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I need to connect two circuit boards - one with a microcontroller, power circuitry, display, etc to another with several sensors on. The connection will require somewhere between 4-10 lines - 5V power, ground, I2C data and clock, possibly plus some interrupts. The circuit boards will be mounted at right angles with respect to each other.

The crucial problem is that the board with the sensors on will need to move constantly with respect to the board with the microcontroller on. The degree of movement will be small - only a few centimetres, and it will be mainly in one axis (perpendicular to the sensor board, parallel to the microcontroller board). The movement will be irregular in nature - the movement will respond to the sensor input. The movement will be driven by a powerful servo, so the speed and acceleration of the movement may be quite jerky / fast.

So what kind of connection can I use between the boards that will be reliable for this kind of movement? I've seen various kinds of flat flex connection used in consumer products. Is that kind of connection reliable over years of many millions of movements every day? What kind of connection would be used in more reliable applications like automotive or aerospace?

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If you need frequent motion you should specify FPC designed for dynamic applications. The treatment of the copper is different from ordinary FPCs to keep it from cracking and there are constraints on the copper thickness and there are other constraints (single-sided is best, gentle radius that spreads out the flexing, and so on).

Most good FPC makers will have a detailed design guide that can assist in your design. Think of consumer applications such as moving print heads or optical read heads that float in suspensions.

I've done this for a spacecraft instrument that needed frequent motion between a sensor and the electronics package- pretty straightforward once you understand the parameters.

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Automotive has this problem for the ABS wheel sensors. Their solution is a long lobe of thin, stranded copper wire in a thick rubber hose. That way most of the force is applied to the rubber instead of the conductors.

I would cut down the actual conductors to the power ones and do all else through radio. Or use a scheme where you can use the power wires for communication, too.

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  • \$\begingroup\$ Are there products available with the ABS-sensor connection tech? Or is it something that I would have to have custom manufactured? \$\endgroup\$ – Tom Bull Sep 29 at 9:01
  • \$\begingroup\$ Also, radio communications isn't a great fit, as far as I can see, for a low-cost, low-latency, high-reliability application like this. Unless you can recommend a solution that fits those requirements (I think that's really a different question though!) \$\endgroup\$ – Tom Bull Sep 29 at 9:03
  • \$\begingroup\$ You have to contact a cable manufacturer to find a thick rubber coated cable with comparatively thin conductors, but they should be available easily. \$\endgroup\$ – Janka Sep 29 at 9:09
  • \$\begingroup\$ There are converters in the market which put both ethernet and POE on two wires. Search e.g. for "2 wire poe ethernet". \$\endgroup\$ – Janka Sep 29 at 9:14
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You have few options:

  1. FPC, more specifically, Polyimide (Kapton) FPC. It provides the flexibility and reliability for such applications and the connectors can be very small or none at all. You can see such circuits in printers, phones and laptops.
  2. Standard Ribbon cable - larger connectors, larger wires, cheaper to buy. Connectors are usually crimped to the cable.
  3. Discrete wires - this is the least reliable but most flexible option, as the movement will apply some forces on the solder joints.
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All of the major cable manufacturers, and some specialty cable manufacturers have products specifically designed for such applications. You need to consult with them with your specific requirements in hand.

I once had an application that needed to connect to a GPS receiver mounted on a moving platform, and I was able to find a super-flexible round cable with a silicone jacket that met my needs. It wasn't cheap, however!

motion platform

The cable is from Misumi Corp., and the connectors are a mix of Glenair (larger) and Hirose (smaller) circular twist-lock units. The two upper connectors are on the moving platform, while the lower one is part of the fixed assembly. The actual strain relief at either end of the part of the cable that flexes is handled by simple zip ties; once that's taken care of, the actual connectors used don't matter that much.

I needed 30 cm (12") of motion in a very rugged package — you probably don't need anything quite this elaborate.

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  • \$\begingroup\$ What connector/connection would you reccomend to go with this kind of flexible cable? \$\endgroup\$ – Tom Bull Sep 29 at 9:07
  • \$\begingroup\$ See edit above. \$\endgroup\$ – Dave Tweed Sep 29 at 20:16
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Another variant on stranded copper wire is braided copper wire. One application where this is commonly used, admittedly for millimetres rather than centimetres of travel - is the connection to loudspeaker voice coils.

This demonstrates a high cyclic life is possible.

As Janka suggests, you may adopt/adapt automotive practice (flexible tubes) for insulation and design to minimise the number of conductors (encoding over the power conductors, or "phantom power")

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  • \$\begingroup\$ What connector/connection would you reccomend to go with this kind of wire? \$\endgroup\$ – Tom Bull Sep 29 at 9:07

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