Keep distance! — positioning of AOPD according to ISO 13855

An Active Opto-electronic Protective Device (AOPD), commonly referred to as an optical curtain or scanner, can be used to protect humans from hazardous machine components in accordance with the ISO 13855 standard, harmonized with the Machinery Directive, which ensures compliance with the essential safety requirements for machinery. The article discusses methods of determining the required distance between the AOPD and the hazardous elements, taking into account the device’s properties such as size, resolution, and the direction of approach by humans (orthogonal or parallel to the detection zone).

What is what

For the purposes of this discussion, an AOPD is synonymous with a detection zone, which is a flat geometric shape (in the case of curtains, it will always be a rectangle or several rectangles) scanned by light beams. It has the property of detecting the presence of any object within it that is not smaller than the so-called detection capability, d. The scanner differs from the curtain only in that the beams radiate from a central point, so the distance between them will increase linearly with the distance from the scanner, whereas in the curtain, the beams are parallel. We are disregarding any other differences here, which otherwise have a significant impact on the device’s applicability in a specific case.

The direction of approach of a person can be parallel or orthogonal to the detection zone. It can also be at any other angle, of course, but the standard distinguishes only these two cases. If the angle between the direction of approach of a person and the detection plane is less than 30°, the access should be regarded as parallel; otherwise — as orthogonal.

AOPD is not a physical barrier; its operation is based on sending a stop signal to the machine after detecting the approach of an object (implicitly a human). For such protection to be effective, detection must occur at a sufficient distance for the machine (i.e., its hazardous actions) to come to a stop in time.

Preventing the machine from starting while a person is within the detection area will be discussed in a separate article.

Basic formulas

Knowing the approach speed of a person, K, it is possible to determine the minimum distance, S, that the person covers in a time not shorter than the overall system response time, T:

S = K × T + C,

where C is the distance of possible intrusion before detection (which will be discussed in individual cases).

NB. In the upcoming edition of ISO 13855 standard, S will be referred to as the separating distance.

Overall response time T refers to the total time from the occurrence of an AOPD intrusion (not from the moment when the stop signal is sent by the AOPD) to the cessation of hazardous movements. It is therefore the sum of the response time of the protective device, t₁ and the stopping time of the machine, t₂:

T = t₁ + t₂.

NB. In the upcoming version of ISO 13855, the above formula has expanded to the form T = ti + tl + to + td + tr + tm + tf — someone really wanted to demonstrate their knowledge of the subject.

For the speed K, the standard proposes two values — 1.6 m/s or 2 m/s, with the second value applicable only for short distances (up to 0.5 m). These values are not calculated or based on anthropological measurements, but they have been adopted and continue to be used out of habit. However, since the norm is harmonized with the machinery directive, it is advisable to adhere to these values.

At a design stage, you can try to estimate the time T by considering the control system properties (t₁) and the inertia of the machine and brake efficiency (t₂). However, the reliable results can be obtained through measuring the actual machine. The standard recommends conducting at least 10 measurements and calculating the value of time T based on them:

Tₑ = E + 3 σ,

where E represents the mean of the measured values (T₁, T₂, …, Tₙ), and σ represents the standard deviation. Statistically, for a normal distribution, 99.7% of the population has a value not greater than Tₑ.

If any of the measured values is greater than Tₑ, the maximum value should be taken into account in the calculations. Otherwise, Tₑ should be used:

T = max(Tₑ, T₁, T₂, …, Tₙ).

Orthogonally through

The basic configuration of an optical curtain is to use it in place of a interlocking guard, meaning that the detection zone covers the opening. Depending on the detection threshold of the AOPD, it is possible (in theory) to insert fingers or even an entire arm between the light beams without the device detecting it.

detection capability	undetected intrusion distance
d ≤ 14 mm	C = 0
14 < d ≤ 40 mm	C = 8 × (d — 14)
d > 40 mm	C = 850 mm

NB. In the upcoming version of the ISO 13855, for a detection threshold in the range of 20 < d ≤ 55 mm, the intrusion depth C is calculated using the formula C = 3.4 × (d — 7).

