Advances in automation systems have greatly increased the productivity of manufacturers. Machine control using the understanding and inspection of the specifications of each product and the position detection of machine parts has made it possible to increase the speed and quality of equipment.
At the same time, safety has been improved, significantly reducing the need for operators to operate the machine. But automated machines are not autonomous. Whether it's material pinching or part/component failure, the operator needs to see the situation and remedy the situation. For this reason, operators and maintenance personnel must:Potentially hazardous areas of the machine must be approached.
Solving these production-related issues must, of course, be done in a safe manner. Advances in safety control systems have made this possible.
Commonly accepted best practices for machine safety system design include: Associated tasks, such as foreseeable misuse and failure of parts/components. It always includes completing a risk assessment taking into account The safety system must not cause damage or premature wear of parts/components.
However, this damage can be caused by "a stop command that can occur at any point in the machine cycle" or "a re-supply of pneumatic energy that causes rapid movement of each part/component". Premature wear increases the frequency of breakdowns and maintenance-related work, resulting in increased operator access to the machine.
Earlier pneumatic safety consisted of several main parts/components that stopped and controlled the movement of the machine. As such, it was very common to use a closed center valve to secure the cylinder . This valve traps pressure on both sides of the cylinder, generally to the desired effect. However, this approach ignores three important issues . The three are: 1) a slow or stuck valve, 2) testing the centering of the valve body depending on the spring action, and 3) the effects of leakage when using spool valves. All three of these problems can cause dangerous cylinder movement.
Slow valve response can cause cylinder movement to last longer than expected. In normal operation, a 5/3 closed center valve may shift from one side to the other without using the center position, except during a safety event, and if the center position has not been tried, the valve will normally Like the operation of , there is the possibility of simply shifting. The closed center valve seals the pressure on both sides of the cylinder, but if the leakage on one side is large, the cylinder will start to move , and if the cylinder is vertically movable, the closed center valve will maintain the pressure on the top of the cylinder. and create a potentially dangerous situation.
A slow valve movement takes longer to reach the center position, which increases the time it takes for the machine to stop. Also, in many cases where the centered position is only used for safety stops and it is not regularly verified that the valve actually moves to the centered position when both solenoids are off, in this case: What happens if the spring inside the valve is broken?
Also, a leak in either the cylinder line or the cylinder piston seal can create an unbalanced pressure condition within the cylinder, resulting in unexpected movement.
This has led to the use of electrically controlled pneumatic exhaust valves in combination with 5/3 open center valves or 5/2 spring return valves . Exhaust valves are typically 3/2 normally closed valves used to remove air pressure from the downstream side of the system . Since these exhaust valves are still used as part of safety systems today, they must meet the same safety category requirements (or performance levels) as any other safety-related system. This exhaust valve and directional control valve arrangement removes all pneumatic energy from the system so that if the valve fails, the pneumatic energy will not keep the machine running.
For speeds and loads that require faster response and immediate stop, pilot operated check valves are used as needed. With this method of use, the pneumatic supply is removed from both cylinder lines and pilot operated check valves hold the cylinders in place by trapping pressure within them. A horizontally mounted cylinder traps pressure on both sides of it, whereas a vertically mounted cylinder, where gravity is a factor, usually only needs to contain pressure on the underside of the cylinder.
This 3/2 High Control Reliability Exhaust Valve , 5/2 Spring Return or 5/3 Open Center Cylinder Valve , and Pilot Operated Check Valve are the most effective safety circuits used in automation equipment. The ultimate goal is whether the cylinder is fully extended, fully retracted, or in an intermediate position. To make the machine safer so that it can be stopped at any point in the cycle.
There are two main reasons for stopping a machine. One is production related outages and the other is maintenance outages. Any production-related issue should undergo a risk assessment to ensure that it is in and remains safe to perform the required tasks. The problem during maintenance is that lockouts are required, mechanical block procedures must be followed to keep the machine from moving, and selectively containment pressure must be released for reasons that affect safe shutdown. It is not possible.
After completing the work, the next step is to restart the machine safely. Re-applying air pressure must not cause unexpected machine movement or damage to the machine. In the old days, it was said, "When in doubt, control with meter-out." By using flow control to reduce the flow of air exiting the cylinder, the velocity of the cylinder can be controlled no matter how quickly air pressure is applied from the other side.
This is especially useful for preventing meter-in slipstick problems caused by a combination of friction, flow, volume and load. This meter-out assumption is not required for safety systems that remove air pressure from all or part of the primary air supply and recinder. With this safety system, when air pressure is reapplied, or during the first cycle of the valve and cylinder, there is no compressed air left in the cylinder that requires meter-out control leading to cylinder runaway.
