| The existing crane type derusting equipment,which has a large structure and is difficult to transport,and the climbing type derusting equipment,which has poor load-bearing capacity and cannot handle complex surfaces.To remedy the defects,our research group has developed a sandblasting and derusting parallel robot for bridge steel box girder based on stewart parallel structure,which consists of parallel operation mechanism,lifting mechanism and mobile platform,etc.It has the advantages of high rigidity,strong load-bearing capacity and good mobility,and can achieve arbitrary movement,free lifting,six degrees of freedom position precise motion operation,etc.The mobile platform is an 8-wheeled structure,which is stable and convenient to switch direction.The operation site of steel box girder sandblasting and derusting is steel sand pile road which has low adherence,so the mobile platform is prone to slip when driving on this road.This paper focuses on a dynamics model of mobile platform of sandblasting and derusting parallel robot that takes into account the slip characteristics and resistance of steel sand pile road,and a compoundcontrol of slip rate error and trajectory tracking error to achieves high performance motion control of the mobile platform.The main works accomplished in this paper is as follows:(1)The kinematic analysis of the mobile platform of the sandblasting and derusting parallel robot is carried out by the mass point motion analysis method,and the relationship among the linear velocity and the front wheel angular velocity of the mobile platform and the position of the world coordinate system is derived,and the incomplete constraint equation of the mobile platform is also derived.(2)A bekker pressure-bearing model of the driving wheel is established according to the physical characteristics of steel sand,and the stress on the contact surface of the wheel sand in this model is double integrated to obtain the resistance of the steel sand when the mobile platform is moving.What's more,the"weakening"of the driving force in this model result from slip rate error is calculated by the slip characteristics curve of the steel grit pavement steel sand pile road.Thus,a dynamics model of mobile platform of sandblasting and derusting parallel robot that takes into account the slip characteristics and resistance of steel sand pile road is established,this model can more accurately characterise the dynamics of the mobile platform which moving on the steel sand pile road.(3)Based on the dynamics model of the mobile platform established in(2),the general design of the Anti-Sliping Trajectory tracking Control(ASTC) method is first carried out.In this control method,the actual acceleration value of the vehicle body can be obtained through accelerometer and the wheel speed value can be obtained through the encoder,and then slip rate can be calculated from the above two measurements.In this way,the control method can determine if the mobile platform is slip,and use different control methods according to whether the platform is slip or not to achieve high performance trajectory tracking motion control of the mobile platform.When the mobile platform is not sliping(?=?_d),a Steel-sand-resistance Compensation based Smooth Sliding Mode trajectory tracking Control(SCSSMC)is proposed;When the mobile platform is completely sliping(?=1),a Slip-rate error Smooth Sliding Mode Control(SSSMC)is proposed;When the mobile platform is incompletely sliping(?_d<?<1),a compoundcontrol method for SCSSMC and SSSMC with variable fuzzy weight coefficients is proposed.(4)The MATLAB simulation test that the mobile platform is not slip is carried out to compare SCSSMC and conventional sliding mode trajectory tracking control which switch by symbolic function,the simulation results show that:comparing to conventional sliding mode trajectory tracking control,SCSSMC not only has excellent robustness,but also can achieve"jitter-free"motor torque output with guaranteed system robustness which is more valuable for practical engineering applications.What's more,MATLAB simulation tests that the mobile platform is complete slip(?=1) and incomplete idling(?=0.9,?=0.6 and ?=0.3) are carried to compare SCSSMC and ASTC,the simulation results show that:comparing to SCSSMC,the slip rate error of ASTC can converge to zero in a limited time when the mobile platform is in incomplete or even complete slip,thus significantly reducing the dynamic and steady-state error of trajectory tracking due to idle.(5)According to the control requirements of the sandblasting and derusting parallel robot,a distributed computer control system based on the"upper computer(IPC-610L)+lower computer(CK3M)"is used to build the experimental platform of the mobile platform of sandblasting and derusting parallel robot.And then the anti-sliping trajectory tracking control experiment is completed based on this experimental platform.The experimental results further verify the effectiveness and superiority of the designed control method. |
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