Load Moment Indicator (LMI) Systems For Precise And Safe Crane Operation

Overview of Load Moment Indicator Systems

Parts of cranes with their basic working (D.E., n.d.):-

1. Boom:- It is the most essential part of the crane. It is the arm of the crane which lifts and lowers the heavy things/ materials. It is probably the outstanding element of the crane. It is long comprising telescopic or fixed arm which lifts and moves the object.
2. The Counter weights: They are mostly near the exterior of crane’s cab. They are used to prevent the unbalancing of the crane. Balancing is very important factor for the crane because it is directly related with the human safety. The counterweights are detachable as they are placed at the back of the crane. Based upon the boom radius, load weight and operational boom angle the formula has been derived which gives the value of counterweight needed to be placed.
3. Jib: – The have lattice like structure which provides freedom to boom to extend. It is an extension of the boom and also allows the cab to rotate and turn. Hydraulic motors are employed to perform the tasks.
4. The rotex gear:- They are installed under the crane’s cabin. They are used to rotate the apparatus from single standpoint only.
5. Operator’s Cab (crane cabin):-  It is at right on top of the rotex gear and it is attached to the top of the deck. Carne cab turns in desired direction. It is installed with the electrical cable, equipments, control panel with also includes different sophisticated sensors and the most operator’s chair. From the cabin the operator controls the crane by the use of joy stick which is also available in the cabin which enables the left to right and forward to backward motion of the boom. Inside a cabin a foot pedals are also provided to control the rotex gear pump and the telescopic action of the boom.
6. Outriggers:- They provides balancing and stability which is necessary to operate the crane and prevent the crane from overturning and leaning. The basic parts of the outriggers are:-
7. The beam or leg.
8. The pad or foot.
9. Sometimes the float.
10. Sensors:- As the name predicts it is a device which senses /  detects the events or changes in the environment and sends a signal to other electronic device to convert the event into the readable form by the frequent use of computer system. It generally works on binary codes that are decided by decoders and then the signal is made in readable form. The use of sensors makes the conventional crane a smart crane which is not only precision worthy but also takes into account environment safety, human safety and provides good productivity.

The sensors that are used in boom cranes are all grouped and may be termed as Load Moment Indicator (LMI). The typical Load Moment Indicator system employes the measurement of crane parameters which includes the boom length, boom angle, load, outriggers position, and temperature.  

It has been designed to provide the essential information that are required to operate the machine within the permissible designed limit to the crane operator. As mentioned above it comprises of different systems i.e. the sensors that provides the information regarding the angle boom, working radius, rated load and the total weight that is been lifted by the crane.LMI system is provided by an audible high db sound alarm, a warning light and automatic locking system which locks the system or turn off the crane under some failure or some unavoidable accidents.  

LMI is a sophisticated device which requires skilled operator with good judgment skill. The operator shall ensure that all warnings and instructions supplied are fully understood and observed before going to work on the crane because the responsibility for safe crane operation is in the hand of operator only.

Not only this proper functioning of LMI system depends upon the way inspection and observance is done as LMI system requires daily inspection because sometimes cranes are to work in harsh environment that can affect the precision of sensors. An Inspection and Calibration Verification Certificate is required to use the cranes.

Load Moment indicator consists of a central microprocessor unit, operating console, pressure transducers, length / angle sensor and anti-two block switches. The whole LMI system works on comparison between the designed value and the value used in real time. When the crane crosses the designed limit than the overload warning alarm gets activated and operator get information about the overload condition and acts according to the given standard instructions.

Some sensors designed for measurement used in the cranes are as follows:-

1. Boom Length / angle measuring sensor:-  Mostly they comes in combination of two transducers fitted in one box. The box is fitted at the base section of the boom. It measures the length and the angle of the boom.

