IAS Introduces next generation wireless sensor, RF90II.

Based on years of experience with wireless devices in the field, IAS set out to solve a few fundamental problems that are typical with outdoor wireless sensor design.

Range / power compromise:

Amplitude of an RF signal varies with the distance it must travel logarithmically. That means as a simple example, that if you double that transmission distance, you don’t cut the signal strength in half, but perhaps reduce the signal strength to something closer to one fourth of it’s original strength. Similarly simply increasing the power of the transmission does not yield a linear increase in potential distance, and in fact, increasing the transmission power may also increase some transmission/reception problems such as reflection and refraction, i.e. noise.

The four things RF90II does to optimize range with respect to transmission power:

1. Lower signaling bit rate. Many wireless devices intended for outdoor use will utilize bit rates in the 150K to 250K range, while quoting sensitivity/distance numbers that are only realistically achievable with much lower bit rates. For general usage, there are good reasons a vendor might choose a higher bit rate, but since the RF90II is designed from ground up for remote sensing application, it can afford to transmit at a bit rate of roughly 10k bits per second to achieve maximum signal-to-noise ratio. This is roughly equivalent to 9600 baud in RS232 terms.

2. Introduce a backward error correction scheme. In simple terms, the RF90II waits to receive an acknowledgement from a base station for each wireless sensor data transmission that it makes. The RF90II will attempt each wireless sensor data transmission up to 3 times, but will go back to sleep as soon as it receives the 1st acknowledgement.

3. Introduce a forward error correction scheme. The RF90II utilized a 16-bit checksum for each packet so that the receiving base station can verify the packet correctness. Due to the nature of RF transmissions, there can be bit errors in the received packet, and in that case, the base station will not acknowledge that packet, forcing the sending RF90II to retransmit. In order to reduce the probability of bit errors, thereby reducing retransmission and saving power, the RF90II computes a correction block for the original packet, and transmits the packet in the block correcting form. The receiving base station, then reverses the computation, thereby correcting individual single-bit errors, and then checks the checksum for correctness. This is known as forward error correction, since the information needed to correct the packet is forwarded along with the original data.

4. RF90II Intelligent Repeater. There a different approaches to improving RF transmission/reception between a transmitter and receiver. Using specialized antenna’s, introducing various error-correcting schemes, improving line-of-sight are all valid techniques. Another technique is to utilize an RF repeater. The simplest example of a repeater is a radio that retransmits any packet that it receives. Many repeaters function this way, but this can also introduce additional problems with respect to wireless transmission/reception. Since wireless agents that form a network are generally communicating using the same frequency band(s), simple repeaters are putting more transmissions into air, and adding more noise into the communication channel. The RF90II ‘listens’ to each wireless sensor transmission and only gets involved in transmission if it fails to detect any acknowledgement from a base station. Additionally, multiple RF90II Intelligent Repeaters can collaborate. Each repeater tracks the signal strength from each wireless transmission that it receives, and periodically shares this information with all other repeaters. By using this information, each repeater can decide for any given transmitter if it is the optimal repeater to handle a given non-acknowledge transmission.

Enclosure integrity.

Wireless sensors are built out of fairly sensitive electronic components. In order to protect these from outdoor industrial/agricultural environments, enclosure integrity is critical. Two of the main challenges to maintaining that integrity are, case penetrations and case opening operations. The RF90II design has several features specifically intended to reduce or eliminate either penetrations or opens.

1. Internal antenna: The RF90II uses an internal Grounded Line Planar Antenna from Linx Technologies built directly onto the PCB. This both eliminates an RF connection which can reduce signal strength, but also eliminates a case penetration for the antenna.

2. External sensor connector: The RF90II uses a Binder IP67 Snap-In circular connector for interfacing with the external sensor. While this can not eliminate a case penetration, it does not reduce the overall IP67 protection level.

3. Battery: One of the main reason to open the enclosure is to change or measure the battery. In order to reduce the need to open the case, the RF90II minimize battery usage so that a single battery can provide multiple years of use before needing to be change. Also, the RF90II reports the battery voltage on each sensor transmission so that battery level can be monitored externally.

4. Enclosure Lid Tension: Small enclosures for wireless devices are typically closed with screws. Improper tensioning of the screws can result in an incomplete seal. The RF90II enclosure utilizes stainless-steel, quick-release fasteners that are spring-loaded to always apply the correct tension when closed.

Additionally, the RF90II has an integrated Temperature/RH sensor built directly on the PCB, so that the internal moisture/temperature of the enclosure can be remotely monitored.

“Pairing” wireless sensors and base stations:

Some vendors require a physical pairing process for each pair of wireless sensor to base station. The design of the RF90II and integration into a base station such as the IAS XR3000 eliminates much of the need for that arduous process.

There are two design features in the RF90II to aid in the connecting or mapping of RF90II wireless sensors to a base station such as the IAS XR3000.

1. Frequency of transmission. The RF90II transmits approximately every 60 seconds, so that the automatic registration process at the XR3000 is much quicker.

2. By setting the 2nd DIP switch on the RF90II PCB to match the lowest 8 bits of the XR3000 Base Station MAC ID, that specific RF90II will only be acknowledged and monitored by the matching XR3000.
Additionally, a single RF90II can optionally be monitored by multiple XR3000 base stations if desired.