How "All-Air" Web Guide Systems Work

The All-Air Web Guide is an ingenious concept … here’s how it works.

The Coast All-Air control system is extremely accurate and provides a simplified, dependable and trouble-free way to guide. The system is proportional (rate of correction is in proportion to web error) and the response to error is virtually instantaneous, providing a smooth and quick response. Since the All-Air systems run entirely on low-pressure plant air, Coast web guides are intrinsically explosion-proof and ideal for applications in sensitive environments (see diagram below).

Simplicity! How the All-Air system works …

Plant air is connected to the friction-free All-Air Servo Controller through a two-stage cartridge/coalescing air filter and precision low-pressure regulator/gauge. A small portion of the air is reduced in pressure before going to both sides of the air-flow sensor. A constant stream of air continually bleeds from the sensor’s opposing nozzle and signal orifices keeping them free of dust and other foreign matter.

As web misalignment (error) occurs, the edge of the web moves in or out of the sensor. This causes the slightly higher nozzle air pressure to increase or decrease the backpressure in the signal air coming from the All-Air Servo Controller.

The change in pressure creates an imbalance between the opposing forces of the air diaphragm and the bias spring in the All-Air Servo Controller causing the shuttle to move.

Movement of the shuttle directs the somewhat higher pressure actuation air to the proper end of the air cylinder. This causes the cylinder’s piston rod to move the guide the precise amount required to realign the web at the air-flow sensor.

With the web edge back in its proper position at the air-flow sensor, the guiding system is again balanced, completing the control loop. Sensing and correction continue to take place as web misalignment (error) occurs.

All-Air Servo Controller

The proportional Servo Controller converts the low pressure signal from the Air-Flow Sensor into a higher actuator pressure.

Plant air is filtered and regulated down to a nominal operating pressure as it enters the Servo Controller. A small amount of the incoming air is routed to the nozzle side of the Sensor, and a smaller amount goes to the signal side (to monitor the web). See air-flow diagram.

As the web wanders away from the Sensor, the slightly higher nozzle air creates back pressure in the signal side of the system, allowing the diaphragm to overcome the opposing force of the bias spring. The increase in pressure pushes the shuttle toward the bias knob, sending the proper amount of air to the blank end (back) of the actuator. When the web partially or entirely blocks the sensor, the reduced pressure allows the bias spring force to overcome the weakened diaphragm and push the shuttle in the opposite direction, sending the proper amount of air to the rod end (front) of the actuator.
The resulting proportional pressure signals the actuator to automatically reposition the web until its position in the sensor brings the shuttle back to neutral. No action is required by the operator.

Air Cylinder

The Air Cylinder provides the force for moving the guiding assembly to align the web. Cylinder diameters range from three to eight inches depending on the application. A three, four, or five-inch diameter cylinder is typically used for intermediate guiding, and when additional thrust is required for shifting an unwind or rewind stand, a six or eight-inch diameter cylinder can be used. The length of the cylinder’s stroke is determined by the maximum linear correction the guide must make.

Air Does It All

In effect, the Sensor, Servo Controller and Air Cylinder—along with a moving web of material—combine to form a closed-loop proportional control system. The Servo Controller converts and amplifies the signal from the sensor to a higher air pressure and sends the desired correction to the Air Cylinder. The cylinder then strokes to reposition the guiding assembly and accurately aligns the moving web.