Omron CJ1M Load Cell Filling: 4-20mA Scaling and Ladder Logic

Gravimetric filling of 1 L milk bottles is a classic high-precision, high-throughput automation cell. The core challenge is closing a milk fill valve in the few hundred milliseconds it takes for the in-flight stream to settle, while keeping every bottle inside a tight legal-for-trade tolerance. This reference walks through a complete implementation on an Omron CJ1M platform using a DACell CB1-K015 compression load cell, a SM110 digital weight indicator/transmitter, a CJ1W analog input module, and a step-by-step ladder program with two-stage (coarse + fine) valve control and in-flight weight compensation.

1. System Overview and Application Envelope

The target machine is a linear or rotary bottle filler for fresh milk. The rated throughput assumed here is 30 bottles per minute per station (two-second cycle), with one head per station. Every bottle arrives empty, is clamped under a fill nozzle, is tared with the bottle on the load cell, is filled, is verified, is capped, and is released downstream.

Operating envelope used for the worked example:

2. Hardware Bill of Materials and Signal Chain

The signal chain must be sized so that the total combined error (cell non-linearity, indicator noise, ADC quantisation) stays under one third of the legal tolerance. The parts below are the recommended minimum.

2.1 Signal-flow diagram

3. CJ1M CPU and Analog Input Module Selection

The CJ1M mid-range family accepts a wide mix of special I/O on the same rack. The minimum configuration for a single-head filler:

• 1 × CJ1M-CPU13 (10 ksteps, two built-in pulse outputs unused here).
• 1 × CJ1W-PA205R power supply, 5 V / 24 V.
• 1 × CJ1W-AD041-V1 analog input (4 channels, set channel 1 to 4-20 mA).
• 1 × CJ1W-OC211 or CJ1W-OD211 16-point output card for valves, cap release, bottle-stop.
• 1 × CJ1W-ID211 16-point input card for prox sensors, photocells, E-stops, valve feedback.
• Optional CJ1W-ETN21 Ethernet for HMI / SCADA recipe download.

Refer to the CJ1M Series Catalog (Cat. No. P059) for slot count, current budget, and supported special I/O. Analog ranges are set per channel using the four range bits in the I/O table of CX-Programmer; 4-20 mA uses input range code 0000 hex on the CJ1W-AD041-V1.

4. Wiring Topology: Load Cell to PLC

4.1 Load cell cable

Use 4-conductor shielded cable between CB1-K015 and SM110. Tie shield to ground at the indicator end only. Avoid running the load cell cable in the same tray as VFD output cables; cross at 90° if unavoidable.

4.2 Indicator to analog card

SM110 typically provides 4-20 mA on a two-wire loop powered from the analog card. The CJ1W-AD041-V1 supplies loop power when set to current mode. Wire:

Set the CJ1W-AD041-V1 input range for CH1 to 4-20 mA in CX-Programmer I/O Table. Confirm with a milliamp calibrator before powering the cell that the loop reads 4.000 mA on the empty platform and 20.000 mA on a 15 kg test load.

5. Analog Scaling: 4-20 mA to Engineering Units

Omron analog modules return a signed 16-bit integer where 4 mA maps to 0 and 20 mA maps to 27648. The end-of-range and fault markers are:

The rated engineering span is 27648 counts. The single scaling constant used in ladder is therefore:

Weight[g] = (Raw_counts ÷ 27648) × Full_Scale_Weight[g]

For a 15 kg full scale with the empty platform tared out:

Weight[g] = Raw × 15000 ÷ 27648 = Raw × 0.54254

5.1 Why the integer constant matters

The CJ1M does not have a built-in floating-point engine. Scaling is done with integer math. To preserve one decimal place of resolution (0.1 g) over 15 000 g of span, keep the math as:

Weight_dg = (Raw × 150000) ÷ 27648    (units: decigrams, 1 dg = 0.1 g)

This keeps the numerator 24-bit (max 16 588 800), well within the 32-bit signed range the CJ1M uses for multiply (B × C). For finer work on a 5 kg cell use (Raw × 50000) ÷ 27648, giving 0.1 g resolution on 5 000 g.

