The Cylinder That Lagged
We used a solenoid valve (article 90) on a fast cycle (article 171). The PLC (article 181) sent the signal (to switch the valve). But the cylinder moved 50 ms late (the cycle was tight). The problem: the valve’s response time. A solenoid valve (article 90) takes time to switch (the coil energizes, the spool moves). A standard valve has a 10–20 ms response. For a fast cycle (the cylinder must move in 50 ms), the valve’s 20 ms lag is 40% of the cycle. We switched to a fast valve (a pilot valve, or a high-speed solenoid, 2 ms response). The lag dropped to 2 ms. The cylinder kept the cycle. The mistake was using a standard valve (20 ms) on a fast cycle.
Pneumatic valve response time is the delay between the signal and the air flow. This article covers the effect on a fast cycle.
The Response Time
A solenoid valve (article 90) has two delays:
- Electrical (coil): The coil energizes (1–5 ms). The magnetic field builds.
- Mechanical (spool): The spool moves (5–15 ms). The air flows.
Total: 10–20 ms (standard valve). A fast valve (a “high-speed” solenoid) has 2–5 ms. A pneumatic pilot valve (a small air pilot, article 90) has 10 ms (it’s air, not electrical).
Step 1: The Cycle (Article 130)
For a fast cycle (article 130), every millisecond counts. If the cycle is 50 ms (the cylinder must extend in 50 ms), a 20 ms valve lag eats 40%. The cylinder only has 30 ms to move. For a slow cycle (500 ms), the 20 ms lag is 4% (negligible). Match the valve speed to the cycle.
The response time rule: For a fast cycle (under 100 ms), use a fast valve (2–5 ms). The cylinder that lagged had a standard valve (20 ms) on a 50 ms cycle. Use a high-speed valve (2 ms). For a slow cycle (500 ms), a standard valve is fine.
Step 2: Tubing Length (Article 117)
The tubing (air line) adds delay (the air travels from the valve to the cylinder). A long tube (3 m) has a 10 ms delay (the air pressure builds over the distance). For a fast cycle, mount the valve close to the cylinder (short tube). The air doesn’t travel far (fast response).
Step 3: The Flow Control (Article 124)
The flow control (meter-out, article 124) slows the cylinder (on purpose). For a fast cycle, open the flow control (fully open, no restriction). A restricted flow control (even if the valve is fast) slows the cylinder.
Step 4: The Regulator (Article 149)
The supply pressure (article 149) affects the response. A low pressure (4 bar) has a slower response (the air builds slowly). A high pressure (6 bar) is faster. For a fast cycle, run at 6 bar (not 4 bar).
| Valve Type | Response Time |
|---|---|
| Standard solenoid (article 90) | 10–20 ms |
| High-speed solenoid | 2–5 ms |
| Pneumatic pilot (article 90) | 10 ms |
| Long tubing (3 m) | +10 ms |
A Response Time Checklist
- What is the cycle time? (Article 130?)
- Is it fast? (Under 100 ms?)
- What is the valve response? (ms?)
- For fast: high-speed valve? (2–5 ms?)
- Is the valve close to the cylinder? (Short tube?)
- Is the flow control open? (Article 124?)
- Is the supply pressure high? (6 bar? Article 149?)
- Does the cylinder lag? (Oscilloscope?)
- Is the PLC signal fast? (Article 181?)
- Is the cycle OK? (Meets takt? Article 130?)
The Bottom Line
Pneumatic valve response time matters on a fast cycle. The cylinder that lagged had a standard valve (20 ms) on a 50 ms cycle. Use a high-speed valve (2–5 ms). Mount the valve close (short tube). Run at 6 bar. The cylinder that kept the cycle wasn’t the fastest bore — it had a fast valve.
