Control Using RF Communication

RF refers to the frequencies that fall with in the electromagnetic spectrum associated with radio wave propagation. RF current creates electromagnetic fields when applied to an antenna that propagate the applied signal through space. Electromagnetic wave based communications have been utilized for many decades especially for wireless voice communications and data communications. The frequency of RF signal is inversely proportional to the wavelength of the field. The rate of oscillation for the radio frequencies is in the range about 30 KHz to 300 GHz.

RF waves that have been modulated to contain information are called RF signals. These RF signals have some behaviors that can be predicted and detected and they can interface with other signals. Antennas must be used for receiving the radio signals. These antennas will pick up more number of radio signals at a time. By using radio tuners particular frequencies can be picked up. There are some free bands available which are used for remote controlling applications. These are also called ISM (Industrial, Scientific and Medical) bands. The most attractive frequency band is 434 MHz

The payload data needs to be modulated on the RF carrier. Two simple modulation techniques Amplitude shift Keying (ASK) and Frequency shift keying (FSK) are popular for this. For power consumption reasons, ASK is mostly implemented as ON-OFF keying (OOK). The challenge is finding an antenna design or concept which represents a perfect compromise between cost and performance. A clear RF design is necessary for meeting regulations.

Bidirectional links for RF remote control:

High end remote controls may be used based on bidirectional RF links. In addition to the link for remote controller to the controlled device there is an additional link backwards from device to controller. This backward link may be used for securing the robustness of the remote link by using handshake protocols and giving feedback to the user. Bidirectional RF links are implemented using RF transceiver ICs which include an RF receiver and RF transmitter sharing one single PLL and one single antenna.

Protocols for RF communication:

RF remote control protocol uses device addresses and commands to represent the information. Each RF remote control requires a unique ID means each transmitter world wide has unique ID. Hence the bit length reserved for RF IDs is longer (For example 32 bits to 40 bits length).

For improved robustness of the RF link Cyclic Redundancy Check (CRC) values are often generated and transmitted as part of the frame. The receiver may clearly identify any bit errors by recalculating the CRC values of the received data frame and compared with the one generated before transmission. The transmitter’s battery charging level may be signalized with a complete 4-bit or 8-bit data field representing the measured battery voltage. The systems allow one way communication between two nodes namely transmission and reception.

The RF modules have been used in conjunction with a set of four channel encoder and decoder ICs. HT-12E and HT-12D or HT-640 and HT-648 are the most commonly used encoders and decoders respectively in RF communication. The encoder is used for encoding transmission data while reception is decoded by decoder. Encoder will be used for transmitting the data serially instead of sending parallel. These signals are serially transmitted through RF to the reception point. Decoder is used for decoding the serial data at the receiver and coverts as parallel data.

Applications of RF communication:

RF communication mainly used for wireless data, voice transfer applications, and home automation applications, remote control applications and in industry oriented applications.

For example, in home automation applications we can use RF controlled switches instead of conventional switches. For this purpose a RF remote can be used for controlling lights and other devices without moving to other places. This application is mostly useful for physically handicapped people. In industry oriented applications for controlling robots and vehicles RF communication can be used. The robot vehicles are generally used in risky operations which can’t be performed by humans. For this a transmitting unit is needed for controlling the robot vehicles movement.

RF transmitting unit to control Robot vehicle

Robot vehicle unit controlled by RF transmission unit

Because of many reasons transmission through RF is better than IR (infrared). Firstly signal through RF can travel larger distances making it suitable for longer range applications. IR mostly operates in line of sight mode, but RF signals can travel even when there is an obstruction between transmitter and receiver. RF transmission is having high reliability than infrared remote communications. RF communications use a specific frequency, but IR will not use specific range and they will affected by other IR emitting sources.