3.2. Wireless actuator design
The wireless actuator consists of three blocks: power management, control unit with transceiver, and the set of relays turning on/off actuators. The block diagram of wireless actuator is shown in Fig. 5. The sensor node actuates the gas valve wirelessly relying on ZigBee/IEEE 802.15.4 standard. Data transmission is realized at2.4 GHz frequency with bit rate up to 250 kB/s using BACnet protocol [11]. This communications protocol was designed for building automation and control networks. It enables the communication of building automation and control systems for applications such as Heating, Ventilating, and Air-conditioning Control (HVAC), access control, fire detection systems and their associated equipment. The goal of BACnet is to provide the mechanisms for computerized building automation devices to exchange messages, regardless the service they perform.
The operation and control of wireless actuator shown in Fig. 6,is enabled by software algorithms implemented and programmed on onboard Micro Processing Unit (MCU) STM32F102C6. A user can upload software to MCU using JTAG interface. For precise control of input signals in the micro- and nanoseconds range, the wireless actuator has to be calibrated properly. The calibration notices are recorded in EEPROM M95640 which is connected to MCU using SPI.
Apart from the information on calibration, the memory chip stores the information on the occurring events, e.g. emergency.
Wireless communication of actuator is enabled, as on the sensor node, by ETRX3 chip interfaced with MCU by UART. The transceiver is operated by AT commands from MCU.
For the wireless actuator stationary power supply is used. Power management block converts 220 V AC-based power signal into DC24 V one (3 A maximum) and then divides this primary DC output into three DC sub voltages. The first sub DC source can be adjusted between 7 and 20 V and is used to control gas valve through a Relay1. The output voltage can be adjusted by a voltage divider and digital potentiometer controlled by MCU. The valve is 5/4 , the actuation voltage is 20 V (its standby voltage is 7 V) and pickup current is about 1 A.
The second sub DC/DC output is fixed at 12 V and serves to supply and control the relay set with other actuators, e.g. sound alarm and/or ventilation, that are not used in this work. The last sub DC/DC generates 3.3 V to supply control unit that includes MCU with memory and radio ZigBee transceiver.