It should also be added that for d > 70 mm, we do not refer to AOPD as optical curtains, but rather as optical barriers. For barriers, the distance between the optical beams must be d ≤ 400 mm, because at greater distances, according to the standard, it is possible to pass between the beams with the whole body.

In the case of scanners, the detection threshold should be taken at the farthest point from the scanner.

Having calculated the values of T and C, we assume K = 2 m/s and substitute everything into the formula S = K × T + C. If S > 50 cm, we can assume K = 1.6 m/s and calculate smaller values of S, but not less than 50 cm.

Regardless of the calculation results, S cannot be less than 10 cm.

Orthogonally over

The upper edge of the AOPD detection zone must be positioned at a height of b ≥ 90 cm to prevent it from being crossed (according to the standard). However, it will still be possible to reach above the AOPD, with less possibility the higher the height b. The standard defines this relationship in the form of a table, where for a given height of the protective field’s center (b) and for a given height (a) at which the hazardous element is located, the horizontal penetration distances C are provided (referred to here as Cro, short for Reach Over).

In the table below, only the maximum values for a given height of the AOPD are considered, corresponding to the hazardous element located at the same height as the upper edge of the detection zone (i.e., a ≈ b).

b [mm]	max C [mm]
1000	1200
1200	1100
1300	900
1400	850
1600	750
1800	550
2000	400
2200	250
2400	100
2600	0

For values not included in the table, the less favourable value from the neighbouring values should be chosen. For example, if b = 150 cm, C should be assumed as 85 cm.
If the depth of penetration C through the AOPD detection zone is different from the reaching distance over it, the larger of these values should be taken into account for the calculations of S.

Parallelly (over)

Everything as before, but C depends on the height H of the detection zone above the floor:^[1]vertical access can be encountered when the curtain is horizontal, for example, protecting a horizontal opening in the enclosure; it is difficult, however, to imagine parallel access when the … Continue reading

C = 1200 [mm] — 0.4 H,

where H ≤ 300 mm (or H ≤ 200 mm if children may have access to the machine), otherwise full-body access under the detection zone is possible.

In addition, the height of the detection zone is related to the detection threshold of the AOPD.

H ≥ 15 (d — 50 [mm]).

As a result, it follows that d ≤ H/15 + 50 mm, which means d ≤ 70 mm (because H ≤ 300). This is where the “range” of the scanners comes from — it is the distance at which d = 70 mm.
The detection zone must have a minimum width to prevent it from being crossed — similar to the minimum width of pressure-sensitive mats (7.1:2). The required width of the AOPD should be significantly smaller than that of the mats if H > 0, but the standard does not take this into account, so we need to rely o the value determined for the mats, i.e., 75 cm.

Limits

Certainly, there are people who can move faster than 2 m/s, and many individuals can jump higher than 90 cm.^[2]the world record being 245 cm Harmonized standards, including ISO 13855, do not specify requirements for absolute safety but rather for the level of safety required by the directive. If we install a curtain according to the standard, but someone like Chuck Norris appears, they may have an accident, but we can confidently write in the accident report that „the accident occurred in compliance with regulations”.^[3]as Mr. Jan Przybylski used to say, who introduced me to the world of occupational health and safety many years ago

The standard does not directly define the intrusion distances between the optical beam and a fixed obstacle (such as the floor), but it suggests (6.3:10) values similar to the reach distances between beams. However, it is clear that the physical obstacle allows for greater intrusion distances, as one can press against it. Therefore, it is better (safer) to use the reach distances through openings specified in ISO 13857. This will result in an overestimation, but at least we will remain on the safe side.

Lastly, it is important not to apply the standard blindly. In one facility, experts recommended installing a light curtain at a distance of 12 m (twelve metres!) from the machine because that’s what the formula gave them.

Image by wayhomestudio on Freepik. Translated with Aria.