Simulation of the Hybtor Robot

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1 Simulation of the Hybtor Robot Pekka Aarnio, Kari Koskinen and Sami Salmi Information and Computer Systems in Automation Helsinki University of Technology ABSTRACT A dynamic rigid body simulation model of a hybrid locomotion robot Hybtor is presented. Load balancing and stability issues are studied using simulations. Normal walking mode and hybrid locomotion mode is simulated and ground reaction forces and joint torque values in different legs are analyzed. We also present how simulation model can be used to verify the control software of Hybtor. 1. INTRODUCTION In this paper we present how a hybrid locomotion robot Hybtor is studied using simulations during its development. Hybtor is a platform for Work Partner robot, which is a special service robot under development in Intelligent Machines and Special Robotics Institute. It has four legs for walking and wheels on its feet to access enough speed on even ground in wheel drive mode. On uneven terrain it can walk and step over stones or other barriers. The power for leg joint motion, as well as for the wheels, is got from actuators with brushless ECmotors. A combustion engine is used for battery charging in order to obtain a continuous working capability of several hours. Hybtor construction is more precisely presented in reference (1). Our main goal in simulations, and in virtual prototyping as a whole, is to support development of complex system that need knowledge from different engineering disciplines. Operation and control of the robot can be studied and verified first with this computerized model, and a part of testing can be carried out before all the components of the physical prototype are ready and integrated. One goal of the simulations is to visualize and demonstrate different locomotion patterns. Another goal is to analyze the actuator motor torque needed in different attitudes in virtual test fields. Comparisons are made between position control and simple force control strategies (2). Balance of Hybtor robot can also be examined by simulations. The simulation model is also used to verify the control software. Hybtor control software can be connected to the simulation model and the effects of the control commands monitored.

2 2. SIMULATION MODEL OF HYBTOR The Hybtor platform is modeled as a robot device consisting of rigid body links without any flexibility. The moving links are connected to each other with ideal revolute joints. One joint corresponds to one degree of freedom. Two kinds of models has been build: 1. Kinematic model 2. Dynamic model The first model has only kinematics properties and it is used for locomotion visualization and monitoring purposes. Kinematic model can be simulated in real-time. The dynamic model is obtained from the kinematic model by adding dynamics properties to the robot model such as mass, inertia moments, joint friction as well as by extending the virtual environment with gravitation field and ground contact forces. This type of model is used in torque analysis, stability analysis and control algorithm testing. Dynamic simulations with ground contact modeling can not be run in real-time. The model of the Hybtor robot has been built using the Envision TR/ Deneb Inc. simulation software (3). Envision program is originally designed for industrial robot simulations and offline programming. Model building using Envision is started with the 3D graphic modeling of the device parts. Earlier made CAD drawings of the parts of Hybtor were imported to the modeling tool and link base coordinates and auxiliary coordinates were defined according to Denavit-Hartenberg convention. Graphical model of Hybtor is presented in Fig.1. The model device consists of parts which can be interconnected to each other with joints. The model has articulated main body and legs with three revolute joints between links, which are the hip joint, the thigh joint and the knee joint. The fourth revolute joint is for the wheel. Figure 1 The Hybtor model in a virtual test field

3 3. SIMULATION PIPELINE During simulation robot devices joint motion is commanded at the higher level GSL-program of that device. GSL is the Graphical Simulation Language of the Envision program. These motion commands initiate the so called motion pipeline calculations (3). Most calculation work is done in the inner control loop. Control module calculates the joint torque to be given as input to the physical model of the robot. The build-in forward dynamics module calculates the actual position, velocity and acceleration of the robot links under gravity and other external forces. The Envision software packet does not have any special module for ground contacts, so that a ground contact model is defined and embedded in to the control loop. Ground contact forces have to be calculated every few milliseconds which defines quite high control loop frequency. 4. MODELING GROUND CONTACTS When the foot of the robot leg, or the tire of the wheel, makes contact with the ground stiff constraint forces are generated. Three orthogonal force components are modeled: vertical, longitudinal and lateral. A simple spring damper model has been used for these forces. The resultant of all the support forces to the four legs must compensate the gravitation and other inertial forces towards the ground in order to keep the robot vehicle on the ground surface. In this simple model the contact stiffness corresponds both the elasticity of the ground and of the wheel tire. A separate ground model with different soil properties has not been defined. The longitudinal force component is made up of two components: rolling resistance and traction (or braking) force. The traction force is dependent on both the longitudinal velocity of the wheel center point and the wheel rotation velocity. Traction forces are necessary so that wheel can generate enough driving force to move the whole body of the robot. The braking force is used as leg propulsion force during walking and together with the lateral frictional forces it also prevents the vehicle from sliding to unwanted directions. Tire deformation dependent forces like aligning torque is not considered in this model because the speed of the robot is relatively low. (4) 5. HYBTOR MODEL SIMULATIONS 5.1 Virtual test fields Two virtual test fields are used in Hybtor simulations. The first test field can be modified so that the virtual robot can drive or walk up a slope or over holes or blocks of different height. In this test field forces and torque are analyzed at specific situations. The second test field consists of humps of different size and height serving as a virtual unstructured outside environment. This field is used as a random simulation environment for wheel driven tests. In this field the attitude of the robot is changing in random fashion. The behavior of the robot depends on the ground conditions and the control algorithm. The simulation runs using dynamic model are much slower than real time. Simulations are

