ZPA; zero pressure accumulation transport system

A zero pressure accumulation (ZPA) transport system is a system that allows the products to be brought to a stop one after the other, without them pressing up against each other. This is done to prevent damage to the products, for example. ZPA is mostly used for the buffering of products before removal or processing. Each product is transported as far as possible towards the end of the line. When a product is removed, the buffered products are moved forward so that the removal position is occupied again.

ZPA control options
- Control using electrical/pneumatic switch components
- Control using only pneumatic switch components

Serial electrical circuit
A serial electrical circuit can be implemented using:
- conventional electrical switches, proximity switches or optical switch elements (photocells);
- pneumatic 3/2 solenoid valves (Nearly Open).

Operation of a serial electrical circuit

When there is no product above photocell 1, the switch in the photocell is open. At that moment it no longer provides a voltage to (solenoid) valve 1 and photocells 2 and 3. The compressed air from the main air supply goes via valve 1 (ports 1A and 1C) to cylinders L1 and L2. The rollers' couplings are then engaged and the product is transported. The same happens for cylinders L3, L4, L5 and L6. If a product arrives above photocells 3 and 2, it will then be moved on because there is no voltage across these photocells.
If there is a product above photocell 1, the switch in the photocell will be closed and will provide a voltage to valve 1 and photocell 2. Valve 1 closes; the compressed air can no longer reach cylinders L1 and L2. The rollers' couplings are disengaged and the product comes to a standstill.
There is now a voltage across photocell 2. If there is also a product above photocell 2, the switch will close and provide a voltage to valve 2 and photocell 3. Valve 2 closes and this product stops moving as well. And so forth.
If the product above photocell 1 is removed, the switch opens and the voltage drops. All the valves open, the couplings are engaged and the products are again transported.

Circuits
The switching elements must be suitable for connection in series. This is dependent on the permissible voltage and current through the switching element and the loss within the switch. When using conventional switches, it is possible to place several in series. When using photocells it depends on the type of output. The most common photocells have a PNP output, passing an attenuated input voltage through to the output. The signal is reduced so far that the voltage becomes too low to use as a power source. Photocells with a relay output do not have this problem and up to thirty photocells can be connected together.

Pneumatic serial circuit
The distinguishing feature of the diagram below is its non-standard use of 3/2-valves. The vent ports have two functions; venting, and keeping the air cylinders pressurized when there is no product at the previous product station. The drive may only be disengaged when a product depresses the last valve and comes to a standstill.
At any product station, drive must be present if there is air pressure. This way, the air pressure can provide the force for engaging the drive and the drive will be disengaged when the air pressure drops away.

Operation of a pneumatic serial circuit

The air pressure from the main supply goes via ports 1a and 1c (of valve 1) to air cylinders L1 and L2. The couplings are then connected and the product is transported. The air also passes via dump valve 1 to port 2b. This is how the air pressure from the main supply also reaches air cylinders L3 and L4, via ports 2a and 2c (of valve 2). These couplings are also engaged and the product is transported. Here too, the air passes via dump valve 2 to port 3b. And so forth.

If a passing product depresses valve 2 while valve 1 is not depressed, then cylinder L3 and L4 are pressurized through ports 2b and 2c. The couplings stay engaged and the product is transported further. When product 1 depresses valve 1, cylinders L1 and L2 no longer receive any air pressure. The couplings are disengaged and product 1 does not move. At the same time, port 2b also loses its pressure. When product 2 depresses valve 2, cylinders 3 and 4 no longer receive any air pressure (also not via port 2b). The couplings are disengaged and product 2 does not move. The air pressure from cylinders L3 and L4 can escape via ports 2c and 2b to dump valve 1. If product 1 is removed, all the air cylinders get repressurized and the products are transported again.

Warning
The 3/2-valves must have three connection points and be suitable for the intended purpose, for example a roller-actuated slide valve with spring return.
Dump valves are used to prevent pressure waves from occurring in the system due to depressurizing cylinders. Without the dump valves, all the cylinders would have to be vented via the exhaust of the first valve.