In the system there is reeling drum that drives the potentiometer which acts as the length transducer. A length cable on the drum which is a part of transducer is two conductor cable first is screen and second is live. The length cable is connected to the anti-two-block switch installed at the boom head and to a slip ring body in the reel. As for angle transducer, it is fitted in a small box containing oil. A pendulum is used to drive the axle of the angle potentiometer. 

Components and Working of Load Moment Indicator Systems

Voltage Output signal for the length transducer:- 0.5 V to 4.5 V (Approx.).

Voltage Output signal for the angle transducer:- 1.875 V to 3.125 V (Approx.).

1. Load measuring transducer/sensor:- There are many sensors used for measuring the load lift by the crane some sensors are :-
2. Load pins:- They are the transducers which are designed to measure the force being transferred through the pins of the truss members used in the crane. Through the forces observed on the pins a signal is decoded into the weight lifted by the cranes.
3. Tensiometer:- It is operated as, the load bearing cables passed through a series of numerous sheaves which are designed for measuring the force applied to the middle sheave. Based on this information the weight of the load which is being lifted by the crane can be determined.
4. Microcell sensor:- They are also used for load measurement but the only problem is that they are designed to be applied to the exterior of the structure and it has to face unavoidable environment conditions. They are very sensitive to the temperature especially its is sensitivity is challenged during direct exposure to the sunlight.
5. Strain sensors:- These are sensors which are mounted over the crane and gives accurate result of weight lifted by the crane. It works on the principle of strain gauge. The problem with this sensor is that of micro cell sensor but still it is used because of its accuracy.
6. Pressure transducers:- As we all know that in 21^stcentury almost every heavy duty cranes uses hydraulic power to perform heavy tasks easily. So considering this the pressure transducer is used to determine the load lifted by the crane. The pressure transducer converts the hydraulic pressure signal into an electric digital/ analog voltage signal. Most of the time two pressure transducers are connected, the first one is connected to the piston of the lift cylinder and the second is connected to the rod. The four conductor double shielded cable connects the pressure transducer to the central unit. The central unit is a microprocessor which is responsible for controlling the pressure by comparing the designed value of pressure with the real time pressure value.

Voltage required for this transducer to work is 5V.

Voltage required for the output signal is 0V for 0 pressure and 1V for pressure upto 5000psi (Approx.).

1. Temperature measuring sensor:- Series of temperature sensors are designed in order to sense the cabin temperature , hydraulic oil temperature etc. and at last the system is provided by thermostat system in order to suppress the fire which can be caused due to some failure of the element, short circuit etc.  The precision temperature sensors are used to set the temperature range and giving warning alert signal in case the threshold temperature is reached or exceeded.
2. Outrigger movement measuring sensor:- As discussed above the outriggers are used for stability and balancing of the crane. So an outrigger pad monitoring system which includes plurality of outriggers having sensors are used to determine the stability of the system. A crane control system utilizes the measured load on the outriggers with positional information for the crane boom to determine if the crane boom is in a side-load condition. The outrigger pad monitoring system may be used during the setup of the crane and to verify the proper operation of a rated capacity limiter.

All above transducers/ sensors are the collective Load Moment Indicator systems that are used in the cranes for its precise working and making the conventional cranes a smart crane. Apart from this there are some important equipments used in LMI system that are discuss below:-

1. Anti-Two-Block Switch:- It controls the load block and describes the relationship with the boom head. During working condition it is always closed but when there is strike between hook block and the weight the circuit gets opened and disengages the relay output with the lockout solenoid valves. A 5k resistor is placed between ground and the contact switch in order to check the damage. The weight at the anti two block switch keeps the switch closed until unless the hook block strikes the weight.
2. Console:- It displays the information (geometrical) such as boom length and the boom angle, boom working radius and the boom head height. It comprises of the warning light and a alarm for overload condition. It also provides pre warning light. The total permissible moment percentage is shown by analog instrument which is installed. It has the duty-selection switch for crane configurations and Reeving of the hook block switch.. 

