LOGO! OBA8 Two-Tier Oven Controller: PT100 Setpoint Changeover

LOGO! OBA8 Two-Tier Oven Controller with Single PT100 and Multiplexed Setpoint

1. Application Summary

A two-tier heating profile is the most common oven sequence in light industrial and laboratory service: warm to setpoint 1, hold for a fixed dwell, then ramp to setpoint 2 and hold for a longer dwell. The temperature is read by a single PT100 RTD, but two distinct operating setpoints must be presented to one PI controller. The cleanest implementation on a Siemens LOGO! 8 OBA8 is to feed the PI controller's setpoint input from an Analog Multiplexer (FB B15) and drive the mux selector from an on-delay timer chain. The process value comes from a single AM2 PT100 module.

The OBA8 generation is required because it ships with the analog gain/offset function block, a software-configurable PT100 input on the AM2 module, and sufficient block memory to hold a PI block, an analog mux, three on-delay timers, and a weekly timer in one program. Earlier LOGO! variants (OBA5, OBA6, OBA7) do not all expose the same blocks in the same place, so the program below must be checked against the block catalog of the target device.

2. Prerequisites

3. Hardware Wiring: PT100, AM2, and SSR

The OBA8 base unit exposes AI1–AI4 as 0–10 V inputs only. The PT100 must terminate on the AM2 PT100 or AM2 RTD module's AI3 / AI4 terminals, which are software-configurable for PT100, NI1000, or 0–10 V. Use 4-wire PT100 wiring to cancel lead resistance on long oven runs:

Set the AM2 DIP switch to PT100 (not 0–10 V, not 0/4–20 mA). The AM2 channels then read directly in °C with a range of −50 to +200 °C on PT100 and −50 to +400 °C on NI1000, and the engineering value appears in LOGO! as a scaled integer 0–1000 corresponding to the displayed range.

For the heater output, choose one of two options:

1. On/off control (Q1 digital output): simplest, but cycles the heater aggressively near setpoint. Acceptable for small ovens with high thermal mass.
2. PWM control (Q1 configured as PWM in LOGO!): the OBA8 supports PWM on digital outputs 1, 2, 3, 4, 5, 6, 7, 8. Period set to 2–10 s for thermal systems. Drives the SSR gate for proportional duty-cycle heating.

4. Sensor Configuration in LOGO! Soft Comfort

Open Tools → Parameter VM Mapping or double-click the AM2 input to confirm the engineering range. For PT100 on AM2 AI3 the value block presents 0–1000 corresponding to −50 to +200 °C. The integer is already a usable process value in tenths of a degree (0 = −50 °C, 1000 = +200 °C, 0.1 °C per LSB).

To convert to physical units the PI block can accept either raw counts or °C. The cleaner engineering workflow is to keep the value as the PI block's process value (PV) in tenths of a degree and enter the setpoint in the same units. For a 35 °C setpoint enter SP = 350 (35.0 °C). This avoids extra scaling blocks.

5. Control Architecture: PI Block with Multiplexed Setpoint

The system uses five logical blocks. The Analog Multiplexer is the key — it is what makes two setpoints feed one PI controller without changing any constants at runtime.

6. Step-by-Step Program Build in LOGO! Soft Comfort

1. Insert the AM2 PT100 input block. Drop the AM2 module on the project tree. Click AI3 and confirm the value range in Properties → Analog Inputs is set to PT100. The block exposes its live value at address AI3.
2. Add On-Delay Timer T1 (FB B03). Trigger = I1 (operator Start, or power-on edge). Time constant T = 02:00:00 (2 hours). Output Q1 is the stage-change flag.
3. Add On-Delay Timer T2 (FB B03). Trigger = T1.Q (so T2 only starts when stage 2 begins). Time constant T = 04:00:00 (4 hours). Output Q2 latches an end-of-cycle flag that turns the heater off.
4. Add the Analog Multiplexer (FB B15). Inputs: V1 = constant 350 (35.0 °C), V2 = constant 750 (75.0 °C), V3 = 0, V4 = 0. Selector S = T1.Q. Output AQ1 is the active setpoint in tenths of a degree.
5. Add the PI Controller (FB B14). SP = AQ from Mux, PV = AI3, output AQ1 drives the PWM block. Set KC and TI per the tuning section below.
6. Enable PWM on Q1. In the OBA8 output properties, switch Q1 to PWM mode. Period = 4 s for a typical 1–3 kW oven. The PI output scaled 0–1000 maps directly to 0–100% duty.
7. Wire the heater interlock. Multiply T2.Q and the PI output through an AND block so the heater is de-energized at the end of stage 2 regardless of what the PI requests.

