Abstract—Hybrid locomotion is advantageous over a single type of locomotion in terms of maneuverability. However robots with the ability of hybrid locomotion using either wheels or legs are usually complicated and heavy. This paper introduces a cylinder-shaped robot that is capable of two types of locomotion, jumping and rolling, on the ground. This paper reports design and experimental results of the robot that can execute the two kinds of locomotion using only one actuator. Experimental results show that the robot with the diameter of 180 mm can obtain the rolling speed of 600 mm/sec and the jumping height of 160 mm.

I. INTRODUCTION

When developing a mobile robot, choosing a locomotion type is one of the important issues. Rolling robots using wheels are known to be unable to move over an obstacle that is higher than half of their wheel radius [1]. Jumping robots may be able to reach up to ten times its body height [2], but wheeled robots can usually move faster than legged ones in hard and flat surface conditions [3]. Therefore, a combination of multiple locomotion types can be an attractive choice in order to take advantage of each type.

Several efforts to incorporate jumping, wheeling, and rolling in one robot have been reported. Jollbot [4] is an example of the robots that employ both jumping and rolling. Although the robot can execute both motion styles, the performance is not quite remarkable. This 300 mm diameter spherical robot may jump up to 0.6 times its height while the rolling ability is weak. The Scout robot [5] is another example into which wheeling and jumping are integrated. The robot is quite skillful in both jumping and wheeling. This 40 mm diameter robot may obtain the jumping height of 100 mm and run at 310 mm/sec. Besides, this robot can easily steer to change the moving direction. Complexity in the structure is a disadvantage of the robot, which is due to the use of many actuators. Another disadvantage is from the robot’s jumping mechanism with a small tip spring that requires a rigid contact surface, and hence it may not work properly when the ground is soft. Sugiyama and his co-workers introduced SMA crawling and jumping robots [6]. The robots use SMA as actuator and deformation principle for rolling. This work suggests a new way of using unconventional actuators for development of rolling and jumping robots.

Manuscript received March 07, 2012. This work was supported by National Research Foundation of Korea Grant funded by the Korean Government (2009-0077778) and Leading Foreign Research Institute Recruitment Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (MEST) (2011-00260).

Thanhtam Ho and Sangyoon Lee are with the Department of Mechanical Design and Production Engineering, Konkuk University, Seoul, Korea (phone: +822-450-3731; e-mail: slee@konkuk.ac.kr).

Usually the number of actuators on a robot is larger than or equal to the number of locomotion gaits the robot employs. Therefore, in order to improve the maneuverability, adding actuators is firstly considered. However, increasing the number of actuators causes the increase of weight, size, and complexity. The performance can be reduced as a result. Besides, power consumption is proportional to the number of actuators. This paper presents the development of a robot that can locomote by means of jumping and rolling actively using only one actuator. The proposed robot may roll or jump over an obstacle while driven by only one tiny DC motor.

II. ROBOT DESIGN

The objective of robot design is to create a mechanism that can perform both jumping and rolling motion continuously with only one actuator. For that purpose, a cylinder is chosen as the exterior shape of the robot. The cylinder cage can work as a wheel in rolling and as a balance cage recovering the robot stable posture after jumping. Rolling motion is generated by the change of center of gravity (COG). Jumping locomotion is inspired by the jumping principle found in insects.

Fig. 1 shows the overall robot structure from the side view. The robot is made of four basic parts: (1) frame with leg, (2) main spring, (3) body base, and (4) slider. The frame contains a cylindrical shell welded with a rod which is called the robot leg. The rod is used as a pillar on which the body base hangs. The body base is linked to the rod by a prismatic joint, along which the main spring is connected. One terminal of the spring is attached to the rod and another one is joined to the body base. A major component of the slider is a DC motor with a pinion attached to the shaft end.

Fig. 1. Overall design of the jumping-rolling robot, seen from side view.

