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Basic Concepts
Siemens PLC is well-known as the “brain” that controls various devices in factories. The smart sensor can be understood as the PLC’s “eyes” and “ears”, responsible for collecting various data. The combination of both enables automation systems to be more intelligent, respond faster, and operate more efficiently.
Hardware Connection
Connecting smart sensors to Siemens PLC can be done in several ways:
- Analog input: For example, 4-20mA or 0-10V signals, suitable for continuous parameters like temperature and pressure.
- Digital input: This refers to switch signals, either 0 or 1, suitable for detecting the presence or position of objects.
- Communication interfaces: RS485, PROFIBUS, Ethernet, etc., which can transmit more information and simplify wiring.
For example, if we use a smart temperature sensor connected to the PLC’s analog input via 4-20mA, 20mA corresponds to 100℃, and 4mA corresponds to 0℃, with a linear relationship in between. The PLC can then calculate the actual temperature based on the current value.
PLC Program Example
Let’s see how to read this temperature value in the Siemens S7-1200:
// Network 1: Read analog value and convert to actual temperature// Assume using AI_0MOVE "AI_0", MD100 // Move the value of AI_0 to MD100ITD MD100, MD104 // INT to DINTDTR MD104, MD108 // DINT to REALMUL_R MD108, 6.25, MD112 // 6.25 = 100 / (20-4), temperature range/current rangeSUB_R MD112, 25.0, MD116 // 25 = 6.25 * 4, correct 0℃ corresponding value// MD116 now contains the actual temperature in degrees CelsiusThis code first reads the analog value, then performs a series of conversions, and finally obtains the actual temperature. Note , don’t forget to set the correct analog range in the hardware configuration!
Practical Application Case
I previously assisted in renovating the cold storage control system at a food factory, using Siemens S7-1200 paired with a smart temperature sensor. It not only accurately controls the temperature but also automatically adjusts the cooling power based on the cold storage door’s open/close status, saving a lot of electricity.
Moreover, we added a humidity sensor that communicates with the PLC via MODBUS. This allows for comprehensive monitoring of the cold storage environment and can predict condensation, enabling the dehumidification equipment to start in advance. The boss exclaimed, “Intelligence is really appealing!”
Common Problems and Solutions
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Signal Fluctuation: Sometimes the analog signal may fluctuate, causing unstable readings. The solution is to add a smoothing filter in the PLC program, such as averaging 10 sampling values.
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Communication Interruption: When using bus communication, interruptions may occasionally occur. It is recommended to include communication monitoring in the program; if no data is received for several cycles, an alarm should be triggered.
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Sensor Failure: Smart sensors can occasionally malfunction. A reasonable range can be set, and if the reading exceeds this range, it can be assumed that the sensor may be faulty, triggering an alarm in time.
Precautions
- Wiring must be carefully checked , especially for analog signals; incorrect connections may damage the PLC input port.
- For critical parameters, it is best to use high-precision sensors; precision = efficiency = money!
- The alarm values set in the program should allow for some margin; do not set them too rigidly to avoid false alarms.
Practical Suggestion: Find an old temperature sensor and PLC to practice with, programming to achieve temperature display and alarm functions. Gradually add humidity, pressure, and other parameters to create a small environmental monitoring system. Remember, only by hands-on practice can you truly master it!
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