Wireless controls have been widely accepted and embraced in the industrial community. Widespread use of monitoring devices in the process industry, the deployment of RFID components in a variety of industry segments, and the demonstrated performance of a large, installed base of the technologies serve as evidence of their viability.
While there has been a large selection of wireless devices available for monitoring, transmitting, receiving, and processing continuously variable parameters like flow, temperature, and pressure, there has been little available in the way of wireless switches which facilitate simple “on-off” signals for machine start/stop control, presence/position sensing, counting, alarm signaling, and other desired digital control events.
Functionality has improved, however, with the development and introduction of wireless digital contact and non-contact switches, sensors, and related application accessories.
Available wireless digital control components designed expressly for industrial environments include:
- Mechanical position/limit switches
- Non-contact magnetic and inductive sensors
- Photo-optical sensors
- Pull-wire switches
- Pushbutton, selector, and key-operated command switches
- Foot switches
- Door handle systems with integral pushbutton control functions
- Universal wireless transmitters
- Receivers and repeaters
The realization of wireless digital control components has been made possible by the blending of existing technologies with a number of notable technical developments. Among the advancements are:
- The design and production of robust, energyharvesting mechanisms (energy generators) capable of meeting the duty cycle and lifeexpectancy requirements typically encountered in industrial applications. One effective technique is shown in Figure 1. Here a permanent magnet is moved through an electrical coil by the motion of the switch actuator. The movement creates sufficient electrical energy to transmit an encrypted, wireless telegram to a “paired” receiver within the signal’s range. The generators, designed with mechanical life expectancies in excess of 1 million operations, eliminate the need for a battery power source in a variety of mechanically-actuated switches, including pull-wire, pushbutton, key-operated, or foot switches.
- The design of rugged, universal wireless transmitters capable of powering a family of inductive sensors and wirelessly transmitting their change-in-state to a remote receiver. The transmitters can also be used with conventional position-monitoring devices having a dry contact or PNP output.
- The use of high-energy-density, long-life lithium batteries (where mechanical energy generators are not feasible).
- The application and use of noise-immune wireless frequencies (e.g., 868 MHz, 915 MHz, and 2.4 GHz) that are nationally- or globally-accepted standards in industrial control applications. (See Table 1)
- The design of transmitter electronics that require little power and permit bidirectional communications between transmitters and receivers. Transmission protocols feature short telegrams for minimal transmission- time durations and optimal noise immunity. (See Table 2)
The control products offer a range of potential benefits:
- Lower installation costs: The elimination of materials — cable, connectors, conduit, cable tray, junction boxes, and strain reliefs, for example — and their associated installation labor can result in significant savings (see Figure 2). Such economies can be even greater for switches and sensors that are located relatively far from their termination point, or which require armored, shielded, or other premium-priced cabling.
- Reduced maintenance costs: The removal of cabling also reduces the potential for cable damage and related repair or maintenance costs. This benefit can be especially valuable where cables are susceptible to damage due to repeated bending, flexing, vibration, or other mechanical stress.
- Increased system uptime: As maintenance requirements are reduced, system uptime and productivity can be increased.
- Flexible control system “topology”: The availability of wireless switches and sensors enables the control system designer to place digital inputs in remote and hard-to-access-with-cable locations. In addition, switches can be easily added to an existing machine or work cell to realize an ancillary control or monitoring function.
- New potential technologies for selected EX applications: The inherent, low-power operating characteristics permit use of ATEX-IECEx-certified wireless switches/ sensors in explosion-hazardous areas, without the need for intrinsically- safe barriers, encapsulation, or other costly protection methods.
- New potential technologies for applications where wiring or wiring maintenance is deemed impractical or not economically feasible: Examples include remote locations where distributed electrical power is not readily available, rotary equipment requiring slip rings or flexible cables, and places where cable runs are extremely long or the installation cost is considered excessive.
- Reduced personnel costs: Use of wireless switches may also reduce personnel costs in applications in which digital information is collected locally by human messenger. Examples are local in-situ production counts and assembly station stock status.
Since these devices are relatively new, many potential applications have yet to be identified and realized. Among the applications already experiencing one or more of these benefits are:
- Remote point presence or position sensing
- Assembly station inventory management systems
- Fire vent position monitoring
- Door, hatch, and access port position monitoring
- Hopper flap/diverter monitoring
- Safety shower/eyewash station alarm signaling
- Rotary machinery
- Automatic door control (see Figure 3)
- Explosive environments
Application: Automotive Assembly Line Inventory Management System
In this application, assembly line workers at this location require several different components to complete each assembly. In the past, employees would place a “stock requirement” card in a pick-up location as additional parts were required. Periodically, a “roving messenger” would retrieve the cards and take them to the stock room, at which point replenishment parts would be dispatched to the assembly cell.
By placing wireless limit switches beneath the part bins located on gravity feed bin tracks, the “messenger” has been eliminated. Each track typically contains multiple bins of one of the required assembly components. As the leading bin on each track empties, it is physically removed to allow the next bin in the track to slide into the first bin position (see Figure 4).
When the last bin of parts in each track slides into the first position, the second position in the track becomes vacant. The absence of a bin in the feed track changes the state of the wireless limit switch, sending a wireless signal to the central stock location. The signal alerts the stock room of an emerging need for additional quantities of the specific part associated with the received signal. Required parts are then dispatched to the assembly cell.
Benefits of the wireless technology included low installation costs, personnel cost savings (from eliminating the “messenger”), faster stock replenishment, reduced assembly cell downtime, and increased productivity.
Application: Tank Truck ‘Filler Pipe’ Position Monitoring
Similar wireless limit switches proved of value in a refinery’s tank truck filling facility. Here the goal was to assure that the neck of the fuel filling pipe was properly positioned above the opening in the tank truck being loaded — before beginning the filling operation.
Originally, filling pipe position was monitored using conventional cabled limit switches. While the solution was functional, the electrical cables from the switches to the control panel quickly became worn or were damaged by the frequent movement and/or from chafing by the fuel pipe. Replacing the “cabled” switches with wireless, batteryless, functional equivalents eliminated the problem. Maintenance was greatly reduced, with no batteries or cables to replace. The wireless technology also lowered installation costs, since no cables were run from the switches to the control cabinet. The switches enabled safe operation in an explosion-risk area.
Emerging Applications and Developments
With increasing awareness of the technology, creative OEMs and end-users continue to find and implement new applications. Among these are:
- Remote manual valve position monitoring in both hazardous and conventional environments
- Vehicle presence monitoring in large distribution centers
- Protection of dies in stamping operations
- Position monitoring of flaps and doors in grain handling and air conditioning systems
- Elimination of cables for position sensors in flexible conveyor systems
In addition, the applications are stimulating the development of new components and capabilities. Developments include new bus-compatible receivers for integration with Modbus, Ethernet, and other popular protocols. Listenbefore- talk sensors further enhance reliability in applications with a high population of like-frequency devices.
This article was written by Peter Engstrom, Managing Director at STEUTE Industrial Controls, Inc. (Ridgefield, CT). For more information, Click Here