Wireless sensors for mobile machines
Wireless sensors for mobile machines
For quick and straightforward retrofits of machines, wireless-based sensors are the ideal solution. All safety features can also be implemented without costly and time consuming cable fanning.
Having decided for a wireless solution, the next decision to be made is in which radio band. Since sensors must operate independently of their location and be easily installable, the only radio bands which come into question are those which do not require approval. To avoid possible low frequency interference from industrial switching equipment, motors etc., a frequency well above 100 MHz is advisable. Global license-free broadcasting frequencies (ISM radio bands) include 433 MHz (Europe/Asia), 868 MHz (Europe/Asia), 915 MHz (America), 2.4 GHz and 5 GHz.
The 2.4 GHz frequency band is currently the best license-free compromise between range and transmission rate. Despite a large number of protocol families such as Wi-Fi, Bluetooth, WirelessHART, ZigBee etc., the choice of actual physical interfaces is quickly reduced to three broadcasting standards: IEEE 802.11, IEEE 802.15.1, and IEEE 802.15.4.
Systems based on the IEEE 802.11 standard achieve very high transmission rates with an acceptable range but also have very high energy requirements.
The IEEE 802.15.1 broadcasting standard is used by Bluetooth technology and transfers data over very small distances (max. 10 m) with low energy requirements. Class 1 type devices with a maximum transmission power of 100 mW achieve distances of up to 100 m but have correspondingly high energy requirements. The disadvantage of Bluetooth is its complex profile structure and restriction of 8 active devices per network cell.
Hirschmann solution
Wireless sensors and systems from Hirschmann Automation and Control GmbH are based on the lower ZigBee layers according to the IEEE 802.15.4 standard.
This broadcasting standard restricts itself to intermittent transmission of small data packets with extremely low energy consumption: for the majority of the time, connected nodes (up to 254 per network) are idle but quickly wake up from their so-called sleep mode to enter their active mode, when required to do so. While Bluetooth nodes need between 3 and 10 seconds to recommence transmission, ZigBee nodes need less than 15 milliseconds. The modulation method used is quadrature phase modulation (DSSS/QPSK).
The maximum line-of-sight range for Hirschmann wireless sensor systems is approximately 300 m. For deployment in mobile machines, the maximum usable range of the network is guaranteed to be around 50 m, which takes into account possible reflections at metal surfaces and interference from other systems.
Hirschmann wireless sensor family
Crane retrofit requirements keep the number of wireless sensors to a manageable level. This gives rise to straightforward network topologies and removes the need for Hirschmann wireless sensor systems to implement a dedicated layer 3 protocol. All sensors are directly bound to their target system and communicate in star topology with this receiver. Hirschmann has limited the number of sensors supported per target system and network cell to 7 in order to minimise the latency time for network access.
Hirschmann's wireless sensor product line includes a wind speed sensor (iSENS WSS-W1), angle sensor (gSENS WGF-W1), anti-2-block switch (iSENS HES-W1) and two predefined load cells with capacities of 6 and 20 tonnes respectively (fSENS KMD-W1). Please enquire for details about other available load cells. All sensors use standard alkaline or lithium batteries which guarantee a lifetime of at least 12 months when used in typical applications.
iSCOUT PRS85 multi-sensor display
Hirschmann's iSCOUT PRS85 console terminal comprises a graphical multi-sensor display. The display can be used to configure alarm settings for up to 7 sensors. A percentage of the maximum allowed value can be selected as the display unit for the measured values of force, angle and wind speed. If this maximum is exceeded, the system can be programmed to shut down or generate an external alarm using built-in relays. In addition, regular “health checks” are carried out to calculate and display the remaining battery capacity of all connected sensors.
Transceiver modules for CANopen operation
The iFLEX TRS10 transceiver module provides the required interface for connecting up to four wireless sensors to a system bus via CANopen. The sensor data conversion is transparent which means the sensors behave as virtual CAN nodes and the measured values are made directly available to the user. Configuration of the sensor network also takes place via the CAN system bus. The iFLEX TRS14 transceiver allows the wireless transmitted sensor data to be converted into current or voltage values for subsequent interfacing to analogue systems.
Low energy consumption with high safety
The deployment of wireless sensors presents a fundamental challenge: battery power must not be wasted to ensure maximum battery lifetime. The typical working phases of a crane are useful in regard to this point: during its dynamic phase the measured values of force and speed vary greatly over time, while during its long idle periods or periods with static loading, the values remain constant. In contrast to static loading, the dynamic phase requires measurement values to be transmitted as quickly as possible in order to avoid large jumps in the calculation of parameters such as load moment.
Hirschmann has solved the conflict between battery lifetime and transmission speeds by individually adjusting the transmission speed to match the rate of change of the measured values. The result can be summarised as follows: the more dynamic the process, the faster the transmission speed of measurement values. The sensor actively fetches and updates its data. Measurement values are not transmitted sequentially.
Average reaction time: 15 ms
Triggering the crane’s anti-2 block switch represents one of the most critical changes in a measurement value. Extensive tests show reaction times for Hirschmann systems to lie between 7 and 50 ms. The most frequently measured values are in the range 12 to 22 ms, with an average reaction time of 15 ms.
Hook control
For mechanical reasons, wired installations can be very difficult to realise in practice. For example, it may be required to monitor the hook block for hook misalignment, particularly in large tower cranes with double jib and double winder. Solution: the straightforward installation of a wireless angle sensor.
A wireless load cell can be used to measure the load on the crane hook precisely and directly. Advantage: a conversion of indirect forces is avoided.
CONTROL unlimited
September 2009