Conveyor Speed via Proximity Pulses on Siemens S7-300

Conveyor Speed via Proximity Pulses on Siemens S7-300

A single inductive proximity switch mounted on the head or tail roller of a conveyor is the cheapest possible tachometer. It emits one pulse per revolution, the PLC counts those pulses inside a fixed time window, and the line speed in m/s is reduced to one multiplication and one division. The challenge on the S7-300 platform is not the math but the choices you make around the input filter time, the cycle in which you count, and how the program copes with the motor start-up transient. This reference walks through a CPU 317F-2 PN/DP implementation with a 30 cm roller, the relevant S7-300 signal chain, the frequency limits of the standard SM 321 digital input modules, working SCL code, an optional FM 350-1 high-speed counter upgrade path, and a clean migration to IO-Link when the conveyor is rebuilt with Sirius and ET 200SP.

1. System Overview

Application: monitoring a driven conveyor belt (belt linear speed expected 0.3–3 m/s typical, peaks up to 5 m/s), with a 3-wire PNP inductive proximity mounted once per revolution of the drive roller. The PLC is a Siemens CPU 317F-2 PN/DP (order number 6ES7317-2EK14-0AB0, firmware V3.3) backed by a standard SM 321 6ES7321-1BH02-0AA0 16DI/24 V module. The pulse is a single 24 V rising edge per revolution, so the entire feature reduces to a counter, a 10 s sampling window, and one real multiplication.

2. Prerequisites

3. Belt Speed Mathematics

Given a roller of effective diameter D and one pulse per revolution (PPR = 1), the belt linear distance travelled per pulse equals the roller circumference:

C = π · D   [m/rev]     v = (N / T) · C / PPR   [m/s]

With D = 0.30 m the roller circumference is 0.9425 m per revolution. For higher resolution you can mount a multi-tooth wheel (a sprocket or a custom disc with 4–12 ferrous targets) and use PPR = number of teeth; the formula above is unchanged. To make the engineering case to operations, the table below maps line speed to pulse frequency and the number of pulses captured inside a 10 s sampling window for PPR = 1.

For belt speeds up to 5 m/s, 53 pulses per 10 s — a pulse period of 189 ms — is comfortably above the floor imposed by any DI filter you might configure. Speed resolution is approximately 0.094 m/s per pulse in a 10 s window. If 0.1 m/s granularity is too coarse, drop the window to 1 s and live with the noise, or increase PPR to 4 and the resolution becomes 0.024 m/s for the same 10 s window.

4. Frequency Limits of Standard Digital Inputs

The SM 321 family supports selectable input filter times. On the 6ES7321-1BH02-0AA0, the HW config in TIA Portal exposes 0.1 ms, 0.5 ms, 3 ms, and 15 ms filter options. The maximum pulse frequency that the input can capture is roughly 1 / (2 · t_filter); below that figure the rising edges are passed cleanly to the process image. Above it, edges are clipped, the counter under-reads, and the computed speed is silently too low.

For a 1 PPR roller at 5 m/s the pulse rate is barely 5.3 Hz, so even the 15 ms filter (33 Hz ceiling) is more than adequate. The reason to choose the 3 ms or 15 ms filter is noise immunity: long cable runs alongside a Sirius 3RW soft-starter or a PowerFlex/Sinamics VFD inject common-mode noise that can corrupt fast 0.1 ms edges. For a 1 PPR target with cable length ≤ 25 m, use 3 ms. For a multi-tooth wheel with PPR = 10, the pulse rate is 10× higher, so the filter must be 0.5 ms or the counter will under-read.

5. Wiring the Inductive Proximity to SM 321

Wire the PNP output of the proximity to a 24 V input channel of the SM 321, sharing the same PSU that powers the sensor. The standard 3-wire schematic uses brown (+24 V) from the PSU, blue (0 V) to PSU 0 V, and black (NO output) to the SM 321 input terminal. Use shielded cable (e.g. LiYCY 3 x 0.34 mm²) and bond the shield at the cabinet entry gland only; the SM 321 channel itself is not referenced to shield.

6. Time-Window Pulse Counting in S7-300

Two implementation strategies exist. The first uses a hardware counter on the CPU or an FM 350-1, and the second uses the standard process image plus a software counter in OB1 or a cyclic interrupt OB (OB10…OB17, or OB35 at 100 ms). For a 1 PPR sensor at 5 m/s, the software counter is fully adequate because the shortest period (189 ms) is an order of magnitude longer than the typical OB1 scan of 5–15 ms. The rule of thumb is: a software counter in OB1 is safe when PPR × v / C < 1 / (4 · t_OB1). For 1 PPR and 0.3 m roller this evaluates to v < 1 / (4 · 0.015 · 0.9425) ≈ 17.7 m/s. Above that, switch to the FM 350-1.

7. Startup Delay and Latch Logic

A conveyor driven by a soft-started motor (Sirius 3RW, Sinamics V20, or PowerFlex) takes 2–8 seconds to reach speed. Counting pulses in the first 1–2 seconds would yield a low, false speed. The standard solution is a TON timer (in IEC 61131-3, the SFB 4 / IEC_TIMER_0_1) armed on the motor start command, after which pulse accumulation is enabled and the sample window is held open for 10 s. On every window expiry, the latched count is divided by window time and multiplied by the circumference, then the counter is reset and the next window begins.

8. SCL Implementation on CPU 317F

The FB below encapsulates the gate, the counter, and the speed calculation. It can be called once per OB1 cycle or once per OB35 tick. Constants are exposed in the instance DB so they can be tuned from the HMI without re-loading the program.

