Safety edge interface

Brief introduction and history

The local Makerspace in my region needed safety edge interfaces for an automatic bi-folding gate. There are lots of sufficient safety edge interface/controllers available to purchase, but they tend to be a bit expensive (for a valid reason) and I decided to make my own.

See rendered image of assembled safety edge interface PCB below:

Photos of assembled product is available under directory photos-public.

What is a "safety edge interface/controller"?

This safety edge interface monitors the status of two individual safety edge circuits. From google Gemini: A safety edge circuit (also called a pressure-sensitive edge or contact edge circuit) is a fail-safe electric sensing system designed to prevent crushing, shearing, or trapping hazards on automated moving equipment—such as industrial doors, gates, machine guards, and automated guided vehicles (AGVs).

See example of application in image below:

See simplified functional overview below:

If the interface/controller measures a resistance higher than the termination resistance, the safety edge circuit is considered broken. If the measured resistance is lower than the termination resistance, the edge is considered activated.

Important safety notice!!

If you are located within European Union, your automatic gate and equipment must follow part of the EU Machinery Directive (https://en.wikipedia.org/wiki/Machinery_Directive). There are likely other safety standards applicable as well. No effort has been made to make sure this safety edge controller conforms to any of the standards. A known required functionality is the requirement for monitoring and control of output relay status. This safety edge controller does NOT have active monitoring of the output relays. If the output relay fails welded shut (activated), your safety edge will not be functional.

Safety edge interface device information

Project design targets

• Easy to troubleshoot -> simple analog design (no programmable logic)
• Easy to repair -> commonly available electrical components selected and components placed easily accessible on printed circuit board.

Electrical schematics

See exported electrical schematics for a good overview of device full functionality. Electrical schematics and circuit board has been created with software Kicad 10.0.** (https://www.kicad.org/).

Bill of material (BOM) for printed circuit board

Information with LCSC part numbers (for low cost PCB assembly via JLCPCB) or TME part numbers or Digikey part numbers are included in the KiCad schematic under respective schematic symbol. Full BOM can be viewed via "Tools -> Bulk edit symbol fields" (see screenshot below) or exported via "Tools -> Generate Bill Of Materials".

My version of the printed circuit board was manufactured by JLCPCB. JLCPCB also assembled all "Basic" type (low assembly cost) components available in their store and remaining components were manually purchased and assembled by me in order to minimize the total project cost. Hence the reason for multiple separate distributors. Keep in mind that if you need to change - for example - a LED indicator to an indicator with different brightness, you might need to change the related current limiting resistor to a different value.

Printed circuit board is designed for and verified to fit in enclosure Gainta G221CMF-IP67 (https://www.gainta.com/en/g221cmf-ip67.html). Gainta enclosure G221MF/G221CMF seems to have identical dimensions. Recommended PCB mounting screws to use with the listed enclosure are included in BOM.
Gainta enclosure G221-IP67/G221/G221C (without mounting flange) seems to have identical internal dimensions, but does include 4 pcs additional M3x5mm insert nuts in the bottom plate. Mounting the PCB in this enclosure would require additional metric type mounting screw.

Power consumption

Fully assembled safety edge interface consumes ~12 mA @ 24 VDC power supply with no safety edges connected and both safety edge channels in FAULT mode. TODO: Measure and add current consumption with relay(s) activated.

Video demonstrating functionality

A video will be recorded and uploaded to YouTube as soon as the safety edge interface has been installed at the target gate.

Further comments/restrictions/useful information

• This safety edge interface is built for a safety edge with fixed termination resistance of 8.2kΩ (seems to be a common standard). Resistor R8 + R9 is designed to match your safety edge termination resistance in order for the designed compared Voltage levels (see schematic) to be functional.
• Safety edge is galvanically separated from power supply circuit in order to prevent an eventual "ground fault" for power supply and interfacing control circuit in case of isolation issues with safety edge (due to - for example - outdoor environment).
• The relay interface circuit is optional (see electrical schematic). If the PNP output (edge OK = output HIGH, edge activated/fault = output LOW) is sufficient for your needs, you can skip populating the relays and related control circuit.

Improvement suggestions

• Verify if the final product does conform to any of commonly required safety standards.
• The printed circuit board arrangement and design can be vastly improved :)

Support and contributions?

Have you detected a mistake somewhere in the project or want to improve it. Send me a pull request or message. My intention is that most of the uploaded and written information should be self explanatory, but feel free to request minor support related to this or similar project(s).