By Michael Stephens & Michelle Todd
By Michael Stephens & Michelle Todd, BEI Industrial Encoders
Encoders come in a wide variety of sizes, accuracies, and ranges based upon just as wide a variety of mechanical and optical technologies. And you can pay as much or little as you wish. But all encoders have one limitation in common; wires and connectors that interface the measuring head to signal conditioning electronics and read-out devices. In most cases, machine designers can work around the annoyances and limitations that numerous wires and unreliable connectors pose. But in some installations, such as cranes, rotating tables, and mobile applications, it certainly would be nice if wires and connectors could be eliminated. Unfortunately, that never was very easy to accomplish – until recently.
SwiftComm is licensed to use a 5.6 dBi gain dipole antenna with a reverse polarity connector.
The antennas are mounted directly to the mating connectors of both the transmitter and receiver modules. Every 600 microseconds, the transmitter sends a data packet to the receiver with the current encoder data. The SET-UP indicator flashes each time an acknowledgement packet is lost. The SET-UP indicator is a measure of signal quality. The normal flash rate is 1 to 2 flashes per second or a 99% packet success rate. This feature is used during initial setup to optimize the location of the antennas, clear or avoid obstructions, and minimize interference sources.
Now, a new encoder interface called SwiftCommTM overcomes this problem; it is wireless and can provide all the range, accuracy, and robustness that you heart desires. Despite the most common and obvious fears surrounding wireless devices, this one has put them all to rest. The downsides have been addressed.
At the top of the list of concerns is wireless encoder reliability in the face of interference. The Swiftcomm system is designed specifically for critical motion control applications where radio frequency interference (RFI) can be a problem. The system uses 2.4 GHz Adaptive Frequency Hopping Protocol (AFHP) to select one of several available RF bands that are interference free to improve reliability. But the world is not totally forgiving and in some remote cases, a little interference will crop up in a reasonably quiet band. When a packet of information gets lost in most typical wireless systems, the controller asks that the information be resent. This protocol slows the data flow from transmitter to receiver and adds an indeterminate amount of time delay before valid data are received. Over Zigbee or Bluetooth systems, this latency can degrade time-sensitive data, and random interference can destroy whole packets.
How to interconnect a simple system
SwiftComm has flexible I/O electronics that lets it connect to numerous industrial sensors and control systems. Simply connect the transmitter to the sensor, the receiver to control system, and apply power. The security codes are not printed on the devices’ labels; they are safe at the BEI factory. If a replacement module is needed, the modules’ serial number is supplied to BEI to match the original pairs’ security code.
By comparison, SwiftComm tracks the encoder output and overcomes any data loss due to an interrupted link. It processes historical data trends and fills the voids in the encoder’s output information stream (which resulted from the lost packet). When the next valid packet of information comes through, SwiftComm computes possible accumulated errors and sends appropriate corrections to the controller. Even in environments where occasional packets are lost, the system recovers and sends a valid stream of data to the controller.
The LINK and SET-UP lights indicate the quality of the RF connection between the modules. On startup, transmitter and receiver modules search their assigned RF spectrum for another module with the same address. When the modules locate each other, they exchange frequency hopping
sequence and other housekeeping information. After finishing, the LINK light turns on. The transmitter then sends the quadrature signal from the encoder as a packet over the RF connection to the receiver. The receiver reconstructs the encoder signal from the received packet and informs the transmitter of a successful packet exchange.
The SwiftComm hopping algorithm uses 75 ISM channels in a pseudo-random sequence. When it detects interference on any channel, that channel is dropped from the hopping sequence and the system avoids using it in the future. If the available channel list becomes exhausted, previously dropped channels are retested to determine if they are clear to be used.
If you are concerned with security, SwiftComm has taken care of it. The transmitter and receiver are protected with a 40-bit, hard security code. The codes are factory programmed and 40 bits make it tough for hackers to decode the more than 500 billion possible combinations that result. The protocol is proprietary and not available to the public. If that is not enough, data are transmitted with a high-level encryption algorithm and random frequency hopping.
Another fear is latency in some high-speed applications. These systems cannot tolerate the smallest time lag in signal transmission: Delays cause major problems. But SwiftComm is one of the fastest wireless sensor interfaces available; it relays data between transmitter and receiver every 600 microseconds. And because the system is a point-to-point configuration, it has minimal inherent latency.
Clean, dry, and far-reaching
Yet another potential problem on the list is outdoor applications where wired systems frequently need to be replaced. SwiftComm satisfies this: the transmitters and receivers are encased in NEMA 4 cast aluminum enclosures, powder coated and gasketed to protect them from windblown dust and rain, splashing water, and ice that might form. Exposure to such harsh environments does not affect them.
What is the maximum distance that the transmitter can reliably transfer encoder data to the receiver? There is nothing new here. Like all wireless systems, the maximum transmission distance depends on the transmitter-receiver environment. For example, on the factory floor, where the environment is filled with electrical noise and metal objects that obstruct the signal path, communications are reliable to about 300 feet. However, outdoors with mostly an unobstructed line-of-sight between receiver and transmitter, communications can reach over 1000 feet.
Finally, you want to know if multiple SwiftComm pairs can operate in the same area without interfering with one another. The answer is yes. Because each pair contains a unique security code, and they use AFHP, the combination ensures they will not interfere. When a certain radio band is being used by one pair, the other pair seamlessly hops to another open band. This significantly helps avoid data interruptions due to RF interference.
BEI Industrial Encoders
:: Design World ::
Filed Under: Wireless, Electronics • electrical, Encoders (rotary) + resolvers, Motion control • motor controls, Networks • connectivity • fieldbuses, Wireless devices