Single-cable Gravity Compensation For Planetary Rovers And Experimental Researches

To simulate the low-gravity wheel pressure, the rover’s gravity shall becompensated. Traditionally, compensation forces are applied onto center of massesof each moving part. Since planetary rovers are multiple DOF systems, themethod always includes multiple cables, which difficulties to the implementationprocess. Therefore, the first half of the dissertation is contributed to research on theSingle-Cable Gravity Compensation model and its implementations. According tothe demand of lunar exploration program, the gravity compensation system shallcover an area of30m×30m, with a position servo error no more than28mm and atension servo error no more than5‰(1σ). The second half of the dissertation iscontributed to research on the design of the Large-Scale gravity compensationsystem and its experimental researches.The constraint on the gravity compensation forces is named “model of thegravity compensation”, each solution of which forms a gravity compensationscheme. The reason to model gravity compensation is to acquire its solution system,which contains the single-cable solution. To model gravity compensation, thedynamic modeling of planetary rovers is firstly presented, and then the constraintson compensation forces, by comparing the dynamic models in low gravity situationand in compensated situation, are acquired (applicable to all rovers). Specifically,an expression of the compensation model is built for rovers with only rotation DOFand differential DOF. The solution of the model, with respect to topology relation ofplanetary rovers， is acquired according the single joint-chain, the multiplejoint-chains, the bifurcation joint-chains, and the chains coupled by differentialmechanism. Also acquired are the independence of compensation forces solution inmultiple chains and bifurcation chains. The solution system of compensation forcesis then acquired.In the solution system, there are always such solution that includes only onpositive elements and that all other elements are negative. The positive elementcorresponds to a force applied by a cable, and the negative element corresponds to aforce applied by a counterweight, and this solution corresponds to single-cablegravity compensation. By imposing single-cable constraint onto the solution systemof gravity compensation model, the solution system of the single-cable gravitycompensation model is acquired. Two examples are presented according tosix-wheeled and eight-wheeled rocker-bogie rovers. The wheel pressure model isbuilt to prove that single-cable gravity compensation can correctly simulate thewheel pressures. Additionally, software simulations are also presented to prove the single-cable gravity compensation model. To avoid measuring the centers of massof rovers and avoid dissembling the rover, the method to calculate and adjustcompensation forces by the measure of contact forces is brought forward. Alsopresented are the method and platform to measure wheel pressures. To apply a forceto an inner point in the carriage of rovers, the property of parallel congruent bodiesis used to design the parallel suspension platform, which constantly transforms theforce on the similar suspension to the inner point.To implement gravity compensation over a large test field, the design ofLarge-Scale Gravity Compensation System is researched on. Because that theposition system could easily excite the resonance of the girder, a macro-microcoaxial dual-motor-driven system is designed. The system utilize the property thatthe inertia of a double-input system could be changed, and arranging its inertial withrespect to working frequency. Because that the motor are incapable to reducehigh-frequency disturbance, an active-passive tension system is designed. Thesystem tightens the cable with a constant-force mechanism, and therefore reducesthe equivalent stiffness of the cable greatly to absorb impacts. Because that thegirder possess a massive inertia and can hardly be precisely controlled, amacro-micro position system is designed. The girder-trolley system carries thetwo-dimensional servo platform, and the position system can precisely follow therover over a large area. Because that the horizontal resonance of the girder could beeasily excited, the threshold method is employed to control the girder-trolley system,so that the girder-trolley system is either moving or resting, switching slowly andstably between motion and motionless.To prove the property of the gravity compensation system, experimentalresearches are carried out to test the system. To test the tension system, the rover aremade to traverse over hard obstacle on flat terrain and on descending slop, as wellas traverse shallow holes on flat terrain. To test the position system, the rovers aremade to travel the diagonal line of the test field. By analyzing data from the system,the property of the system and the correctness of system design, is verified.This dissertation builds the single-cable gravity compensation model, presentsthe experimental methods of single-cable gravity compensation, and solved keyproblems in Large-Scale Gravity Compensation design. The dissertation finds itsuses in the mobility test on rovers in the lunar exploration program, which appliesgravity compensation on rovers over a large field with high precision.