FUNCTION_BLOCK FB_BeltSpeed
VAR_INPUT
    iStartCmd   : BOOL;   // 1 = motor start request
    iProxPulse  : BOOL;   // prox input (filtered process image)
END_VAR
VAR_OUTPUT
    oSpeedMs    : REAL;   // belt linear speed in m/s
    oRunning    : BOOL;   // 1 = monitoring active (after start delay)
    oCount      : INT;    // pulses in the current window (live)
    oFault      : BOOL;   // 1 = no pulse in window while command active
END_VAR
VAR
    tStartDelay : TON;    // IEC on-delay
    tSample     : TON;    // IEC 10 s window
    rCircum     : REAL;   // pi * D, in m
    cnt         : INT;
    bLatchEdge  : BOOL;
END_VAR
VAR CONSTANT
    T_START_DELAY : TIME := T#10s;
    T_SAMPLE      : TIME := T#10s;
    D_ROLLER_M    : REAL := 0.30;
    PPR           : INT  := 1;
END_VAR

BEGIN
    // roller circumference per revolution
    rCircum := 3.14159265 * D_ROLLER_M;

    // gate: arm on start command, hold 10 s
    tStartDelay(IN := iStartCmd, PT := T_START_DELAY);
    oRunning := tStartDelay.Q;

    // count edges only when running
    IF oRunning AND iProxPulse THEN
        cnt := cnt + 1;
    END_IF;
    oCount := cnt;

    // 10 s sampling window with rising-edge latch
    tSample(IN := oRunning, PT := T_SAMPLE);
    IF tSample.Q THEN
        // freq [Hz] * circumference [m] / PPR = m/s
        oSpeedMs := (INT_TO_REAL(cnt) / TIME_TO_REAL(T_SAMPLE))
                    * rCircum / INT_TO_REAL(PPR);
        cnt := 0;
        tSample(IN := FALSE);   // re-arm next cycle via oRunning
    END_IF;

    // no-pulse fault: command active, gate open, but count == 0 over 10 s
    oFault := oRunning AND (cnt = 0) AND tSample.Q;
END_FUNCTION_BLOCK

9. High-Speed Counter Alternative (FM 350-1, ET 200S 1Count)

When the application grows — a multi-tooth encoder, a VFD that needs an actual speed feedback channel, or a 5 m/s line with PPR = 60 — the FM 350-1 counter module (6ES7350-1AH03-0AE0) reads up to 500 kHz per channel and exposes the count as well as a hardware-calculated frequency via the CTRL1 block. The ET 200S 1Count 24 V module (6ES7138-4AA01-0AB0) is the distributed alternative on PROFIBUS or PROFINET. Both eliminate the cycle-time dependence of the OB1 counter and report directly in Hz, which the S7-300 then converts to m/s with a single multiplication by the circumference.

10. IO-Link Migration Path

The user’s note about renewing the system with “Sirius and I/O Link” points to the modern replacement. Inductive sensors with IO-Link output (SICK IME12, IFM IG5, Balluff BES M12) report not just the bit but also diagnostic data (count overflow, target visibility, supply voltage). On the S7-300 side, an ET 200SP station with interface module 6ES7155-6AU00-0BN0 plus a CM 4xIO-Link communication module 6ES7557-0AA00-0AB0 brings the data into the CPU via PROFINET. The pulse stream is then delivered as a 32-bit process value with no DI filter in the path, and the SCL in section 8 can be reused without modification — only the source of iProxPulse changes.

11. Verification and Commissioning Procedure

1. Mechanically confirm roller diameter at the lagging surface with a tape measure. Compare to the nameplate; the lagging thickness often adds 5–15 mm to the bare OD.
2. With the motor stopped and the prox energised, place a ferrous target by hand under the prox. Verify the SM 321 input LED toggles and the process image in the online view reflects the change.
3. In the FB instance DB, set a temporary D_ROLLER_M = 0.30 and force iStartCmd = TRUE. Watch oCount increment on each pulse and oSpeedMs update every 10 s.
4. Compare the calculated speed against a hand tachometer on the roller (rev/s × C). Tolerate ± 5% for belt slip on the roller; ± 1% if the roller is directly geared to the motor.
5. Run the conveyor at the maximum design speed (5 m/s in this example). At the 10 s window refresh, verify oCount ≈ 53 and oSpeedMs ≈ 5.0.
6. Stop the conveyor with the command still asserted. Verify oFault rises to TRUE within one window and is logged to the HMI alarm view.
7. Open the Web server of the CPU 317F-2 PN/DP (http://<cpu-ip>) and confirm the oSpeedMs tag is visible in the diagnostics buffer, in case the HMI is unavailable.

12. Troubleshooting Matrix

13. Field-Proven Caveats

• Mount the prox at the drive roller (motor side) rather than the tail roller. The drive roller has no belt slip relative to the shaft; the tail roller can slip on a worn take-up.
• A single 4×4×4 mm cube of mild steel stuck on the roller face is a sufficient target for a typical 4 mm-sniff prox. Bigger targets mean more sensing margin and less missed pulses on a dirty or wet roller.
• If the conveyor is inverted or vertical, add a software debounce of 20–50 ms after the rising edge; gravity-driven chains can chatter the prox at zero speed.
• Route the prox cable in a separate conduit from the Sirius 3RW soft-starter power cables. The 3RW generates sharp edges at 4 kHz typical that capacitively couple into nearby low-voltage cables.
• The CPU 317F-2 PN/DP web server is enabled by default in TIA Portal ≥ V15. Use it for first-line diagnostics even before the HMI is in place.

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