5.2 First-order filter

Raw counts from a 4-20 mA loop are not stable enough to drive a fine-fill cutoff directly. Implement an exponential moving average:

Filtered[n] = (Raw × 1 + Filtered[n-1] × 7) ÷ 8

This is a 1/8th-weight filter. On a 1 ms scan, 8 cycles reach ~75 % step response, fast enough for a 2 s fill cycle but slow enough to kill the 50 Hz noise that the loop picks up from VFD cabinets. Tune the divisor 4 / 8 / 16 in field.

6. Two-Stage Fill Sequence and Setpoints

A single-stage fill cannot meet legal-for-trade accuracy at 30 BPM: the in-flight stream alone at 200 g/s coarse flow is ~5 g. Two stages - bulk coarse and trickle fine - reduce the in-flight mass at cutoff to under 1 g, which the software compensation absorbs.

7. In-Flight Weight Compensation

The mass of milk still falling between the valve seat and the bottle mouth at the instant the valve closes is the largest single error term. With CV_FINE closed, capture the actual weight and compare to the commanded setpoint:

InFlight = Setpoint_Fine − Actual_At_Cutoff

Add this to the next bottle's fine setpoint:

Setpoint_Fine(n+1) = Setpoint_Fine(nominal) + InFlight_n

Bound the value to ± 20 g; if the in-flight mass jumps outside this window, the milk temperature, supply pressure, or valve seat is changing and a recipe correction or alarm is required. On a properly tuned machine the steady-state in-flight term converges to 1-3 g within 5 bottles.

8. Shift Register for Multi-Station Sequencing

For a single station a sequential state machine is enough. For a linear or rotary filler with N stations that share a single CJ1M, drive the sequence with a shift register so each station advances on the conveyor pulse:

Use the Omron SFT(014) instruction. Reset the register on a station-home prox. Clock it with the conveyor index pulse. A single ladder rung drives the state for whichever station is active:

|     SFT(014)  W_bit[1]  W_bit[1]           |  Rung 1: shift on index pulse
|       |          |         |               |
|   IndexPulse  Station1   Station1          |
|                                          |  Rung 2: state machine on active bit
|     LD   W_bit[1]                         |
|     AND  NOT State_Tare                   |
|     AND  BottlePresent_1                  |
|     OUT  Cmd_Tare_1                       |

The shift register pattern is the recommended structure for any N-station filler; per-station coils are gated by the active bit, so a single SFT handles sequencing and the per-station I/O map stays in one place.

9. Tare, Zero, and Weight Stability Detection

The empty bottle mass is 30 g ± 1 g. A pre-fill tare is non-optional. A stability check is required after tare and after the fine cutoff, because the weight reading wobbles for ~150 ms after the valve closes.

9.1 Stability algorithm

Implement a band check over N consecutive scans:

IF ABS(Filtered[n] − Filtered[n−1]) < 1 dg  for 20 scans  → Stable = TRUE

1 dg = 0.1 g is tight enough for a 15 kg cell. Widen the band to 3 dg if the platform is mechanically noisy (long bellows, suspended hopper).

9.2 Tare sequence

1. Bottle prox ON.
2. Wait for Stability = TRUE (no bottle yet, just empty platform).
3. Sample 16 scans of the filtered weight, average into D_Tare_Offset.
4. Subtract D_Tare_Offset from every subsequent reading in the cycle.
5. Set State_Tared = TRUE, advance to Coarse.

10. CX-Programmer I/O Table and Module Setup

After online connection to the CJ1M, the I/O table must reflect the actual rack. CX-Programmer auto-detects slot contents. Set:

For a CJ1W-AD041-V1 the 4-20 mA range writes the 0-27648 count into the channel's input word. Confirm the address in the I/O Table dialog - on a CPU13, the first analog input module on slot 3 lands at CIO 200 + (channel - 1). The CJ1W-AD041 Datasheet lists exact addresses per slot.

11. Ladder Logic Implementation

The example below is a single station. It assumes:

• Raw 4-20 mA value lands in W_AI_Raw each scan.
• Filtered value in W_AI_Filt (1/8 EMA).
• Scaled grams in W_AI_Grams (integer grams, tared).
• Setpoints stored in DM: D_Target=1030, D_CoarseOff=830, D_FineOff=1022.