4 recorded for real time play back visualization. From these animations the level of the motion realism can be visually judged. During simulation important data, like contact forces and joint torque in all four legs, are saved to files for post processing and analysis. Joint torque data analysis shows, for example, how much and for how long time periods the actuator nominal torque is exceeded. Motor nominal torque is allowed to be exceeded but high currents for a long time will heat up the motors and increase the energy consumption (5). 5.2 Load balancing and stability An important goal of the walking robot control is to maintain stability during all kinds of locomotion. The Hybtor robot, with only four legs, can easily become unstable when one leg is lifted up. These unstable situations are observed from the simulation animations. Stability is easier to maintain when the weight load is evenly distributed to as many legs as possible. a b Figure 2 Weight shifting simulations. Hip joint angle 0 degrees (a) and 15 degrees in (b) When one leg wheel for some reason loses contact with the ground, the diagonal leg will tend to lift of the ground also. This means that the whole load has to be carried by the other pair of diagonal legs. This diagonalization effect can rabidly double the load and also the actuator torque needed on the supporting leg joints. This was noticed especially in the uneven random field simulations but it happens also in even plain test field when stiff position control was used and the leg configurations were not exactly the same. The consequences of the diagonalization can not be noticed by a visual study of the simulation animations. It is, however, easily revealed in the post analysis of the saved force and torque data. Hybtor mechanism allows load balancing by weight shifting through inclination of the hip joints. This was simulated in wheel driven mode keeping left hand wheels on 0.25m higher plane. The simulations where done first with zero hip angle and then with hip inclination of 15 degrees. The thigh joint (25 deg) and knee joint (65 deg) angles where the same for all legs. This is represented in Fig. 2a and 2b. The simulation results presented on columns A and B of the Tab. 1 show that the torque differences between the legs are diminished, when the weight is shifted more to the left side legs as expected and nominal torque was not exceeded. This uneven distribution of the joint torque in Hybtor simulation model is due to the rigid body model and stiff position control. In the real Hybtor, tire elasticity and flexible main body construction will soften this effect a lot when moving on a terrain with only small unevenness. However when position control is used, there will be great differences on joint torque needed in different legs. Legs can be made more compliant by using smaller

5 proportional control coefficient. In situations, where it is not necessary to change the leg configuration constantly, Hybtor can lock the joints using special electrical brakes and safe energy that way (1). Table 1 Hybtor joint torque values on weight shifting simulations A and B, in walking mode simulation C and hybrid locomotion mode simulation D. Simulation A B. C D Joint Nominal Min Max Min Max Min Max Min Max Hip Thigh Knee Most important way to actively control stability and to balance the robot platform on uneven terrain is to use force control of the legs (2). In direct force control, the leg joint actuators are commanded to generate torque values, that are calculated from the wanted wheel center point force vector. This direct force control is used for example in hybrid locomotion mode simulations. Another method is hybrid force control method, that means position control to one direction and force control to the orthogonal direction, will be tested on forthcoming simulations.(2) 5.3 Comparison of walking and hybrid locomotion The Hybtor model was simulated at walking mode locomotion and hybrid locomotion mode and the results were compared. The Hybtor model was first walking in a normal wave gait fashion where the leg in transfer phase was lifted about 0.1 m over the ground using position control. For stability reasons the whole body was inclined to the other side about 10 degrees using hip joints. The supporting legs, with wheel brakes on, gave the forward propulsion force. Walking mode is presented in Fig. 3a a b Figure 3 Transfer phase of the rear leg in walking (a) and in hybrid locomotion (b)

6 Ground normal reaction force leg 1 solid leg 2 dashed leg 3 dashdot leg 4 dotted Force [N] Time [ms] Figure 4 Ground normal reaction forces during walking (a) and hybrid locomotion (b) leg 3 Thigh joint (2) torque leg 1 solid leg 2 dashed leg 3 dashdot leg 4 dotted leg 2 leg leg Time [ms] Figure 5 Thigh joint torque during walking (a) and hybrid locomotion (b) In hybrid locomotion mode simulation the supporting legs generate propulsion force, as in walking, but the wheel of the leg in transferring phase is not lifted up but instead force controlled so that the wheel is in contact with the ground during the whole transfer phase. That wheel can rotate freely or can be driven to help forward motion. This way the stability of the robot body is maintained without shifting the weight. (6) Hybrid locomotion mode is schematically presented in Fig 3b. Walking mode simulation C is started by dropping Hybtor from the height of few centimeters over the test field. Between 0.5 and 3.0 seconds the rear right leg (leg 4) is stepping forward about 0.6 m. This means load lightening of the diagonal leg so that most of the load comes quite equally on legs 2 and 3. This can be seen from of the reaction force plotting in the Fig 4a. Because of the current leg configurations there is, however, a big difference in thigh joint