Conclusion

From all above discussions it is clear that mechatronics is the future of the conventional crane. It not only make the crane precise and high productive machine but also it make the crane more safer. Safer in terms of environment and also in terms of human safety as well. The sensors plays a major role for making the cranes more faster and safer and also helps in designing a system known as Load Moment Indicator system. Not only the LMI system can be employed in cranes but the can also be employed in heavy duty machineries as well.

An evaluation of signal conditioning elements that are required to measure the load using strain gauges.

Signal conditioning is generally a manipulation and amplification of some signal that need to be amplified or need some processing so that the information through the signal can be sent for next phase. There are most of the analog signals exhibited by the sensors or the transducers which requires some processing in order to digitalize the signal. Some sensors requires signal conditioning before a data acquisition so that the device can effectively and accurately measure the signal.

For example, thermocouple signals have very small voltage value so it needs to be amplified using signal conditioning before it is digitalized. In the same way there are other sensors, such as resistance temperature detectors (RTDs), thermistors, strain gages, and accelerometers, require excitation to operate.

Strain gauge is a metallic or semiconductor based element which changes its resistance because of the applied load. Strain gauge is a resistive sensor which is used to measure force and torque and acceleration. They are also called as force sensor. They are basically made up of the piezeo-electric ceramics which generates electric field when strained and changes the dimension when electricity is passed through them. When load is kept over the strain gauge apparatus than  exhibits electric field whose value is amplified by using signal conditioning and then voltage unit is calibrated with the load unit and by this the load is measured (L, 1994). 

During signal conditioning there are some requirements that needed to be fulfilled for the strain gauge. The steps needed for the signal conditioning of the strain gauges are:-

1. Bridge Completion:-  For the full bridge strain gauge sensor, the bridge must be completed with the reference resistor. Strain gauge signal conditioning systems provides half bridge completion networks which consists of two highly precise reference resistors. The resistors are matched well and provide a stable reference voltage to the negative input lead of the measurement channel. The current draw form the excitation voltage is minimized by the high resistance of the completed resistors.
2. Bridge excitation: – To power the bridge a constant voltage is provided by the strain gauge signal conditioners. The excitation voltage level is kept around 3V – 10V.
3. Excitation Sensing:-  there is possibility of error in form of voltage drop caused by resistance of wire that are connected for the excitation to the bridge if the strain gauge circuit is located away from the strain conditioners. A remote sensing is installed to some signal conditioners in order to compensate this error.
4. Signal Amplification:- There is relatively very small output of the strain gauge and the bridges (output nearly equal to 10mV/V for most of the strain gauge). Therefore the amplifiers are installed with the signal conditioners, which boost the signal level and increases the measurement resolution and improves the signal to noise ratios.
   1. Bridge balancing (Offset Nulling):- Due to slight variation in resistance among the bridge legs generates some output voltage when no strain is applied on the strain gauge. Therefore a system is installed for balancing the bridge legs for accurate voltage output. In this some software compensation and hardware nulling compensation is also included.
5. Shunt Calibration:- It is a general procedure to verify the output strain value with some predefined mechanical input. Shunt calibration simulates the strain input by changing the resistance of the bridge arm by the known amount. The simulations can be performed by using some mathematical calculations that simulate the strain gauge system. The shunting compensation is done by connecting a large resistor with one leg of the bridge. Than the measure of the output of the bridge is compared with the output of simulated strain value.(Sciammarella Cesar A., 2012)
6. Linearization:-  A hardware and a software are used to convert the output voltage of the strain gauge into strain measurement by some suitable formula analysis. 

References

D.E., D., n.d. Crane Handbook. Ontario: Equipment Research & Development Department.

L, H. R., 1994. Strain gauge Users handbook. California: Lockheed Aeronautical system Company.

Sciammarella Cesar A., S. F. M., 2012. Experimental Mechanics of Solids. 3 ed. West Sussex: John Wiley & Sons. Ltd.

W.Fair, H., 1998. Crane Safety on Construction Sites. s.l.:Construction Division of the American Society of Civil Engineering.a