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Understanding non-contact transmission of power and sensor signals

Understanding non-contact transmission of power and sensor signals


All information and opinions presented in this paper are the authors. Design News online did no editing or confirmation of the information provided.


The power of today's industrial controls - driven by demands for defect-free, unmanned production - is allowing design engineers to put more
sensors and actuators on machines to bring high-level feedback and functionality out to the part/tool interface. There's just one low-tech
obstacle to this: hard wiring, sliding contacts, or plug connections are needed to transmit power and signals out to removable tooling or machine components that require unrestrained motion. This problem can be encountered with dial tables, pallet shuttles, molds, dies, robotic
grippers, packaging machinery, stamping presses, and machine tools - to name just a few. And whether it's a slip ring, power track or simple plug, it has always taken a mechanical contact device - susceptible to dirt, wear, contamination, corrosion, and damage - to effect the connection.

That has all changed with the development of (patent-applied-for) Power Remote sensing technology. Based on passive inductive principles,
non-contact Power Remote systems can transmit both power and sensor signals across an air gap up to 15mm. Available in standard 12VDC and high-power 24VDC versions, they eliminate a "hard" connection to sensors or actuators on tooling, dies, pallets and fixtures to eliminate connect/disconnect time, and repair costs/downtime for damaged cords and connectors. Available in a number of different configurations, a Power Remote system can support up to eight remote sensors and provide one amp of auxiliary power without a physical connection of the devices to the control or power source. Remote power/sensor signal transmission technology increases sensing flexibility for testing and industrial applications, and expands design options beyond hardwiring and mechanical contact limitations. Sealed to IP67 standards, Power Remote components are immune to oil, coolant, metal chips, dust and other contaminants. They also tolerate high levels of shock and vibration. Heavily shielded, this passive coupling system does not generate, nor is it affected by, RF or electromagnetic interference (EMI).

Inductive principles at work

Power Remote technology is based on straightforward inductive principles. Michael Faraday discovered that when a piece of wire
(conductor) cuts the lines of flux from a magnetic field, voltage and current are induced in the wire. Similarly, Power Remotes create a
changing field of magnetic flux with a coil around a ferromagnetic core. When a second coil is put into that changing field, the multiple turns of
the conductor cut the lines of flux and induce a voltage and current that varies as long as the field is changing (alternating current). The interacting lines of flux from both fields act as a two-way bridge passing energy inductively between heads.

In practice, a "local" power remote head with internal coil is mounted in a fixed location and wired to a control or PLC via multiconductor polyurethane cable. The coil in the local head chops and converts the DC supply voltage into an oscillating signal so that it can travel inductively (via flux lines) across a predetermined air gap and centerline offset to a "remote" head with its own internal coil. The remote head is mounted on the moveable or removable component such as dial table, fixture, tooling, mold, etc. When the remote head aligns with the local head, the oscillating signal is converted back into a DC voltage in the remote head, producing drive current to power sensors or other devices.

Sensor signals, essentially voltage outputs, are delivered from the remote head back to the local head in the same manner. Special circuitry
inside the local head changes the inductive energy back to a DC voltage which can be processed by any off-the-shelf control or PLC. Any standard two- or three-wire DC sensor or mechanical switch can be used as a detector including inductive, photoelectric, magnetic, capacitive, ultrasonic, fiber optic and mechanical switches.

The local and remote heads respond only to each other, so no false outputs will result from metal chips or other metallic debris found in
harsh manufacturing environments. Heads are CE-rated and sealed to IP67 for reliable power and signal transmission in environments with oil, coolant, metal chips, dust and other contaminants. Designed for jarring industrial applications, heads are shock-resistant to 50G, and vibration-resistant to 10 50Hz, amplitude 1.5mm. Coils are heavily shielded and specially wound for proper field suppression and control, as well as for protection against EMI and RF interference.

Power Remotes operate at relatively high frequencies, so filtering requirements are low and a large capacitor is not required on the remote
side. Also, because frequency is proportional to flux density, only a small magnetic core is needed for higher frequencies, making the heads
compact and highly efficient, electrically and electronically.

Factors that affect power and signal transmission

The key to successful operation of the detecting sensor(s) is the drive current supplied by the remote head. The amount of drive current is
a function of head size, distance between the heads (air gap) and amount of centerline offset. An optimum combination of axial alignment and air gap distance is necessary for peak system performance, as demonstrated in the transmission operating window shown in Figure 2. The dark area represents alignment requirements to deliver 1A of drive current, while the light area represents requirements for 0.5A of drive current.