11.1 Analog input and filter

|     LD     Always_ON                         |  Read raw counts
|     MOV(021)  CIO_200_CH1   W_AI_Raw        |
|                                              |
|     LD     Always_ON                         |  EMA filter
|     *      W_AI_Raw    1    Tmp1             |
|     *      W_AI_Filt   7    Tmp2             |
|     +      Tmp1  Tmp2    Tmp3                |
|     /      Tmp3  8       W_AI_Filt           |
|                                              |
|     LD     Always_ON                         |  Scale to grams
|     *L     W_AI_Filt  15000   Tmp4          |  Long multiply, 32-bit
|     /L     Tmp4       27648   W_AI_Grams    |

For finer resolution multiply by 150000 and divide by 27648 to get decigrams into W_AI_dGrams. Then divide by 10 to obtain W_AI_Grams for setpoint comparison.

11.2 Tare

|     LD     BottleProx                        |  Bottle present
|     AND    NOT State_Tared                   |
|     AND    Stable                            |
|     MOV(021)  W_AI_Grams  D_Tare_Offset     |
|     SET     State_Tared                      |

11.3 Coarse fill

|     LD     State_Tared                       |
|     AND    State_Coarse                      |
|     AND    NOT State_Fine                    |
|     OUT    Cmd_CV_COARSE                     |  Energize coarse valve
|                                              |
|     LD     State_Coarse                      |
|     AND    (W_AI_Grams GE D_CoarseOff)     |  Reached 830 g
|     SET     State_Fine                       |
|     RSET    State_Coarse                     |

11.4 Fine fill and cutoff

|     LD     State_Fine                         |
|     AND    (W_AI_Grams GE D_FineOff)        |  Reached 1022 g
|     RSET    Cmd_CV_FINE                       |
|     SET     State_Verify                      |
|                                              |
|     LD     State_Verify                       |
|     AND    Stable                             |
|     AND    ABS_Diff LE 50                    |  1030 ± 5 g
|     SET     State_Verified                    |

11.5 In-flight update

|     LD     State_Verify                       |
|     AND    Stable                             |
|     MOV(021)  W_AI_Grams  W_ActualAtCutoff   |
|                                              |
|     -      D_FineOff  W_ActualAtCutoff        |
|     MOV(021)  Result   D_InFlight            |
|                                              |
|     +      D_FineOff  D_InFlight              |  Apply on next bottle
|     MOV(021)  Result   D_FineOff_NEW         |
|     LIMIT check ± 20 g                       |  Alarm if out of band

11.6 Bottle release and station advance

|     LD     State_Verified                     |
|     PULSE  Cmd_CapRelease  200 ms             |
|     PULSE  Cmd_BottleStopRelease 100 ms       |
|     RSET   State_Verified                     |
|     RSET   State_Tared                        |
|     RSET   State_Coarse                       |
|     RSET   State_Fine                         |
|     RSET   State_Verify                       |

12. Commissioning and Calibration Procedure

1. Mechanical check. Bottle-stop closes fully, load cell is mechanically isolated from the conveyor, no contact between bottle and nozzle during fill. Apply a 15 kg test mass; the cell must read 15 000 ± 5 g on the HMI before any software scaling is touched.
2. Loop check. Disconnect the SM110; inject 4.000 / 12.000 / 20.000 mA from a calibrator into the analog card. The HMI raw counts must read 0, 10322, 27648 within ± 2 counts. If off, the analog card is suspect.
3. Zero and span. Reconnect SM110. With no load, execute a software zero. Apply a 5 kg test mass; adjust the scaling multiplier so the HMI reads 5 000 g.
4. Tare test. Place an empty bottle. Confirm the tared weight reads 0 g within 1 second. Lift the bottle; weight returns to ~30 g. Repeat 5 times; if any reading drifts more than 2 g, the SM110 is not stable or the mechanical isolation is leaking.
5. Single-stage dry run. Disable the fine valve; perform 20 fills in coarse only. Record the actual mass and the cutoff timing. Compute the in-flight mass range; this is the value the fine stage must compensate.
6. Two-stage dry run. Enable fine. Adjust the fine shut-off point until the actual mean weight equals the target. The in-flight update should converge within 5 bottles.
7. Throughput run. Run 200 bottles at the rated 30 BPM. Verify the weight histogram is inside ± 5 g at 95 % (2 sigma). If not, see the troubleshooting matrix below.

13. Verification and Acceptance Test

14. Troubleshooting Matrix

15. Frequently Asked Questions