## 1 Introduction  
As cheap but effective defensive weapons, landmines have been widely deployed since World War I. About 100 million landmines remain to be removed in more than 80 countries. These landmines kill or injure approximately 20,000 people every year. More people get poorer because the contaminated land is not suitable for agricultural or industrial production [1; 2; 3; 4]. However, landmine detection has been a historically difficult problem. Landmines are 5-30 cm in diameter (including anti-personnel and anti-tank mines), their casing materials vary from plastic to metal, and the burial environments are very complicated. Under these circumstances, even though many techniques have been developed for landmine detection, current researches indicate that no single device can detect landmine to a real performance level. Metal detection based on electromagnetic induction (EMI) can only detect landmines containing a certain amount of metal components, and it may lead to a high rate of false alarm due to metal shreds that are very common on battlefields [5; 6]. Ground penetrating radar (GPR) images geophysical subsurface by acquiring reflected radar signals that are related to underground dielectric variations and it has a larger detection depth when compared to EMI method, but it also may cause a high false-positive detection rate due to the existences of big rocks, animal burrows, and roots [7; 8]. The nuclear quadrupole resonance method observes the presence of abundant nitrogen in the explosives (TNT and DNT) of the mine, is why this device is very expensive and difficult to operate [9]. Dogs and rats have also been trained to sense the explosive vapor that is diffused from the mine for their remarkable sense of smell, but the training time is long and the detection result is dependent on individual animal's work status [10]. Other methods, like X-ray backscatter and electric impedance tomography (EIT), are limited by the lack of safety involved in X-ray radiation [11] and detection depth [12]. Mine prodders can be traced to World War I and they are still most common supplementary tools in landmine detection because of their adaptability on detecting metal or nonmetal landmines that are buried in complex circumstances [13; 14; 15; 16; 17; 18]. Generally, prodding a landmine with a force of 25 N is unlikely to detonate a mine, though 50 N is regarded to be \"safe\" by some authorities [19]. A conventional prodder is usually a steel stick that is 30 cm in length. After the rain has softened the soil, deminers use the prodder to inspect subsurface of suspect area by poking the ground gently to contact the buried object to sense whether it is a landmine by their senses of touch and sound. Therefore, in recent years, many novel prodders with force feedback have been developed to explore new solutions for landmine detection by combining sensors with prodders [20; 21; 22; 23; 24; 25]. [PERSON] et.al. introduced a sensorized prodder to detect landmines that are buried in shallow soil based on their mechanical feature of big compliance when compared to stone, root, and other clutters [26]. This prodder consists of three main parts: an electromechanical actuator, a thin steel rod, and an accelerometer that is fixed on it. After the steel rod is inserted in to the soil to obtain contact with the buried object to form a prodder-object coupling system, an external electronic unit provides power supply and excitation signals to drive the actuator to stimulate the buried object, and the vibration signal that is measured by the accelerometer mounted on the rod is transmitted to the signal processing unit for characteristic analysis.