Radio transmission is possible thanks to electromagnetic waves. These waves are present all around us, generated naturally in the universe and by humans on earth. They travel with the speed of light – 300,000,000m/sec. – from the sun to earth in eight minutes and across our galaxy, the Milky Way, in 100,000 years.
Heinrich Hertz generated the first man-made electromagnetic waves in an oscillating circuit. The first demonstration of radio remote control was in 1898 when Nicola Tesla showed his innovation for creating radio waves by controlling a small boat. In 1911 the first radio messages were transmitted in the USA.
Heinrich Hertz has the distinction of his name being given to the unit describing the frequency of electromagnetic oscillations (cycles per second). One hertz, or Hz, equals one oscillation per second. The electromagnetic frequencies used in radio communication range from hundreds of kilohertz (1,000Hz or 1kHz) to gigahertz (1,000,000,000Hz or 1GHz) band corresponding to wavelengths of 1,000m down to centimetres. In industrial radio remote controls the frequencies range from 30MHz (megahertz or one million Hz) to 2,300MHz.
Generally speaking, the higher the frequency (or shorter the wavelength) the better the communication between transmitter and receiver. In industrial radio remote control the required range is seldom more than a few hundred metres. The power of the transmitter is therefore mostly measured in milliwatts (0.001W). A mobile phone is 1,000 times more powerful (W) and a broadcasting transmitter a million times more powerful (kW).
The foundation of the present radio control systems was laid during the late 1950s when industry demanded an efficient and safe system for materials handling. The first radio remote crane control was installed 1968 in a tower crane. Cost savings were achieved as the crane operation and the loading could be assigned to one person. The crane operator accepted double duties as the radio control transmitter was light, easy to carry and had easy to manoeuvre joysticks.
The first systems were simple and crane movements were activated by tones on a wave frequency of 27MHz. Some commands to the crane were given by transmitting two or more tone signals. These early systems only had 16 ID codes for identification. Uncontrollable cranes caused accidents as the transmission was not reliable at that time, mainly due to inter-modulation products, and radio control was therefore not popular.
The first radio remote control systems were designed for industrial and forestry applications in the 30MHz band using amplitude modulation (AM). Frequency modulation (FM) was introduced later. The information, or data, being transmitted modulates the carrier wave. AM is unreliable in an industrial environment due to interference problems. The wavelength of a 30MHz wave is 10m, whereas a wave of 400MHz is only 0.75m. This means that the 30MHz signal has a long distance in which the emitted power is low and that the transmitter may have to be moved by up to 1m to resume contact with the receiver. Mobile phones sometimes have the same problem. A system on 400MHz is not subject to this kind of problem as the distance of low power is as short as 1cm.
Antenna efficiency is also important. A 400MHz antenna has an efficiency of up to 65% and the length of a quarter-wave antenna for 400MHz is 17cm. A quarter-wave antenna for 30MHz should be 2.5m long, which is not practical in a portable transmitter unit. By reducing the length of the antenna, the efficiency is decreased to around 10%.
An undisturbed operation within a limited range is sometimes a problem especially in heavy industry.
Radio remote control has made materials handling more efficient but there are disadvantages:
• the risk of unauthorised use of the radio transmitter has to be minimised by educating crane operators and by keeping the transmitter in a lockable cabinet when not in use
• crane operators need an introduction to radio remote control, with special emphasis on safe operation, before commissioning a system
• the ‘feel’ of the crane control from cabin operation is lost with radio remote control.
There are, however, many safety advantages with radio remote control:
• good supervision of crane operation from a safe distance
• the operator may assist or carry out load preparation which minimises the risk of accidents caused by communication problems and efficiency is increased by improving time management
• the operator can leave the control cabin where he may otherwise have been exposed to a hazardous environment of gas, heat, flame, etc.
• good ergonomic design and a light weight transmitter prevent shoulder and back injuries which are common in cabin controlled cranes and with pendant controllers
• the operator has better social contact with fellow workers.
Electronic safety considerations
Carrier wave frequency: national authorities such as the FCC (Federal Communications Commission) in the USA, and other telecommunication organisations, allocate the use of frequency bands for the carrier wave. The 400MHz band is used in many countries but is subject to various national and local standards.
A standard frequency band in Europe is 433.050MHz – 434.790MHz. With a bandwidth of 25kHz 69 frequencies are available which means that there is room for 69 radio remote control systems in the same area without interference from each other.
There is, however, always a risk of interference as the mixture of two frequencies may generate an unwanted third frequency (f1 ± f2 = f3). This third frequency may be the same as used by another radio remote control system. If the signal is strong enough it may cause the crane to stop.
ID codes: these are set individually for each radio remote control system. Each manufacturer might have its own method for ID code setting meaning that the safety levels may be of different qualities. Switches, ID code plugs or software may be used for setting the ID code. There should never be two systems with the same code.
Hamming distance: EN 60 204-32 is a norm specifying that one change in a control function shall result in at least three changes in the data flow.