A Novel Design of a Robot That Can Jump and Roll with a Single Actuator

Thanhtam Ho and Sangyoon Lee

2012 IEEE/RSJ International Conference onIntelligent Robots and SystemsOctober 7-12, 2012. Vilamoura, Algarve, Portugal

978-1-4673-1736-8/12/S31.00 ©2012 IEEE 908

The DC motor is installed on the support to which a limited latch is attached. The latch is designed especially for jumping. The slider is constrained to move along the body base. Translation of the slider is converted from rotation of the DC motor by means of the rack and pinion mechanism. One may note in Fig. 1 that the rack is a component fixed on the body base. The relative motion of the slider along the body base is the key factor to produce both rolling and jumping locomotion of the robot.

A. Rolling mechanism

Fig. 2(a) illustrates the motion of the slider during the rolling locomotion. Since the rack is fixed on the body base, rotation of the pinion produced by the DC motor is converted to translation of the slider along the body. Fig. 2(a) also shows several limit points to which the slider can reach. Point B is the highest position of the slider while L is the lowest one. In order to prevent the latch from locking to the hook during the rolling locomotion, the slider motion is limited in the range A-B instead of L-B (see Fig. 2(a)). Point A indicates the lowest position at which the slider remains higher than the hook and is not locked. The range A-B is called the rolling range. It is noted that in this rolling locomotion mode the relative motion between the body base and the robot leg is small. The robot structure can be modeled to a simpler form as shown in Fig. 2(b). The model has two parts: the slider part and the remainder.

Motion of the slider diverts COG of the whole robot body from the frame center. By the principle of minimum potential energy, the robot is supposed to roll to keep the COG position lower than the frame center. It is obvious that if the slider lies on the right side of the frame center, the robot will roll clockwise and vice versa. One may note that control of the position of the slider is equivalent to controlling the rolling motion.

B. Jumping mechanism

In order to obtain the jumping locomotion, jumping mechanism of insects like the froghopper has been studied and applied. The basic principle is based on the energy storage system. Compared to directly activated jumping mechanisms like our previous jumping robot [7], insect jumping mechanisms possess larger advantage in terms of jumping distance and jumping height. The froghopper which is known as the jumping champion among creatures [8] is one example. During the procedure for jumping, energy generated by the insect’s muscle is gradually stored in a resilient device and quickly released to jump to the air [9]. Rapid release of the stored energy takes an important role in jumping. Since the proposed robot is actuated by only one actuator which will function as the energy generator, energy needs to be released passively.

(a) (b)

Fig. 2. Rolling mechanism: (a) Rack and pinion mechanism; (b) Robot model in rolling.

In the jumping locomotion mode the motion range of the slider is larger than that in the rolling locomotion. The slider is able to move between the highest position B and the lowest position L (see Fig. 2(a)). Here, the latch on the slider and the hook of the leg take important roles. They enable the energy generated by the DC motor to be stored in the spring. Also, these two devices act as the energy releasing mechanism that frees the energy for jumping. In the design of the slider, the latch is linked to the slider support by a revolute joint. One low stiffness torsion spring is inserted to the joint. The latch is called limited latch because its rotation angle is restricted in the range from 0 to 90 degree. The jumping procedure is illustrated in Fig. 3.

Fig. 3 Jumping mechanism:( a) The slider moves downward to lock to the hook; (b) The slider moves upward, compressing the spring; (c) At critical point, the spring is compressed maximally and starts to release; (d) All stored energy is released and the robot starts to jump.

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In order to start a jumping motion, the slider moves downward to the hook position. Due to the pushing force of the hook, the latch rotates around its revolute joint. When the slider continues to go downward to the lowest position L, the latch contacts the hook no longer and it is rotated back to its original position by the torsion spring. The situation is displayed in Fig. 3(a). It is noted in Fig. 3 that vs indicates the velocity of the slider with respect to the body base and vb is the velocity vector of body with respect the robot leg. From the lowest position the slider moves upward. In this direction, the latch is locked to hook (see Fig. 3(b)) and therefore its absolute velocity becomes zero (because the hook is a part of the robot leg which does not move). Rotation of the DC motor moves the body downward with the velocity vb. Since the main spring is connected to the rod and the body base, motion of the body results in the compression of the spring. This is called the energy storage procedure where the energy generated by the DC motor is stored in the main spring.

