The increasing number of human-robot interactions makes it more important than ever to design machines with functional safety. Achieving functional safety demands careful consideration throughout the entire design process.

In our blog series, we discuss how to design a machine with functional safety, ratings, common safety functions and a trend for simplifying system design. First, let's review the steps to take when designing an automated machine with functional safety and apply them to an autonomous ground vehicle (AGV) as an example.

The process of designing functional safety into a machine is well-documented in standards like the Machinery Directive (2006942/EC). To ensure success, follow these steps:

Define specifications and modes of operation
Identify hazards
Assess risks
Reduce risks

Specifications and Modes of Operation

The first step is to define your machine’s required performance characteristics, such as its payload and speed. Additionally, determine aspects of the machine’s setup, runtime and maintenance. These specifications and modes of operation set the stage to identify the hazards to address with functional safety.

These are the key parameters for the AGV in our example:

Top speed of 2 m/s
Payload of up to 50 kg
Responds to a potential collision by limiting speed to 0.2 m/s when 3 meters from the point of impact, and comes to a controlled stop when 1 meter from the impact point if the object is still present.

Hazard Identification

Next, identify possible hazards. It is important to keep in mind that hazard identification should be performed with consideration for every phase in the machine’s lifecycle and not solely during standard operation.

Common hazards include:

Crushing
Severing
Puncturing
Burning
Noise
Entrapment
Entanglement

For our example AGV, the primary hazards are crushing and puncture wounds.

Risk Assessment

This step involves evaluating each hazard’s probability of occurring and its severity. Risk assessment tools can vary by industry and location, but the techniques involved are very similar. Regardless of the methods used, the goal of risk assessment is to quantify the machine’s original risk.

Risk Reduction

The final step is to reduce the original risk to within the range of acceptable residual risk. Your approach to risk reduction should follow the three-level method of implementing safe design, safety functions and information.

Risk reduction steps.

Safe Design

The first degree of risk reduction should be to incorporate safety into the overall design. However, keep in mind that this can detract from overall machine performance. Some interpretations of the process include using physical guarding around a machine to achieve a safe design, but this can compromise space and cost. Other approaches to a safe design could include speed reduction, torque-limited motors and smoothing and rounding surfaces.

In our example, designing the AGV and payload to feature rounded surfaces eliminates the risk of puncture wounds. However, the AGV still possesses high kinetic energy due to its high speed and large payload. So, the risk of crushing has not been reduced.

Safety Functions

The second level of risk reduction is to incorporate safety functions. Like safe design, safety functions can be an effective strategy to decrease a machine’s overall risk. However, safety functions can be considered less invasive to the design and don’t inherently detract from performance. Some safety functions include:

Safe limited speed (SLS)
Safe stop 2 (SS2)
Safe operating stop (SOS)

These safety functions work for the AGV design. SLS ensures that the AGV speed reduces when there is a potential collision with an obstacle and increases the chance of avoiding it. SS2 stops the AGV before impact. SOS ensures the AGV safely holds its position until the obstacle is cleared.

Information

Adding information features to the machine can also help lower risk to within a safe range. Some examples include adding warnings and directives to wear protective equipment. This information can be placed on the machine itself or at the entrance to its operating space.

Meeting Safety Ratings

After selecting safety functions to implement in your design, determine what ratings they must meet. Both PL (ISO 13849-1) and SIL (IEC 62061) ratings are applicable and a safety function should be rated to both standards. To determine the required PL and SIL rating to meet in the event a safety function fails, perform risk analysis based on:

Injury severity due to the hazard
Frequency and duration of hazard exposure
Probability of avoiding the hazard

Below is an example analysis showing how to arrive at a safety rating and its probability of dangerous failure per hour (PFHd) based on these considerations. The highlighted path shows the analysis for the AGV’s SLS, SS2 and SOS functions.

Higher ratings like PLe and SIL3 have a lower chance of danger in the event of failure and therefore offer greater risk reduction. Keep in mind that higher PL and SIL ratings place greater demands on system architecture, diagnostic coverage and the mean time to failure (MTTF) of components.

Example safety rating analysis.

Stay tuned for our next installment where we discuss common safety functions.

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