Dealing With Manufacturing's Crowded Airwaves By: Chris Koepfer Wireless technology is everywhere. From cell phones to Wi-Fi Internet access, the airwaves are increasingly filled with electronic signals. Manufacturers have also seen an increase in wireless technology on the shop floor. As these devices proliferate, the integrity of the individual carrier signal is sometimes compromised by noise and competing frequencies among the various devices. The spindle probe used on Renishaw's new frequency hopping spread spectrum radio transmission is a compact design. The diameter of the unit is 63 mm and its length is 79 mm making it applicable in a variety of machines, especially where Z-axis stroke is minimal. Dropping a cell phone call is an inconvenience, but having an inspection probe on a machine tool miss a hit that defines a workpiece location can be dangerous and costly to a metalworking shop. Renishaw has developed a new system for its spindle probes that overcomes these interference problems. The company calls the system frequency hopping spread spectrum (FHSS) radio transmission. The technology is being applied to inspection probes mounted in the spindles of machine tools. These probes are used for determining the location and orientation of parts and for inspecting size and position of critical part features for verification and process control. These probes work by sensing a series of discrete points on the surface of the component. When the probe’s stylus meets the surface of the part, a trigger signal is generated. This signal must be passed to the machine’s CNC so the position of the machine at that instant can be recorded. Because the probe is spindle mounted, the trigger signal must be sent to the machine’s CNC via a remote transmission system. Traditionally these signals used approved, fixed channel, radio frequencies. When applied on larger travel machine tools and five-axis machines—where the probe is not in line with the receiver—these signals are vulnerable to other radio frequencies and electronic noise in the shop. Spread spectrum transmissions are designed to give more robust communication between the probe and receiver than single transmissions. They do this by distributing their signals across a wider frequency range. The system uses “frequency hopping” to transmit a signal that uses a coded pattern of different frequencies identified to the transmitter and receiver. Varying the signal reduces the chance for same signal interference between the probe and the machine tool CNC, reducing the chance of a missed hit from the probe and the workpiece. The FHSS probe transmission hops between a sequence of frequencies in the 2.4 GHz band: 1) At the first frequency, the signal between the probe and receiver is clear. 2) After the first hop, the frequency switches to a channel within the range of a more powerful device. In this case, the probe signal is overwhelmed and lost. 3) Here the frequency has shifted to a point of overlap with a DSSS (direct sequence spread system, used in WiFi wireless networks) device, transmitting over a wide frequency range but with low power. The probe has enough power to get through while the DSSS device still has plenty of bandwidth to transmit. 4) Clear frequency—this means the transmission is successful. 5) The probe continues hopping sequences that will visit all of the available channels over time. Once matched, the RMP60 probe and the RMI receiver will frequency hop together to overcome any interference present in the shop, with no need to change the channel or frequency band. The allocated band for this system is divided into 79 discrete 1 MHz channels. FHSS makes it possible for multiple probe systems and other industrial equipment to coexist reliably with a significantly reduced chance of interference. The RMP60 probe used on this system is compact, measuring 63 mm in diameter and 76 mm in length, for application on a wide variety of machine tools, even those with restricted Z-axis travels. Radio probe systems allow for flexibility in the positioning and orientation of the receiver, especially where line-of-sight restrictions make optical transmission systems impractical. The RMI receiver is a combined antenna and interface requiring no separate enclosure to be mounted in the CNC cabinet. Conduit routings are simplified with this design making installation faster and making the system well suited to retrofitting on existing machines. As electronic equipment and wireless technology continue to develop, finding ways to maintain reliable contact between the signal sender and receiver, especially in unattended or lightly attended production shops, is an important consideration. This system from Renishaw takes most of the concern about interference away from the probing sequence. Chris Koepfer MMS Online Comment on this article