There is an important note in setting up of the slider and the body base. As shown in Fig. 3(c), the translation axis of the slider along the body is not parallel to that of the body along the robot leg. The distance between two axes gets larger at higher locations. The more the body skids downward, the smaller the overlap area between the latch and the hook becomes. Until this area gets to a point called the critical point which can be seen in Fig. 3(c), the spring is maximally compressed. Right after this moment, the constraint between the latch and the hook is broken and the energy stored in the spring is rapidly released. The released energy shoots the system of the slider and the body upward with a large acceleration. At the end of this process where the spring is totally extended, the body-slider system achieves a velocity vb which is seen in Fig. 3(d). The robot can be remodeled as the two-mass spring based jumping mechanism [10] in which the lower mass m is the cylindrical frame and the upper mass M consist of all other parts. The jumping robot model is displayed in Fig. 4.

The tilt angle 0 of jumping model in Fig. 4 represents the skew angle of the robot leg in jumping mode. One may see in Fig. 1 that the cylindrical shell of the robot is flattened a little at its bottom. This keeps the robot stand stationarily in jumping mode. Let l0 is the maximum displacement of the main spring when it is compressed and k is the spring stiffness. Using the energy conversation laws, the largest jumping height and jumping distance the robot achieves in each jump can be calculated as in (1) and (2).

2 20 0

max 2

cos

2

l kMy

g M m

(1)

20 0

max 2

sin(2 )l kMx

g M m

(2)

Fig. 4. Model of two-mass jumping mechanism.

III. CONTROL METHOD

Unlike wheeled robots where rotation of actuator is directly transferred to wheels, rotation of the DC motor in the proposed robot indirectly controls rolling and jumping. In order to execute autonomous tasks, a two-level control model is considered (see Fig. 5). The high level controller receives tasks from the user or directly interfaces with the environment, decides and gives commands to the low level controller. The low level controller adjusts rotation of the motor in order for the robot to do basic motions including forward rolling, backward rolling, stopping and jumping. On the current robot, the control algorithm is not implemented fully yet, but only the low level controller is developed. The low level controller moves the slider along the rack in order to perform specific motions given by users. The control parameters for the low level controller may be the position or the motion speed of the robot. Both linear and nonlinear controllers such as PID and fuzzy controller can take the role of low level controller. For example, in order for the robot roll forwardly, the slider must be kept in front of the cylinder frame center. For the purpose, a fuzzy control algorithm is utilized. At first the tilt angle and the rolling speed of the robot are measured and then the controller used these parameters to adjust the slider position.

Fig. 5. Control model for the jumping-rolling robot.

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IV. ROBOT PROTOTYPE AND EXPERIMENTS

A. Robot prototype

A prototype of the jumping rolling robot has been fabricated. The prototype’s parameters are selected from the design and simulation results. The prototype is shown in Fig. 6 and its parameters are presented in Table 1.

The robot body and the slider are mostly made of acrylic material while carbon rods are used for the frame and the leg. The diameter of the frame is 180 mm and the robot’s total weight is about 80 gram including the battery and the control board. The control board installed on the robot includes an 8-bit microcontroller, a bluetooth module, two acceleration sensors, one gyroscope sensor and the motor driver. All are packaged in the 10 gram board. The microcontroller is the main processor of the robot which obtains data from sensors and implements the control algorithms. There is a tiny potentiometer installed to measure the rotation of the DC motor. Two limit switches are also added on the motion range of the slider. Two acceleration sensors are installed for the tilt angle measurement while the gyroscope sensor gives the rolling speed.