7 torque between these legs presented in Fig 5a. The biggest torque about 300 Nm is needed in rear left leg (leg 3). The stepping leg thigh joint have a negative torque which is needed to carry the leg up in the air during the step. The same kind of torque characteristics but for different legs recur during the next transfer phase of the front right leg (leg 2) between time period seconds. The extreme values of the joint torque are listed in the column C in Tab. 1. Hybrid locomotion mode simulation D is started in a same way, rear right leg (leg 4) stepping first forward with free wheeling. plotting of hybrid locomotion simulation is presented in Fig 5b. Now the stepping leg is force controlled so that 160 N force is directed towards to ground in order to keep some ground contact and support. About 100 N force component is directed to forward direction. The extreme values of the joint torque are listed in the column D in Tab. 1. Comparison shows, that the hybrid locomotion has characteristics similar to the normal walking. In hybrid locomotion, however, weight load is better balanced on all legs even without weight shift, which was found necessary in walking locomotion mode in order to keep balance. Maximum support force of the three legs is almost 1000 N in walking mode while in hybrid mode all legs had a supporting force between N. In walking mode the thigh joint torque of the stepping leg 4 is seen to be negative because it has to be carried and the torque of the leg 3 goes to 300 Nm exceeding the nominal value. In hybrid mode the torque of the leg 4 in transfer phase stays positive during the forward transfer and even the maximum torque for leg 3 stays below 230 Nm. The knee joint torque values were quite similar in both simulations. 6. TESTING OF THE CONTROL SOFTWARE The simulation model has been used to test and verify the control software of Hybtor. Control software components, like the inverse kinematics algorithm and force control algorithm, of the real Hybtor was directly embedded to the simulation software (5). Testing of the higher level Hybtor control software has been started by using special connection to the simulator (1), (6). The Envision simulation software supports so called Low Level Telerobotics Interface (LLTI), through which the simulation model can be commanded from applications over Ethernet. This connection, presented in Fig.6, can be used in three different modes in monitoring and testing of the real Hybtor platform and control software. 1. Virtual control mode 2. Hybtor monitor mode 3. Control monitor mode In the virtual control mode the control software sends commands only to the virtual model, and the feedback is the model configuration data. This makes it possible to visually verify the operation of the control software. This mode have been used in the tests. In the last two modes the actual Hybtor operation under control is only monitored. In mode two the feedback data from the real Hybtor is send also to the simulator so that the virtual model imitates the configuration of the real Hybtor. In the mode number three the virtual model and the real

8 Hybtor platform follows the same commands and the actual behavior can be compared to the intended virtual behavior. VIRTUAL MODEL HYBTOR HIGH LEVEL CONTROL Simulator interface TCP/IP Comm. CAN Comm. LLTI-user IO / TCP/IP Comm. CAN-bus Ethernet Leg Leg Leg Leg Body HYBTOR PLATFORM Figure 6 Simulator connection to Hybtor main control 7. SUMMARY In this paper we present the dynamic rigid body simulation model of the hybrid locomotion robot Hybtor. Preliminary simulations have been made in analyzing the joint actuator torque needed in wheel driven mode, normal walking mode locomotion and hybrid locomotion mode. Simulation model has been used in verification of the control software and will be used in monitoring the overall control of the real Hybtor platform. The simulations has given us understanding of the behavior of the whole hybrid robot system. REFERENCES 1. Halme A., Leppänen I., Salmi S. Development of WorkPartner-robot - design of actuating and motion control system CLAWAR'99 2 nd International conference on Climbing and Walking Robots Portsmouth, England, Sept Hannu Lehtinen. Force based motion control of a walking machine. Dissertation for the degree of Doctor of Technology at HUT. VTT Publications 179. Espoo, Finland, Telegrip. Motion Pipeline Reference Manual. Deneb Robotics Inc Homer Rahnejat. Multi-Body Dynamics. Vehicles, Machines, and Mechanisms. Society of Automotive Engineers, Inc. USA Sami Ylönen. Control System of the Electromechanical Leg of a Service Robot. Master's Thesis, Automation Technology Laboratory, Helsinki University of Technology. Feb Halme A., Leppänen, Salmi S. Hybrid locomotion of a wheel-legged machine. CLAWAR nd Int. conference on Climbing and Walking Robots Madrid, Spain, 2-4. Oct 2000

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