If In Doubt, Meter Out

“If in doubt, meter out”. This is a phrase you hear a lot in pneumatics. While I would not advocate always choosing meter out speed controllers because of an old saying, there is a reason this old expression is still used so much.

Below is the schematic symbol for a speed controller. This shows a throttle and a check valve. The throttle reduces the speed of the air flowing through it. The diagonal arrow shows that this is variable, or flow can be increased or decreased depending on how many turns the operator has put on the knob.

The check valves prevents air travelling through it in one direction, but allows flow in the other direction. In this case, air flowing from port 1 to 2 cannot get past the check valve, so is forced to go through the throttle and the speed is reduced. When air flows in the opposite direction, from 2 to 1, air can go through the check valve and bypass the throttle, so the speed is not reduced. See image below. The blue line shows the throttled air flow and the red line shows the flow which bypasses the throttle.

If port 2 connects to a cylinder port, then this would be a meter in type, as the air flow entering (going in the cylinder) is throttled, or metered.

The image below shows a basic schematic of two speed controllers attached to a cylinder, which is controlled by a 5/2 valve. This shows two meter in types. The air entering the cylinder ports is throttled, which (in theory, I will come back to this) will reduce the speed of the cylinder rod. This will control the speed of the weight being lifted.

But, when the valve switches position, the weight will (in this case of a vertically mounted cylinder) push the rod down fast, as the air exhausting from the port on the head end of the cylinder is not throttled. This can be unsafe and damage the cylinder.

By using two meter out types of speed controller, the exhaust air at the head end is throttled, so the weight lowers slower and controlled. When the rod is extended, the movement is also controlled as air exhausting the rod end is throttled, so the piston and rod assembly can only move as fast upward as the exhausting air allows.

Meter in speed controllers are much better suited for single acting cylinders (spring extend or retract cylinders) or applications where the load attached to the cylinder rod is constant and has a positive resistance (such as a milling machine).

I mentioned above I would come back to something. Sometimes in pneumatic meter in circuits jerky rod movement an be observed. This is because as air enters a cylinder, pressure builds until the piston seal friction and load weight can be overcome, which can cause the rod to move quickly at first. Air then cannot fill the space inside the barrel quick enough (as the piston rod extends), so the piston rod may stall again until pressure builds again. This is due to the compressibility of air. In scenarios like my meter in example above, where a weight/load is not attached to another constant resistance, this jerky motion may be observed.

(Speed controllers, even pilot operated types, are not a means of drop prevention. If that is required for safety reasons, a locking cylinder or other safe drop prevention method should be considered).

Whether you would choose one type or the other is dependent on your application, and how a load is acting on the cylinder or motor or whatever type of actuator you are controlling. Old sayings can be helpful, but, to ensure an efficient and safe application, it is best to know and understand the system you are working on.