7. Ladder / FBD Excerpt

The function-block diagram in LOGO! Soft Comfort is the cleanest representation. The ladder equivalent of the changeover stage:

|   I1          T1 (On-Delay, T = 2h)        |
|---| |------( B03 )---------T1.Q-------------|
|                                          |
|  T1.Q         T2 (On-Delay, T = 4h)       |
|---| |------( B03 )---------T2.Q-----------+---|
|                                          |   |
|  constant 350 (SP1)                      |   |
|---|>---[ V1 ]--|                          |   |
|               |  Analog Mux (B15)        |   |
|  constant 750 (SP2)        |  S = T1.Q    |   |
|---|>---[ V2 ]--|----------( B15 )--AQ--->|   |
|                                          |   |
|  AI3 (PT100) ----> PV                     |   |
|  Mux.AQ    ----> SP                       |   |
|               |  PI Controller (B14)     |   |
|               |---( B14 )--AQ1-----------+---|
|                                          |   |
|  NOT T2.Q AND PI.AQ1 → Q1 (PWM)     |   |

8. Setpoint Changeover Sequencing and Optional Ramp

A pure step change from setpoint 1 to setpoint 2 will produce a control error spike that the PI controller must then integrate out. For a small oven this is usually acceptable because the thermal mass limits the rate of temperature change. For larger chambers, replace the constant setpoint 2 with an Analog Ramp (FB B11) that ramps from 350 to 750 over a configurable rise time (e.g. 30 min). The Mux then selects between the constant SP1 and the ramp output. This eliminates the step changeover issue and gives a controlled heat-up profile from stage 1 to stage 2.

The state machine driving the changeover is intentionally small: three states held in two on-delay timer outputs and one operator input.

9. Web Server and HMI Visualization

The OBA8 has an integrated web server enabled by default. To expose the process value and active setpoint for remote monitoring, open Tools → Web Editor in LOGO! Soft Comfort and place two display boxes on the default view: one bound to AI3 for the live PT100 reading, one bound to the Mux output. Drag a start button bound to I1 and a reset button bound to a flag that clears T1 and T2 through a reset coil.

For a more capable interface, the OBA8 supports up to 8 S7 connections. A Pro-face HMI, WinCC on a PC, or a simple Weintek panel using the Siemens S7 protocol driver can read the same variables. The minimum variable set to publish is:

10. PI Tuning for Thermal Loads

Use the open-loop step method for first-cut tuning. With the heater at ambient, command a manual PI output of 30% and record the temperature rise rate. The process gain Kp is Δtemp / Δtime per 1% output. The integral time TI is roughly the thermal time constant of the oven (typically 5–30 min for a 1–10 kW chamber). Conservative starting values:

Disable the derivative action in the PI block (TD = 0) — derivative amplifies RTD noise and SSR switching transients on heating elements. If the setpoint changeover produces a 2–4 °C overshoot into stage 2, lower KC by 20% and double TI; if the response is sluggish, raise KC by 25%.

11. Verification and Commissioning Checklist

1. Power up with the heater breaker open. Confirm AI3 reads ambient ± 2 °C with a calibrated reference probe in the oven.
2. Force the Mux selector S = 0 with the LOGO! Soft Comfort online force tool. Confirm SP = 350 (35.0 °C) on the watch table.
3. Force S = 1. Confirm SP = 750 (75.0 °C). The PV should still equal ambient.
4. Close the heater breaker. Start the cycle. Log PV and HeaterPct for 1 hour. The curve should reach 35 °C ± 1 °C within 15–25 min for a typical 2 kW chamber.
5. Let the cycle run past T1.Q. Confirm the changeover: PV should rise smoothly to 75 °C. Acceptable overshoot is <3 °C on the first attempt.
6. Let the cycle reach T2.Q. Confirm heater output drops to 0 and the cycle latches off until I1 is recycled.

12. Troubleshooting Matrix

References to Official Documentation

For deeper detail on the blocks used here, consult the official Siemens LOGO! 8 system manual and the AM2 PT100 module manual on the Siemens Industry Online Support portal. The system manual covers FB B14 (PI Controller), FB B15 (Analog Multiplexer), and the parameter VM mapping for scaled values. The module manual covers DIP switch positions, sensor ranges, and the wiring diagrams used in section 3.