How to select the automatic leveling device for a paver

1. Introduction

When the asphalt paver is in operation, the screed is pulled by the main machine through the traction arms on both sides and remains in a floating state. This floating screed has a preliminary filtering and hysteresis effect on the unevenness of the roadbed and has a certain leveling function, but it can only eliminate the unevenness with a smaller wavelength. If the smoothness of the road surface paving is to be completely unaffected by the base layer, it is necessary to adjust the vertical height of the traction point of the large arm at any time during the paving process according to the unevenness of the base layer, and ensure that the paving elevation Angle remains the initial value. The purpose of the automatic leveling device for ironing plate cultivation lies precisely here.

Automatic leveling devices are divided into contact type and non-contact type. No matter what form the leveling device takes, its working principle is exactly the same: First, the sensor of the leveling device detects the deviation of the paver from the reference plane due to the unevenness of the road. Then, the processor amplifies, calculates, compares and processes this deviation signal, and promptly sends a pulse adjustment signal to act on the corresponding high-frequency solenoid valve to control the lifting of the traction arm leveling cylinder, so that the screed returns to the initial paving elevation Angle. The rational selection of the automatic leveling device of the paver is of great significance to the paving operation.

2. Contact leveling device

The first to appear was the contact-type electronic leveling device, commonly known as the longitudinal slope controller (transverse slope controller). Almost all pavers are equipped with two vertical and one horizontal controllers. Depending on the usage of the paver, a two-longitudinal or one-longitudinal and one-transverse scheme can be adopted. When laying, the longitudinal slope controller slipper (or slider) slides on the reference, so the reference must be set in advance. The reference can be a steel wire rope tensioned along the longitudinal slope of the road, or it can be a paved road surface or curb stone. The cross slope controller is used to detect the lateral slope of the ironing plate and control the lifting and lowering of the large arm leveling cylinder on the side without the longitudinal slope controller. The one longitudinal and one transverse scheme is only used when the paving width is less than 7 meters and it is considered troublesome to set the reference. Usually, the two longitudinal schemes are adopted because the control effect of the longitudinal slope controller is much better than that of the transverse slope controller. Tensioned steel wire ropes, paved road surfaces or curb stones, etc. are real reference points, and the controller is in contact with them, so this is an absolute reference point. Adopting an absolute reference not only ensures the flatness of the paving but also controls the elevation of the laid layer. In actual construction, due to various reasons, the elevation of the roadbed may deviate to some extent. If no correction is made during the laying of the base layer (stabilizing layer), the elevation deviation will be passed on to the surface layer. To ensure that the surface layer elevation meets the design requirements, there will inevitably be a significant deviation in the surface layer thickness. Positive deviation will increase the amount of asphalt mixture used, raising the construction cost. If the negative deviation exceeds the allowable range, the construction quality will be substandard. In severe cases, rework may even be required, which will cause irreparable huge losses. Conversely, if the thickness of the surface layer is guaranteed, the elevation deviation will eventually be passed on to the upper layer, causing a height difference in the road markings, and rework will also be required. Therefore, in the paving construction of the pavement base, a reference steel wire rope plus a longitudinal slope controller must be adopted to further adjust the reasonable deviation between the elevation and thickness, ensuring that the upper layer (wear layer) is paved with the most economical thickness. The installation quality of the reference steel wire rope is entirely dependent on people, and there will always be some errors due to human factors. The longitudinal slope controller may also cause the slipper (or slider) to jump off the reference point for some reason during sliding. Meanwhile, the value detected by the sliding shoe (or sliding rod) is a single point, and the error of this value will be reflected one hundred percent in the lifting and lowering of the leveling cylinder. Therefore, the leveling accuracy of the reference steel wire rope + longitudinal slope sensor scheme is not high, and the smoothness of the road surface paving often fails to meet the design requirements. In the surface layer paving of high-grade highways, this scheme is generally not adopted. When laying the surface layer, it is recommended to adopt the scheme of mechanical floating average beam + longitudinal slope controller. The mechanical floating average beam is assembled from the front beam and the rear beam, and is connected to both sides of the paver through four sets of connecting devices on each side. During operation, it moves forward along with the paver. The front beam is nearly 8 meters long, with a tensioned steel wire rope in the middle. The sliding rod of the longitudinal slope controller is attached to it, and 6 to 8 sets of sliding boots below slide on the road surface to be paved. The rear beam is of the same length. Below are 6 to 8 groups of hollow rubber tires rolling on the layer behind the ironing plate. Thanks to the effect of the spring, both the sliding boots and the tires can reliably contact the road surface. Therefore, the relative elevation at the contact point of the longitudinal slope controller slider is the average elevation of all the support points of the entire beam length, so it is called a mechanical floating average beam. This leveling device completely eliminates many factors that affect the paving flatness in the reference steel wire rope + longitudinal slope controller scheme, significantly improving the paving flatness. The total length of the mechanical floating average beam is up to 18 meters. When the paving bends, there will be residual transverse pull marks on the paving layer. The paver needs to be disassembled and reassembled when turning around or moving to another site, which is labor-intensive, time-consuming and very troublesome. The surface of tires is prone to adhering to asphalt materials and requires dedicated personnel for cleaning. With the introduction of non-contact ultrasonic average beams, the use of mechanical floating average beams + longitudinal slope controllers is expected to be affected. However, due to the fact that high-grade highways have few curves and large curve radii, and pavers do not need to frequently turn around or change locations, the above-mentioned drawbacks are not prominently manifested. In addition, the longitudinal slope controller is a standard configuration that does not need to be purchased separately for the paver, and the price of the mechanical floating average beam is relatively low. Therefore, this longitudinal slope controller combined with the mechanical floating average beam leveling control device is still very popular among users in the construction of high-grade highway pavement.

At present, contact-type electronic leveling devices are gradually evolving from analog to digital. The digital leveling device displays parameters in digital form, which is very intuitive. It is precise, simple and convenient to adjust, but its price is slightly higher than that of the analog type. It won't be long before the existing analog electronic leveling devices are completely replaced by digital ones.

3. Non-contact leveling device

In 2001, China began to introduce non-contact ultrasonic leveling devices, also known as ultrasonic average beams. They operate on the principle of ultrasonic distance measurement: at a certain height above the road surface, an aluminum alloy beam (average beam) is fixed on each side of the traction arm, and multiple sonar sensors (generally four) are arranged downward, with three in the front and one at the back. During the paving operation, the sonar sensor emits sonar signals to the ground serving as the reference benchmark and receives the return signals, thereby calculating the average value above the ground to control the paver.