Siemens LOGO! Vacuum Pump Control: Analog Scaling and Staging

Vacuum Pump Staging Architecture Overview

Sequencing multiple vacuum pumps against a single pressure transmitter is one of the most common small-controller applications in industrial pneumatic and process plants. The reference application described here uses three 230 V contactor-driven pumps (Q1, Q2, Q3) controlled by a Siemens LOGO! logic module, with a WIKA pressure transmitter spanning -1 bar to 0 bar on a 0-10 V output. The controller must stage pumps on as vacuum rises (pressure becomes more negative) and rotate lead/lag runtime to equalize wear.

Pressure setpoint bands in this application are tight (a 0.2 bar window around -1 bar governs the cycling of pumps Q1 and Q2, and a 1.0 bar window governs the addition of Q3). With raw analog sampling at 10-bit resolution and a noisy pneumatic installation, naive threshold comparison produces output chattering at the band edges. The fix is a combination of proper analog scaling, a hysteresis-equipped Analog Threshold Trigger (Schwellwertschalter), and an Average Value block ahead of the comparator. This article walks through the complete implementation in LOGO! Soft Comfort V8.x or V9.x, including firmware considerations for the new LOGO! 9 generation introduced by Siemens.

Hardware Prerequisites and Wiring

The following hardware satisfies the reference design. Order numbers are taken from the Siemens Industry Mall and the LOGO! 9 product family release.

Wire the WIKA transmitter output (signal, +, -) to LOGO! AI1 (terminal I7 on the BM for the 230 V variants, which accepts 0-10 V on inputs I7/I8). Provide a shielded cable and ground the shield at the panel entry only. Each pump contactor coil is driven from a LOGO! Q-output through an interposing relay if the coil inrush exceeds the LOGO! relay rating (typically 10 A resistive, 3 A inductive at 230 V AC).

WIKA Pressure Transmitter Signal Characteristics

The WIKA pressure transmitter in the reference design spans the following electrical and physical range:

The relationship is linear and negative-going:

V_out = 10 * (P - P_min) / (P_max - P_min)  where P_min = -1 bar, P_max = 0 bar
P_bar = -1 + (V_out / 10)
LOGO! raw = (V_out / 10) * 1000 = 0 to 1000
P_bar = (raw / 1000) - 1   (units: bar, gauge)

The native LOGO! internal representation for a 0-10 V input is the integer range 0-1000. To make the engineering value of -1 bar readable in the program, you must perform a polarity inversion using an Analog Amplifier block, an Arithmetic block, or use a signed-physical scaling mode if your firmware supports it. On LOGO! 9 firmware >= V1.9.0, the analog inputs can be configured with signed values directly in the message text or web server.

Analog Signal Scaling in LOGO!

Two practical approaches exist in LOGO! Soft Comfort to convert the raw 0-1000 reading into a -1000 to 0 engineering value (representing -1.000 bar to 0.000 bar in decibar resolution).

Approach 1: Analog Amplifier with Gain and Offset.

Block: B001 (Analog Amplifier)
Input  : AI1 (raw, 0-1000)
Gain   : -1.00
Offset : -1000
Output : AQW1 (signed, -1000 to 0)
Display message: Pressure = AQW1 / 1000  (bar)

Approach 2: Arithmetic block with linear transform.

B002 (Arithmetic)
Formula: Y = ((AI1 - 0) * (-1)) + 0
       = -AI1
Result : 0 to -1000
Optional divide-by-1000 scaling for display.

Both approaches yield the same signed engineering range. Use the Arithmetic block when you also need to combine the pressure value with other analog signals (for example, to compute a running average of multiple transmitters or to apply a square-root extraction for differential pressure).

Threshold Trigger Configuration for Chattering Prevention

The root cause of the original problem described in the application request is the use of a single-edge comparator against a noisy analog signal. The Analog Threshold Trigger (block group "Analog") is the correct function block, because it accepts separate On and Off thresholds. The hysteresis gap between these two values defines the deadband around the setpoint.

Choose a hysteresis gap that is greater than the peak-to-peak measurement noise. For a typical industrial pneumatic system with a 24 ms scan and a 0-10 V signal, expect 5-15 mV RMS of noise. At 10 V full scale = 1 bar, this equates to 5-15 mbar peak-to-peak. A hysteresis of 20 mbar (200 raw units) provides a 2x to 4x safety margin.

Pump Sequencing State Machine

The required staging logic per the original specification is:

Implement the state machine with the following blocks in LOGO! Soft Comfort:

B006 - On-delay 5 min (T1)        : enable alternation of Q1/Q2 lead role
B007 - On-delay 10 min (T2)       : back-up run timer for Q2 at -1 bar band
B008 - Off-delay 30 s (T3)        : minimum run time after staging Q3 on
B009 - RS Flip-Flop (Q1 lead)     : alternates between Q1 and Q2 every 5 min
B010 - RS Flip-Flop (Q2 lead)     : inverse of B009
B011 - AND (Q3 enable)            : B004 output AND NOT(B006 elapsed)
B012 - OR  (Q1 final)             : B003 OR B009
B013 - OR  (Q2 final)             : B003 OR B010 OR B007
B014 - AND (Q3 final)             : B011 AND (Q3 not in 1-min inhibit window)

Wiring rationale: When the threshold trigger B003 trips (P rising above -0.80 bar), Q1 and Q2 latch on through B012 and B013. When P drops back through -1.00 bar and stays, the alternation timer B006/B007 begins cycling the lead/lag role. B008 prevents Q3 from re-cycling during rapid pressure oscillations near the upper setpoint.