Close axial alignment allows increased variation in air gap, while a small (but not too small) air gap distance allows for slight misalignments of head centerlines. As shown in the operating window for this High Power Remote model, heads must be no more than 7mm apart to deliver one amp of drive current. With an air gap less than 3mm, the lines of flux between heads cannot interact properly, neutralizing the electrical induction and preventing power and sensor signals from being transmitted.

The presence of plastic or glass does not hamper power or signal transmission. Plexiglas actually aids in signal transmission because the
permeability of the plastic is slightly better than that of air. Heads can also be spinning as long as they maintain axial alignment and proper air
gap.

Power Remote systems place no special demands on the control. An ordinary in-zone signal must be addressed in the control's application
programming. The in-zone signal tells the control that the heads are properly aligned (within predetermined operating parameters) and signals
are being passed between heads, similar to the system check typical of I/O networks which require this test before operating.

Standard Power Remote system

The Standard Power Remote system transmits 12VDC and 5-100mA of drive current (depending on head size, air gap and offset) to power up to eight sensors. Standard Power Remotes are available in single-, four- and eight-sensor systems.

Heads for single- and four-sensor systems use M18 or M30 tubular nickel-plated brass housings, sending 5-30mA drive current across an air
gap up to 8mm.

Eight-sensor systems use 80mm-square by 40mm-high "puck-style" Polyamide heads, sending 20-30mA of drive current across an air gap up to 15mm. While the Standard Power Remotes deliver ample drive current for proper sensor operation, they are not designed for high-power transmission to drive electrical components such as actuators or solenoids.

Amplifiers - one per head - are required to manage sensor signals, and are available in both PNP and NPN versions. It is possible to have a PNP amplifier on one side and an NPN on the other, but PNP and NPN sensors cannot be mixed together on one side.

To handle the multiple sensor signals, four- and eight-sensor Standard Power Remote systems use frequency-modulated signals. Individual signals on multiple-sensor systems are distinguished from each another through a standard full-duplex serial bit stream, operating on a 10-msec. clock.

High-Power Remote system

Having the ability to transmit 24VDC at one amp to a remote location, the High-Power Remote system allows bi-directional transmission
of up to eight signals for sensors, while providing up to eight outputs to trigger actuators, relays, solenoids, etc. An auxiliary power supply on
the remote side provides a 24VDC output to drive electrical components such as servomotors that demand more power than actuators or relays.

The high-power version has additional circuitry in the local and remote heads for I/O signals. The system uses two coils in each 90mm-square by 45mm-high "puck-style" aluminum head - one for power and another for sensor signals - running at different frequencies, with high
impedance and lower power (almost like a small radio). The high-power version can transmit one amp at 24VDC across an air gap of 4-6mm; 0.5 amp across 3-10mm.

The High-Power Remote can also be used where the sole purpose is the transmission of power without a "hard" connection (conductor, plug,
etc.) between the power source and the appliance. Because no sensors are involved in power-only applications, amplifiers are not needed.
Non-contact power transmission has wide industrial potential, including driving solenoid valves, actuators, safety gating etc.- even trickle-charging batteries for unmanned vehicles.

Application-specific Power Remote Systems

Two of the most widespread uses for Power Remote sensing have led to development of application-specific systems: the T-Slot Power Remote and the Thermal Power Remote. Inherent shock and vibration resistance make Power Remote technology ideal for these two applications.

Developed specifically for the stamping and forming industry, the T-Slot Power Remote system reduces wiring and eliminates the hard
connection for up to four sensors embedded in tooling, dies, pallets and fixtures on stamping presses or machine tools. The aluminum remote head is uniquely designed to be semi-permanently "buried" inside a standard 1-inch T-slot, secured by four setscrews, slightly beneath the work surface. Solid-state PNP (sourcing) active-high outputs are built into the M30 local head housing, which connects directly to the I/O module of the PLC or controller.

Adopted for die management by one of the "Big Three," the T-Slot Power Remote System supports up to four tool-mounted sensors without a hardwired connection to the control or power source. A typical system uses sensors to confirm part/material presence or gas pressure.

The Thermal Power Remote system provides non-contact thermocouple data transmission for plastics molds, forging dies and heat treating furnaces. The system speeds tooling changeover time and eliminates the hassle of thermocouple rewiring/connecting. It allows simultaneous, non-contact temperature data transmission for up to six J-type thermocouples, providing temperature feedback (0-750 degrees C) in applications where hardwired connections are impractical or impossible.

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