TABLE I PROTOTYPE PARAMETERS

Symbol Name Value

D Cylindrical frame diameter 180 mm

ms Frame mass 15 g

mb Body mass 45 g

ms Slider mass 20 g

l0 Spring displacement 50 mm

k Spring stiffness 0.36 N/mm

0 Initial tilt angle in jumping 15o

γ Skew angle between slider axis and body axis

3o

Fig. 6. Robot prototype.

B. Tilt angle measurement

Tilt angle and rolling speed of the robot body are important parameters for controlling the rolling motion. Measuring the rolling speed can be done straight forward using the gyroscope tiny MEMS sensor. However, obtaining the dynamic tilt angle during the rolling motion is quite difficult, especially for the mesoscale machine like our robot. One feasible solution is the fusion of acceleration and gyroscope sensors by the Kalman filter [11, 12]. This method is quite suitable for our robot except one drawback: the implementation of the Kalman filter algorithm may take much time and the total run time for the whole control algorithm can be large. Here, we propose a simple solution which relies on the special kinematic of the rolling motion. We use two tiny acceleration sensors installed at two terminals of the body line for measuring the dynamic tilt angle. The kinematics model of the rolling circle is shown in Fig. 7.

In Fig. 7 S1, S2 are two acceleration sensors, is the tilt angle needed to measure, α is angular acceleration, R is frame radius and a is translation acceleration of the robot. Let s1x, s1z, s2x, s2z are output signals from the x and z axes of two acceleration sensors, respectively. By doing the kinematics analysis of the rolling, the tilt angle of the robot is directly calculated from the sensor outputs as in (3).

2 2 2 2

2 2 2 2

atan2 ,2( ) 2( )

atan2 ,2( ) 2( )

m n n m

a g a g

a g a g

a g a g

(3)

Here m and n are two intermediate variables which can be obtained from the sensor signals (4).

1 2

1 2

2

2

z z

x x

s sm

s sn

(4)

Fig. 7. Rolling kinematics model.

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C. Experiments

Several experiments have been done on the current robot prototype with and without the control algorithm. The purpose of the experiment without control algorithm is to verify the working principle of the robot mechanism. In these experiments, the user controls the sliding of the slider via the bluetooth remote controller. Although it is difficult to synchronize the motion of the slider with the rolling to obtain the smooth motion, this experiment has clarified that the proposed robot may perform both jumping and rolling as expected.

The second experiment was conducted with the low level control algorithm. In this experiment, the user sent the commands like “roll forward”, “roll backward” to the robot and the robot implement the desired moving by itself. Since all calculation are straight forward, the total time the microcontroller needed for calculating the tilt angle and control logic is about 5 milliseconds. In this experiment using a 7.4V-230mAh Lithium-ion polymer battery, the robot may roll continuously more than 60 minutes with the average speed of 600 mm/sec. This result implies that the robot may travel an equivalent distance more than 2 km before battery recharging is required. For the jumping experiment, when the robot obtain its maximum height the distance from its lowest point to the ground is up to 160 mm. For that, the robot center reaches the height of 250 mm. Fig. 8 displays the jumping height and Fig. 9 demonstrates the postures of the robot in jumping and rolling captured from the experiment.

Fig. 8. Jumping height of the robot prototype.

Fig. 9. Robot prototype in jumping and rolling experiment.

V. CONCLUSION

Design and experiments of a novel jumping and rolling robot are presented. The most outstanding feature is the minimum use of actuator. With only one DC motor, the robot is able to perform the jumping and rolling locomotions agilely. Experimental results show that the average rolling speed is up to 0.6 m/sec, which is equivalent to the distance of approximately 2 km. Besides, the robot is able to jump upward nearly as much as its body height. This result is remarkable if one considers that the 7.4V-230mAh battery pack weighs at only 10 grams. A method for measuring the tilt angle is also developed. The method is verified and applied for a control algorithm. Though several problems have been solved for the robot, the robot needs to be improved in multiple ways. First, control of steering is needed to improve the maneuverability of the robot. Second, size and weight of the robot need to be reduced. This is important to increase the jumping distance and rolling speed of the robot.

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