside an area designated by the edge of the bench on which it is mounted.
  • As an additional precaution the working area should be marked out with painted lines or black/yellow striped tape. This is known as an awareness barrier.
  • Statistically the highest incidence of contention between human and robot is when both are accessing the same object. End effectors often have sharp edges, which can cause injury. This hazard can be minimized by fitting a collision sensor.
  • Due diligence: document the hazards and risks you identify and what measures you are taking to protect personnel from those hazards. This can be a reasonable defense in a court of law, depending on your country.
  • Below you will find a form with which to do your own risk assessment of the robot in your application. The form enables you to identify the hazards, the risks and ways of minimizing the risks.

    If you are really serious about being bomb-proof you should read the ANSI standard ANSI R15.06.
    If you are in the USA you should read this link:
    http://www.osha.gov/dts/osta/otm/otm_iv/otm_iv_4.html
    Also
    http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=DIRECTIVES&p_id=1703
    This is a link to the OSHA directive of 1987 but still very relevant. It is actually clear and concise and well worth reading before you use the robot, whether in USA or elsewhere in the world. It is in many ways superior to EU regulations, for example in the EU robots are covered by the machine directive in which the power is shut off in an emergency. As the OSHA directive rightly states:
    Dangerous robot movement is arrested by dynamic braking systems rather than simple power cut-off. Such brakes will counteract the effects of robot arm inertia. Cutting off all power could create hazards such as a sudden dropping of a robot's arm or flinging of a workpiece.
    This is entirely in harmony with our policy of decelerating the motors under software control when the E-stop is pressed or if the software has failed then the motors are energized in a stop position and any inertia is arrested with flywheel diodes.

    There is also a difference in philosophy between EU and USA: in the EU robots must be CE marked and satisfy safety criteria in the machine directive. In the USA the onus is on the use/employer and takes into account the whole system and workplace as well as products and materials. This makes more sense in our view. For example a robot carrying water poses less risk than a robot carrying acid.

    Compared to many industrial robots such as those found in the motor car industry ST robots are safer because they are less powerful. Moreover because the robot is stepper motor driven, once stalled the system raises an error and does not attempt further motion. However that should not stop you being absolutely sure you have taken every precaution possible to ensure the safety of personnel working with the robot. You should take in to account the likelihood of the hazard, how frequently it may appear and the seriouslness of any possible injury. You can:
    1. Name each task, including operator, maintenance, cleanup, and quality tasks.
    2. Identify the hazards associated with each task
    3. Identify the severity, exposure, and probability of the worker sustaining injury. There are criteria in ANSI R15.06
    4. Select applicable safeguards including sensors, barriers, awareness barriers, training etc.
    5. Assuming safeguards are installed validate their selection by repeating preceding steps to verify that all hazards have been addressed and that any remaining risks have been reduced to a reasonable level.

    The ANSI standard suggests that you evaluate such things as the frequency of exposure to the hazard and the severity of possible injury - see table A below:

    This form must be completed by a competent Assessor for any procedure using the robot system before an attempt is made at the procedure by any worker or visitor.

     

    Name and Status of the Assessor:                                                                   Date: 

    Activity being assessed:

     

     

     

    Known or expected hazards associated with the activity:

     

     

     

     

     

    The risk of injury and its severity likely to arise from these hazards:

     

     

     

    Who is at risk?

     

     

    Measure to be taken to reduce the level of risk:

     

     

     

     

     

    Training prerequisites:

     

     

    Level of risk remaining:

     

     

    Emergency action:

     

     

    References if any:

     

     

    Signature of Assessor

     

     

     

    Revision date