Another solution to this problem is to meter-in the cylinders. Cylinder movement can be controlled by controlling the flow of air pressure to the cylinder using a flow control valve (speak on). This method works for most applications except where wear, flow, volume and load cause slipstick problems. Also, if the vertical load is sufficient to overcome the stiction of the cylinder seals, the upper meter-in control device will cause the cylinder to drop due to gravity alone, leaving air pressure on the underside of the cylinder and metering it out. Unless control equipment is used, the desired speed limiting effect may not be obtained.
An alternative to meter-in control is to meter the entire system when air pressure is first applied after a safety event or normal shutdown. this is,"Soft start", where air pressure is slowly increased until an adjustable preset point is reached before full line pressure is delivered to all downstream components. The advantage of this is that downstream components slowly move into place,There may be no need for individual flow control components at every point.Some devices soft-start the entire system, while others soft-start only at the point of use. A soft-start device in combination with meter-out control of the actuators seems at first glance to be the ideal solution, and in some cases it certainly is.
Soft-starting the entire system can be problematic. In the circuit example on the left, where the solenoid pilot valve is downstream, the valve must remain switched off until at least the minimum operating pressure is reached. Otherwise the valve may not switch properly.
This also results in the cylinder taking full pressure immediately when the valve is switched ON, rather than a gradual start. Additionally, if items such as venturi-type vacuum generators are installed, they act like leaky devices in the system, preventing the soft valve from switching to full flow. Also, feeding the suction cups and clamping cylinders through the safety exhaust valve can create an additional risk of dropping material when a safety or emergency stop is initiated. This problem can be solved by using a soft start at the point of use to move the supply to the vacuum generator and clamping cylinder upstream of the safety exhaust valve.
The overall effectiveness of a soft start device is entirely dependent on the actuator valve, the position of the cylinder at stop, and auxiliary devices such as flow regulators and pilot operated check valves.The first consideration is to find out where air pressure is vented or trapped during normal operation of the safety system. The next consideration, as required by the risk assessment, is to find out where air pressure could be removed or trapped during component malfunction.
When pneumatic energy is reapplied (using a direct solenoid valve and assuming no electrical signal is reapplied), all actuators controlled by 5/2 spring return valves are in their non-actuated position . It moves slowly and at the proper speed when the valve is first energized. The machine returns to its normal resting state in an orderly and safe manner. If the spring return valve fails, reapplying air pressure can cause the cylinder to move in the wrong direction, but at a slower rate.
But for many continuous process machines, going back to hibernation is not an option. The cylinder should stop at that position and stay there when pneumatic energy is reapplied. A 5/3 open center valve with pilot operated check is routinely used in these applications and has no effect on the soft start of the entire system. This is because, at rest, the pressure on any valve that impedes downstream flow always rises slowly. This ensures that the pneumatic supply to the actuator valve is at maximum pressure (pressure on at least one side of the cylinder) when the valve is first actuated, unless a soft-start device or meter-in flow control is used at the point of use. ), this causes the machine to start abruptly.
The main difference between meter-in flow control and using point-of-use soft-start is that after a preset start-up pressure is reached, soft-start allows full flow. Also, don't forget the meter-in problem. Slipstick cylinder motion wreaks havoc on mechanical processes. However, when using a soft-start device with a metered-out flow control device at the point of use, the pneumatic energy re-supply and cylinder speed is controlled and does not interfere with the normal smooth cycling of the cylinder. Cylinders are controlled in all aspects of machine operation.
Another point, if you don't use soft start for the whole system, these devices are designed to slowly bypass air pressure downstream until a certain pressure is reached, then open fully to allow full pressure to flow to the valve. This is the point. The flow of this bypass is usually restricted and although adjustable it may exceed the limit and unfortunately pneumatic systems are most often plagued with leaks. In such systems that rely on building up pressure before the valve is fully opened, if the leakage downstream of the soft start valve is equal to or greater than the bypass flow capacity, The weak point is that the soft start valve does not fully open.
Some mechanical processes such as air blowers and vacuum generators also consume compressed air all the time. This air consumption is effectively considered the "leakage" of the soft start system. In such a system, it is imperative that more complex circuits are incorporated to isolate the system's leakage area after soft start is fully open and full flow is available, or until point-of-use equipment is used. Is required.
Installing a soft-start device upstream of the entire machine circuit is often recommended, but in many cases this is not the best solution. On the other hand, the use of soft start in combination with flow control equipment at the point of use limits the initial re-energization of energy as needed to maintain position during safety events and It provides the most consistent solution for speed control of machines that must begin continuous motion once air pressure is reapplied. This is especially true for safety systems that use high control reliability pneumatic exhaust valves and 5/3 open center directional control valves to control cylinder motion.