In the area of parallel robots, the use of lower-mobility parallel manipulators for many tasks requiring fewer than six degrees of freedom (DoF) has drawn a lot of interest. This paper treats one fundamental problem in the study of lower-mobility parallel manipulators: type synthesis. Using reciprocal screw theory, we define the mechanism constraint system and limb constraint system. We then investigate the relations between the mechanism constraint system and the limb constraint system under different geometrical conditions. Three tables describing the relations are presented. Based on the three tables, we propose a constraint-synthesis method for type synthesis of symmetrical lower-mobility parallel manipulators. The method used in the paper to construct the kinematic structure of the limb of lower-mobility parallel manipulators with prescribed DoF is simple and systematic. Examples are included to illustrate the application of the method and some novel lower-mobility parallel manipulators with 3-, 4- and 5-DoF are synthesized.

The Kutzbach–Grübler mobility criterion calculates the degrees of freedom of a general mechanism. However, the criterion can break down for mechanisms with special geometries, and in particular, the class of so-called overconstrained parallel mechanisms. The problem is that the criterion treats all constraints as active, even redundant constraints, which do not affect the mechanism degrees of freedom. In this paper we reveal a number of screw systems of a parallel mechanism, explore their inter-relationship and develop an original theoretical framework to relate these screw systems to motion and constraints of a parallel mechanism to identify the platform constraints, mechanism constraints and redundant constraints. The screw system characteristics and relationships are investigated for physical properties and a new approach to mobility analysis is proposed based on decompositions of motion and constraint screw systems. New versions of the mobility criterion are thus presented to eliminate the redundant constraints and accurately predict the platform degrees of freedom. Several examples of overconstrained mechanisms from the literature illustrate the results.

The lower-mobility parallel mechanism (PM) has fewer than six degrees of freedom (DoFs) and requires fewer links, joints and actuators than its 6-DoF counterpart. Hence, the use of the lower-mobility PM may save much cost in those applications requiring less mobility. In this paper, the type synthesis of symmetrical lower-mobility PMs is performed systematically using the constraint-synthesis method. First, some important theoretical fundamentals of type synthesis of lower-mobility PMs are investigated, including a classification of symmetry of lower-mobility PMs, the identification of instantaneous mechanisms and input selection method. Moreover, the difficult problem of how to correctly apply the general Grübler-Kutzbach criterion to lower-mobility PMs is solved. A mobility analysis of PMs with a closed loop in the limbs is also presented to demonstrate the validity. The type synthesis of 5-DoF and 4-DoF PMs is then dealt with in detail. Enumerations of novel 5-DoF and 4-DoF PMs are presented. The characteristics of constraint and structure of 5-DoF and 4-DoF PMs are also proposed. In addition, some novel 3-DoF PMs are presented.

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Based on the kinematic relationship of rigid body, this paper presents a new principle and method to analyze the singularity of a Stewart parallel manipulator. We study the sufficient and necessary condition that three velocities of three non-collinear points in a body can determine a screw motion. All singularities of the Stewart parallel mechanism are classified into three different linear-complex singularities. The various algebraic and geometrical properties as well as kinematics ones are analyzed in detail in different singular configurations. With the condition above, the singularity loci and distribution characteristics of a 3/6-Stewart parallel manipulator for some orientations of the mobile are studied in an oblique plane and in three-dimensional space.

Although there have been a number of publications focused on heterogeneous of NO2 on mineral particles, most of these studies were focused on β-Al2O3 and performed in the dark. Less was known about the reaction process of NO2 on α-Al2O3, especially the effect of sunlight factor. The heterogeneous reaction between NO2 and α-Al2O3 was investigated by using diffuse reflectance infrared Fourier transform spectrometry. The effects of NO2 and O2 concentrations as well as simulated sunlight were examined, and the reaction mechanism including the consumption of surface OH groups, oxidation process of nitrites, and the formation of water was also discussed in detail. It was observed that the formation rates of nitrates and nitrites were sensitive to NO2 concentrations and O2 concentrations. Nitrite was identified to be an intermediate production and disappeared very soon as [NO2] was up to 4.035 × 10(15) molecules/cm(3). Light played an important role in the changes of the electronic configuration of mineral dust, such as electronic donating ability, surface OH groups orientation, as well as the conversion efficiency between proton acid and nonproton acid, all of which could significantly enhance the heterogeneous reaction process. The reaction order for NO2 and O2 was determined to be 0.960 ± 0.111 and 0.620 ± 0.028, respectively. The uptake coefficient of NO2, which dominated the first step of the heterogeneous reaction, was calculated by the infrared absorbance with the use of ion chromatography and determined to be 9.9 × 10(-10) in the dark and varied from 2.54 to 3.33 × 10(-9) under simulated sunlight from 0.45 to 1.35 mW/cm(2). It was also found that γNO2 was independent of [NO2] and sunlight increased the uptake coefficient by three times, indicating that the heterogeneous reaction between NO2 and α-Al2O3 was enhanced under sunlight.