W4 Sensor Teach Process Issue with PLC

Overview

We have identified a specific issue with the teach process of the W4 sensor when replacing the older WTB4C-3P3464 model or similar  (archived, https://www.sick.com/de/de/catalog/archiv/wtb4c-3p3464/p/p222000) with the newer WTB4FP-84167120A00 variant (or Similar)  (⁣WTB4FP-84167120A00 - W4 | SICK ). This problem affects customers using the B&R Automation Studio 4 environment on an X20CP1585 controller, where the teach process is initiated via an HMI. The issue arises due to a combination of sensor response timing and the existing control code, leading to a failure in the teach execution.

Problem Description

The error occurs when the new WTB4FP-84167120A00 sensor is installed as a replacement. The teach process, triggered through the HMI, fails to complete because the new sensor takes slightly longer to provide the `TeachInState` value compared to the older WTB4C-3P3464 model. The control code, written in Automation Studio 4 and running on a globally deployed X20CP1585 controller, queries the `TeachInState` only once. Since the new sensor’s response is delayed by a few milliseconds, the code does not detect the updated status, causing the teach process to stall between steps 2 and 3.

 Root Cause Analysis

1. Sensor Response Time: The new WTB4FP-84167120A00 sensor exhibits a marginally slower response time for delivering the `TeachInState` compared to the older WTB4C-3P3464. This delay, though small (a few milliseconds), is critical in this context.

2. Code Limitation: The existing control logic reads the `TeachInState` only once per cycle. If the sensor has not yet updated this value by the time of the read, the process fails. Ideally, the code should poll the `TeachInState` repeatedly until the sensor completes the teach operation, but this is not feasible due to the fixed nature of the deployed code.

3. Cycle Time Constraint: The system operates with a 10ms cycle time, which is insufficient for the new sensor to respond reliably. The test showed that increasing the cycle time to 20ms or higher eliminates the issue, as it allows enough time for the sensor to provide the `TeachInState`. However, modifying the cycle time is not an option since the code is locked on widely used X20CP1585 controllers.

Observed Behavior and Testing

Customer Scenario: The error manifests only with the new WTB4FP-84167120A00 sensor and a 10ms cycle time. The older WTB4C-3P3464 sensor worked flawlessly under the same conditions, indicating the issue is tied to the new sensor’s response characteristics.

Testing Results: When the cycle time was adjusted to 20ms or greater in a controlled environment, the teach process was executed successfully with the new sensor. At 10ms, the failure rate was 100% with the new sensor, while the old sensor performed reliably.

Root Cause Combination: The problem is a synergy of the new sensor’s slightly delayed `TeachInState` update and the code’s single-read approach, exacerbated by the 10ms cycle time constraint.

Solution

To resolve this issue without altering the existing code, a special variant of the WTB4FP-84167120A00 sensor has been developed. This new variant provides the `TeachInState` value more rapidly, aligning its response time with the 10ms cycle requirement of the current system. Key points:

Implementation: Customers can replace the problematic WTB4FP-84167120A00 sensor with this optimized variant. Contact our support team for availability and ordering details.

Compatibility: The new variant is designed to be a drop-in replacement, requiring no changes to the HMI or control logic.

Recommendations

Upgrade Path: If replacing a sensor, ensure the optimized WTB4FP-84167120A00 variant is used to guarantee compatibility with the 10ms cycle time.

Workaround: For sites unable to immediately source the new variant, temporarily increasing the cycle time to 20ms (where feasible) can serve as a stopgap measure until the upgraded sensor is installed.

Preventive Note: Avoid using the standard WTB4FP-84167120A00 sensor in systems with a 10ms cycle time and single-read `TeachInState` logic unless the optimized version is confirmed.

Additional Considerations

Since the X20CP1585 controllers are deployed worldwide and code modification is not possible, this sensor upgrade is the most practical solution. The issue highlights the importance of matching sensor response times to system cycle constraints, especially in legacy systems with fixed logic.

For further assistance or to order the optimized sensor variant, please contact our support team. We are committed to ensuring seamless operation of your automation systems.