Wireless Discrete Control Applications

Update: I received feedback that several portions of this post were incorrect and not current so after consulting with our team of experts, here’s a modified version of the post.

Original post:Last week in a post, Wireless Device-Based Control Loops, we highlighted PID control with wireless field devices. PID is not the only type of control that can be done across a wireless field network. I caught up with Emerson’s Tracy Niebeling, who manages some of the Rosemount brand wireless devices, to talk about some of the other ways process manufacturers are applying IEC 62591 WirelessHART devices in control applications.

Numerous discrete control applications can be automated via a wireless sensor network. Tracy highlighted the Rosemount 702 wireless discrete transmitter as a device that can be used for these discrete control applications. The 702 has two channels that can be configured as digital inputs (DIs) or digital outputs (DOs) in any combination—2 DIs, 1 DI / 1 DO, 2 DOs.

Configured as inputs, the 702 accepts dry contact inputs. Some example input switches include proximity, GO Switch, float, pressure, and flow. Additionally, the 702 can detect momentary discrete inputs of 10 milliseconds or more in duration, regardless of the wireless update rate. At each wireless update, the device reports current discrete state along with an accumulating count of close-open cycles for each input channel. The 702 reports the state of these inputs back to the control system, SCADA, or PLC connected to the wireless gateway or redundant wireless gateway pair.

Configured as outputs, the channels can drive small loads up to 26VDC and 100mA the 702 acts as a switch to an externally powered 24VDC loop to switch the load on or off. An interposing relay can be used for circuits beyond 26VDC and 100mA to drive larger output loads. Typical applications include valve control, motor control, pump control, lights, and alarms to name a few.

At the Emerson Exchange event in Nashville last October, Tracy co-presented on the subject of wireless control solutions. For some control applications, they recommended a pair of redundant wireless gateways connected to the plant Ethernet control network. The gateways form a private network using the secondary Ethernet port. Both gateways can see the network of wireless field devices, and the redundant gateway operates on standby, ready to jump in and take over network responsibilities in the event of a primary gateway failure.

A simple control application is to configure a channel on the 702 to drive switch on a circuit such as a flashing light. The control system sends an on or off command to the wireless gateway which then transmits it wirelessly to the 702. The 702 energizes or de-energizes switches on or off the output channel circuit to turn the light on or off.

In their presentation, they provided additional examples including level control, remote operator button, pump start, and plunger lift on a gas production well.

To describe one example, let’s take the circulating tank pump control system. A control system is connected to the wireless gateway. The wireless network includes a 702 discrete transmitter reading a local on/off switch. Another 702 (or the second channel of the existing 702) is connected to the pump motor.

When the on/off switch is turned on, it is read by the 702 and transmitted back to the control system through the wireless gateway. After logic in the controller verifies the pump is turned on, the control system sends a start signal to the 702 in order to switch on the circuit to the pump’s motor, and then flow into and out of the tank begins. This simple example of wireless control can be further enhanced with wireless indicators for flow, level, and even the position of the valves.

Where wired infrastructure is not already present, the opportunity to make local control visible back to the control room operators becomes easier with wireless networks. These are just a few examples of how you might incorporate wireless-based control into your plant.

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