speed control circuits (throttling circuits)

SPEED CONTROL CIRCUITS (THROTTLING CIRCUITS)

1. Speed Control of a Pneumatic Cylinder

• In many pneumatic applications, it is required to operate different components with different speeds. Therefore speed control of pneumatic cylinders are necessary.

• For regulating the speed of pneumatic cylinders, the flow control valves are used. Also similar to the methods discussed under hydraulic circuits, the different speed control methods are (i) meter-in circuit, (ii) meter-out circuit, and (iii) bleed-off circuit.

2. Meter-in Speed Control of Pneumatic Cylinder

(Supply Air Throttling Method of Speed Control)

1. Circuit

Fig.13.9 illustrates a meter-in circuit that can be used to control the speed of an air cylinder using a flow control valve.

2. Operation

Extension: When 4/2 DC valve is shifted to its left mode, air flows to the blind end of the cylinder via the flow control valve. This air flow extends the cylinder. Here the extending speed of the cylinder depends on the setting of the flow control valve. Thus the extending speed can be regulated by just regulating the air flow in the flow control valve.

Retraction: When the 4/2 DC valve is shifted to its right mode, air flows to the rod end of the cylinder and hence the cylinder retracts. The air from the blind end of the cylinder is exhausted to the atmosphere through the check valve as well as the flow control valve.

3. Meter-out Speed Control of Pneumatic Cylinder

(Exhaust Air Throttling Method of Speed Control)

1. Circuit

Fig.13.10 illustrates a meter-out circuit to control the speed of an air cylinder.

2. Operation

The operation of meter-out circuit is very much similar to that of the meter-in circuit. The only difference is that meter-out flow control system controls the air flow rate out of the cylinder. In other words, meter-out circuit controls the retracting speed of the air cylinder.

4. Difference between Meter-in and Meter-out Speed Control Methods 

Table 13.1 provides the difference between meter-in and meter-out speed control methods. 

Table 13.1. Supply air throttling Vs. Exhaust air throttling

Meter-in

(Supply air throttling)

1. In supply air throttling, air entering the cylinder is throttled in each direction of motion of cylinder.

2. Exhaust air can pass freely.

3. In this case, piston is not loaded with two cushions of air when cylinder is in motion.

4. Smooth motion of piston is not possible. 

5. Used for small cylinders.

Meter-out

(Exhaust air throttling)

1. In exhaust air throttling, exhaust air leaving the cylinder is throttled in both the directions of motion of the cylinder. 

2. The supply air can pass freely.

3. In this case, piston is loaded with two cushions of air when the cylinder is in motion.

4. Smooth motion of piston is possible. 

5. Widely used in many practical applications.

5. Speed Control of Double-Acting Pneumatic Cylinder with Two Flow Control Valves

1. Circuit

Fig.13.11 illustrates a speed control circuit of double-acting air cylinders using two flow control valves.

2. Operation

In this circuit, the two flow control valves-one at the inlet line and other at the outlet line of the pneumatic cylinder-are used to regulate the speed of both the extending and retracting motion of the cylinder.