Average Value Filter and TOFF Debouncing

To further suppress high-frequency noise on the analog input before the threshold trigger, insert an Average Value block (Mittelwert) between AI1 and the scaling amplifier. Recommended configuration:

B015 - Average Value
Sample time (ST) : 200 ms   (5 samples per 1.0 s LOGO! scan window)
Number of samples (N) : 8   (4 s moving window)
Output : to B001 (Analog Amplifier for scaling)

For digital output chattering caused by mechanical relays or contactor bounce, add an Off-delay (TOFF) timer immediately after each threshold trigger output:

B016 - Off-delay 2.0 s on Q1 output
B017 - Off-delay 2.0 s on Q2 output
B018 - Off-delay 5.0 s on Q3 output (longer to protect from rapid stage-out)

This dual-stage filter (analog moving average + digital off-delay) reliably eliminates chattering on installations with up to 30 mV RMS noise on the 0-10 V signal. The total loop delay introduced is approximately N * ST + TOFF = 4 s + 2 s = 6 s, which is acceptable for vacuum system pressure time constants of 10-60 s.

Alternative Approaches: PI Controller and PWM

The original question mentioned attempting PI controller and PWM functions. These are appropriate when modulating a variable-speed pump or a proportional valve rather than staging discrete contactors. For the discrete three-pump application, the threshold trigger + off-delay + alternation timer approach is the correct choice. The PI controller is recommended only if the system is later upgraded to include a VFD-driven pump.

If a PI controller is required:

B020 - PI Controller
SP (setpoint)   : -800  (engineering: -0.80 bar)
PV (process var): AQW1 (scaled pressure)
KP              : 0.50
TN (integral)   : 30 s
Direction       : Direct (output rises as PV drops below SP)
Manual mode     : Connect to digital input I1 for maintenance bypass

If PWM is required for a throttle valve:

B021 - PWM
Period          : 10 s
Minimum on-time : 0.5 s
Input           : B020 analog output (0-100%)

For the discrete staging application described, leave the PI and PWM blocks out of the program. They introduce unnecessary complexity and can actually worsen relay chattering because the output slews continuously across the trigger band.

LOGO! Soft Comfort Implementation Steps

1. Open LOGO! Soft Comfort V9.0 and select the target hardware: File > Select Hardware > LOGO! 9 BM 230 RCE.
2. Wire the WIKA transmitter output to analog input I7 on the BM. Confirm in the right-side "Inputs" panel that AI1 reads a stable voltage when the line is pressurised.
3. Insert B015 (Average Value) and route its output to B001 (Analog Amplifier with gain -1, offset -1000).
4. Insert B003, B004, B005 (Analog Threshold Trigger blocks) and enter the On/Off threshold values from the table above.
5. Insert B006 (On-delay 5 min) and B007 (On-delay 10 min) for alternation timing.
6. Insert B008 (Off-delay 30 s) for minimum-run protection.
7. Insert RS flip-flops B009 and B010 to implement the lead/lag alternation between Q1 and Q2.
8. Build the AND/OR network B011 to B014 to derive the final coil drive signals for Q1, Q2, Q3.
9. Insert B016, B017, B018 (Off-delay blocks) on each output to provide mechanical debounce.
10. Assign outputs Q1, Q2, Q3 on the LOGO! base module to the B014 outputs.
11. Add a Display message (LOGO! onboard display or LOGO! TDE) showing the live pressure value and the pump status. Example text: P = [AQW1 / 1000] bar Q1:[B012] Q2:[B013] Q3:[B014].
12. Save the project as a .lsc file, then transfer to the LOGO! via Ethernet or LOGO! USB cable.
13. Run in simulation mode first (Tools > Simulation) to verify the logic against the pressure waveform from your pneumatic system.

Commissioning and Verification Procedure

1. With the system at atmospheric pressure (0 bar), confirm AI1 reads 1000 raw units and the display shows 0.00 bar.
2. Apply -0.50 bar with a calibrated pressure standard. Confirm the display reads -0.50 bar within +/-0.02 bar.
3. Slowly decrease pressure through -0.80 bar. Verify Q1 and Q2 energise; verify Q3 stays off.
4. Continue to -1.00 bar. Verify Q3 turns off and the lead/lag timer begins. Confirm after 5 minutes the active Q1/Q2 pair swaps.
5. Return pressure to 0 bar. Verify all three pumps restart in the order Q1, Q2, Q3 with the 2 s off-delay between staging events.
6. Log the LOGO! web server or TDE display data over a 24-hour burn-in. Confirm no spurious cycling events and that pump runtime is balanced within +/-10% between Q1 and Q2.
7. Record the firmware version reported on the LOGO! display (Menu > Diagnostics > FW Version). Document the LOGO! Soft Comfort project revision number and date in the maintenance log.

Troubleshooting Matrix

Specifications Summary

Frequently Asked Questions