Redundancy control in the receiver: the system shall incorporate a means to prevent it responding to external or disturbed signals. Using two separate single-chip microprocessors with differential programming using redundant control has proven to be the safest technique. To make sure that the microprocessors are functioning correctly, electronic watchdog circuits supervise their operation. This system is self-supervising i.e. if the system fails the crane will stop and can be restarted only after repair.
Verified data transmission: the data may be transmitted in blocks from the transmitter to the receiver. These blocks consist of an ID code, operational data and a control polynomial. Each data block is verified.
For a simple explanation of the verification code, visualise a train on a railway station as the data signal to the receiver on a crane. This train is a locomotive and a number of carriages. The locomotive has a unique ID code and carries information on how long the message is. It is also the obvious starting point. Each carriage has a number of seats and there is an equal number of seats in each carriage. Every seat (the data holder) is booked for crane functions (up, east, north, etc.). If the seat is occupied, an ‘on’ command for this function will be received when the train reaches the crane receiver. The last carriage is reserved for the train guard.
Before any driving action can start, the station masters (the microprocessors in the crane receiver) have to confirm that the data is correct. Stationmasters walking down each side of the train will check the occupancy of each seat and at the same time the train guard will check from inside the train, moving towards the last carriage where he will be able to present a check sum to the station masters.
The stationmasters will check with their own results and if everything turns out correctly, they will shake hands and the train (i.e. the crane) can start. The control data train is sent continuously (repeatedly) during operation. Information is sent 20 to 30 times a second depending on the length of the information signal.
Control commands, initiated by joysticks or push buttons, are processed in the transmitter logic card before the RF card transmits them. The command signal is received by the receiver RF card and processed in the receiver logic card before it reaches the output relays.
Some systems might have a special input to a computer processor that can monitor the status of the main relay. If the main relay contacts have been welded closed due to overheating, the processors can detect this and take the appropriate action. This is one way of making a system safer but not necessarily of making it more reliable as the risk factor is down to the level of an electromechanical component which is normally one in three million operations.
Electromechanical safety
The crane’s main relay is energised only when the receiver accepts the verified data signal. If the signal is not recognised the crane will stop automatically.
Joysticks shall have clearly defined notches or positions for controlling speed steps and a low static pressure to maintain a selected speed. The zero positions are to be well defined to prevent the operator from activating a function by mistake. Transmitter push buttons shall have well defined speed steps and a low static pressure to maintain the speed.
Buttons, joysticks and switches shall be protected against inadvertent operation. Two-hands are recommended for functions to release magnet and vacuum lifters and to open grab tongs. Active and automatic stops shall halt all crane movements and the brakes shall be activated in less than 500ms after the stop button has been pressed. An active stop signal may be transmitted actively or via a processor and there should be a zero position interlock that prevents re-start of a system where a relay has been welded in the closed position.
Safety in tandem operation: some lifting operations require two or three cranes to work together at the same time. In a safe system one operator must assume responsibility of the radio remote control system by an active command. This operator is then in command until he actively relinquishes responsibility. Thereafter another operator can take over by an active request. The system must also be safe after a mains power failure.
Selecting the right transmitter
If the duration of operation is more than 15 minutes an hour a joystick transmitter is recommended unless a hand-held transmitter is required due to limited space in the working area. A push-button transmitter may in this case cause fatigue to the operator’s fingers.
Joysticks should be of robust construction and there should be well defined speed steps with little static pressure to the operator’s fingers. This gives a comfortable and ergonomically correct driving style.
The transmitter should be carried in a waist belt, shoulder strap or holster for safe and ergonomic control.
Cost aspects
Service friendly systems will reduce down time. Standardised components will minimise inventory. Standard batteries in the transmitter will reduce cost and inventory. Programmable systems will reduce cost in down time and inventory. Moving parts with a long working life will reduce repair costs.
For some operators radio remote control is still a strange piece of equipment whereas pendant control is far more acceptable. Suffice to say that a pendant control is like a dog: when you pull its tail, the dog barks. A radio remote control is exactly the same thing but without the dog.
Safety of radio remote control
All radio remote controls have to be approved and several countries have special demands for the security of hoisting machines. Within the European Union there are the directives and EN norms such as EN 60 204-32, EN 954-1 and EN 50082-2 specifying minimum requirements. The CE mark is only of value if the customer finds out which norms the equipment is tested for and evaluates whether these norms are enough to ensure safe operation. Naturally this places a big responsibility on the purchaser: he or she must have the competence to evaluate the system, not just the price tag.
The prEN 13557 Controls and control stations is slowly emerging from the working groups and is the most comprehensive norm for these areas of control stations. Rapid technological development in the area means that regulations tend to become obsolete before they have even been put into practice which will create a lack of faith in the value of the regulations and of the CE mark. Safety is the most important issue in this business and the updating process for these regulations should be speeded up.
European manufacturers should form a group of experts to support and evaluate the suggestions for regulations from the working groups and authorities to support a correct and up to date platform of rules.
