13 January 2009
However, there are obvious risk factors involved in wiring sensors to moving machines. Each requires a cable for power as well as for data, plus there are wiring terminals and I/O modules also located in the manufacturing cell. The cables have to be flexible to allow for the robot’s movement, so wiring and cable guides require a great amount of time for planning, assembly, installation, and commissioning.*******************************************RIGHT: Wireless proximity sensors clustered at the end of the robot arm receive power by induction from the primary loop (blue edges to the manufacturing cell) whose power supply is shown on the floor at the lower left. The sensors transmit their data via 2.4 GHz frequency hopping radio to an input module in the switch cabinet on the right, which is connected to the control system.*******************************************The best cable, as they say, is one that doesn’t have to be installed. Thinking along these lines, ABB Stotz-Kontakt presented at Hannover Fair the first pre-production samples of a new wireless proximity switch that promises to completely eliminate the task of cabling sensors on robots and production machines, or anywhere inside the manufacturing cell, for that matter. The wireless proximity switches provide the same basic functions as conventional inductive proximity switches. However, they do not require any cable connections for the power supply and data transfer between sensors and machine control system, which opens up completely new possibilities for the construction of manufacturing machines.In the initial offer there are four different size sensors M8, M12, M18, and M30; each can be either flush or non-flush mounted. The nominal switching distances range from 1.5 mm to 15 mm. The cylindrical shaped sensor heads are fitted into a communication module that is the same size for all sensors; it is approximately the shape of a cube 50 mm on a side (see photo). The communication module is connected to the sensor head like a standard sensor plug. The sensors and communication modules are designed for protection class IP67 and operate in ambient temperatures of –25 to 55 deg.POWER SUPPLY, COMMUNICATIONSAt one point the designers considered using batteries, but the idea was soon discarded, says Wolfgang Zimmermann, Sensor Product Manager with Heidelberg-based ABB Stotz-Kontakt. Frequent battery changes would be too much of a burden for the customer, he says.Instead of batteries, the module obtains the energy it needs for the sensor function and radio communications by induction from the electromagnetic supply loop installed in the manufacturing cell (the blue lines in the diagram).The sensors are designed for low power: 6 mW for each. The electromagnetic field used for transfer of power is produced by a power supply connected to primary loops that surround the manufacturing cell. The minimum size power loop has a planar dimension of 1 m x 1 m. For larger areas, loops can be constructed up to 3 m x 6 m. This results in a minimum supplied volume of 1 m x 1 m x 1m up to 3 m x 3 m x 6 m. As long as the sensors are inside the orthogonal volume defined by these primary loops, they will receive sufficient power.************************************The IP67 communications module is connected to the sensor head like a standard sensor plug. It receives its power by induction and transmits radio data from the proximity sensor, which is partially visible at the rear.************************************A 120 kHz signal in the primary loop induces the operational current in the sensors. The round, high-frequency-cable has one conductor approximately 10 mm² in diameter. This conductor is made of many thin wires, which are insulated against each other by enamel. This is necessary to prevent the skin-effect of the current with the frequency of 120 kHz. Each primary loop has one winding of this cable.Communication takes place in the 2.4 GHz ISM band. It uses frequency hopping for secure data transfer, according to the standard ETS 300328. Up to 60 sensors communicate with one input module, which in turn communicates the sensor signals via two antennas and transmits these to the machine control system via a FieldBusPlug (FBP) A maximum of 5 input modules – up to 300 wireless proximity switches – can be used in one machine cell.GREATER SAFETYDuring assembly and operation, the sensors can be clearly identified by pressing a button at the proximity switch and the input module. The initialisation of a new proximity switch during commissioning takes place simply at the touch of a button. Contrary to conventional proximity switches, a continuous presence check, in which each sensor indicates correct functioning to the input module twice every second, guarantees a timely fault recognition. Mr. Zimmermann says this is a “level of reliability previously unknown.”A typical enclosed manufacturing cell in an automobile plant can contain from 30 to 200 proximity switches, claims Mr. Zimmermann, which all have to be fitted with power supply and signal transmission cables for each sensor. “Complex assembly machines sometimes need modifying to customer requirements up until its very manufacture and this can prove to be particularly expensive due to the wiring,” he said. “In operation, especially with robots, there is the risk that in the course of time the cable might fray at critical points.”And, he points out, removed with the cables is also a fault source which occurs in rough ambient conditions. This could be aggressive media such as oils, greases, metal chips, and drill emulsions or simply high humidity. --Michael Babb, Control Engineering Europe
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