Wireless sensors

Some sensors sit where no cable can reach: on a rotary table, a crane, a moving carriage or a remote tank. A wireless sensor sends its signal by radio to a receiver in the panel, which gives the PLC an ordinary input signal.

How it works

The sensor (or a transmitter module connected to a normal sensor or switch) sends its state as a short radio telegram several times per second. The receiver in the control panel decodes it and switches an output wired to a PLC input, or sends the data over a network.

Power

• Battery: years of life if it only sends on change plus a periodic heartbeat.
• Energy harvesting: the energy of pressing a switch, a small solar cell or vibration powers the transmission. No battery at all.
• 24 V supply: for sensors that have power but no signal cable.

Link monitoring and the safe state

A cable either works or is visibly broken. A radio link can fade silently. That is why good wireless systems send a heartbeat: if the receiver hears nothing for a set time, it switches its output to the safe state (usually OFF) and sets a link-alarm output. Your PLC program must treat "link lost" as a fault, never as a valid sensor state.

Range and interference

Metal machines, walls and other radio systems reduce range. Typical systems use 868/915 MHz or 2.4 GHz with a few tens of metres indoors. Mount antennas high, outside metal cabinets, and check signal strength (RSSI) during commissioning.

Where wireless is (and is not) used

• Good: position switches on rotating or travelling machines, door and gate switches, tank levels far away, temporary installations.
• Avoid for fast signals (radio adds latency) and for safety functions, unless the system is certified for safety.