1

REACTION FORCE/TORQUE SENSING WHEEL SYSTEMFOR IN-SITU MONITORING ON LOOSE SOIL
Shoya Higaa, Kenji Nagaokab, and Kazuya YoshidacaTohoku University, shoya@astro.mech.tohoku.ac.jp
Tohoku University, nagaoka@astro.mech.tohoku.ac.jpcTohoku University, yoshida@astro.mech.tohoku.ac.jpbAbstract
Lunar/planetary exploration rovers are required to travel on loose soil. In such environments, wheel sinkage andslippage easily occur and adversely affect the mobility performance of the rovers. Therefore, we need to consider thewheel–soil interaction for appropriate mobility control. However, soil characteristics of the target environment of rovers
are unknown and heterogeneous. Thus, unexpected wheel sinkage and slippage occur with rovers. It is expected thatsuch phenomena can be avoided by appropriately estimating wheel conditions. Therefore, we proposed a sensing wheel
system that estimates wheel sinkage/slippage and soil reaction force/torque. Previous work focused on an online
estimation method of wheel sinkage/slippage using a time-of-flight (ToF) camera; through this method, online estimatesof wheel sinkage/slippage could be made. In this study, we expand the previous system in order to develop a real-time
sensing function of soil reaction force/torque. The proposed system can measure the reaction forces/torques by fixing a
6-axis force/torque (F/T) sensor onto the wheel axis. Traction tests were performed under several wheel configurations
and traction load conditions to validate the projected wheel forces/torques. Experimental results showed that the sensing
wheel precisely measured the drawbar pull, normal force, and wheel torque in real time.
Keywords: lunar/planetary rover, sensing wheel system, reaction force/torque

1. Introduction
Lunar/planetary exploration missions have been performed using unmanned mobile robots (rovers). Rovers allow
for the direct investigation on the surface of a target celestial body and provide considerable scientific returns. Therefore,rovers will play a significant role in future lunar/planetary exploration missions. A major challenge is that the target
environments of rovers are covered with fine granular regolith, where gravel and rocks are also widespread. Despite
these environmental challenges, wheeled rovers have been utilized owing to their efficient mobility performance andhigh mechanical reliability. However, the rover struggles to follow the desired route because the rover traverses withwheel slippage and sinkage when it travels on deformable terrains, such as loose soil. Once the rover encounters
increasing wheel sinkage and slippage, it becomes exceptionally difficult to reduce these wheel sinkage and slippage
conditions. Therefore, rover operators have avoided steep-sloped, sandy terrains whenever possible. However, the mostinteresting areas for scientists are generally in challenging environments, such as on the rims and inside of craters. In
order to explore such challenging targets, early detection of increases in wheel sinkage and slippage is essential. Large
wheel sinkage and slippage are induced by a limitation of the conventional wheel–soil interaction model, unknown
terrain characteristics, and other unpredictable factors. Therefore, to prevent lunar/planetary rovers from lapsing into a
The authors are solely responsible for the content of this technical presentation. The technical presentation does not necessarily reflect
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Fig. 1 Conceptual diagram of a wheel system for wheel performance estimation.

serious-slip condition, the current wheel–soil interaction model must be reconsidered, and the terrain characteristics ofthe target environments should be re-evaluated in situ.
To precisely clarify the wheel–soil interaction, we measured the stress distribution of a wheel with and without
grousers (Higa et al., 2016b, 2015). These experimental results are significant achievements for remodeling theconventional theory using flat and homogeneous soil conditions; however, real terrain conditions include rough andheterogeneous soil, and thus, there is uncertainty in the mobility prediction of the rover. Typical rovers can obtain wheel
torque and environmental information using optical cameras. Wheel torque can be estimated by the motor current of thewheel, and optical cameras can be used to estimate wheel conditions. The exploration targets of future rovers will
include more challenging areas than past and ongoing missions. To travel safely on deformable terrain with wheeledvehicles, estimating each wheel performance online is important. Therefore, in this study, we have proposed a systemfor online wheel performance estimation.
There have been several related studies that estimated wheel performance (wheel sinkage/slippage andforces/torques acting on a wheel). Iagnemma et al. addressed the online estimation of terrain parameters by identifying
the force acting on the wheel in the conventional model (Iagnemma et al., 2004). This study was accomplished bysensing the torque and reaction forces applied to a mobile robot by the terrain. Terrain forces and parameter estimates
were also conducted by using classical wheel–soil interaction models (Ray, 2009). Nagatani et al. estimated the drawbar
pull accurately by directly measuring the normal stress distribution beneath a wheel using pressure sensor arrays
attached to the wheel surface (Nagatani et al., 2009). Estimations of wheel sinkage have been performed by determining
the wheel contact angle (Shirai and Ishigami, 2014). Iizuka et al. proposed a sensing wheel that directly detects wheel
sinkage using strain gages (Iizuka et al., 2014a, 2014b). Nagatani et al. studied a visual odometry system using atelecentric camera on loose soil (Nagatani et al., 2010). Estimations of wheel sinkage/slippage based on images of theside surface of the traveling wheel have also been studied (Hegde et al., 2013; Milella et al., 2006; Reina et al., 2006).
Wheel slippage detection has been performed by measuring the wheel motor current (Ojeda et al., 2006). These related
works only addressed estimations of either the wheel sinkage/slippage or the reaction force acting on the wheel basedon the conventional wheel–soil interaction model. In addition, it was difficult to implement these studies to the wheelsof the rover. However, to introduce the online wheel performance estimation in a real mission, consideration of bothwheel sinkage/slippage and reaction force/torque estimates is essential. Therefore, we have developed a sensing wheel
system of wheel sinkage/slippage and reaction force/torque that can be implemented to each wheel of the rovers infuture missions. Our previous work focused on estimating wheel sinkage/slippage online (Higa et al., 2016a). This
paper reports reaction force/torque sensing using the wheel system that we developed.

2. Concept of a Sensing Wheel System for Wheel Performance Estimation
Figure 1 shows a conceptual diagram of the proposed system for wheel performance estimation. The system iscomposed of a wheel (driven by a rotary motor), a time-of-flight (ToF) camera, and a 6-axis force/torque (F/T) sensor.
The ToF camera is used for online estimation of wheel sinkage and slippage. The ToF camera is a depth sensor andProceedings of the 19th International & 14th European-African Regional Conference of the ISTVS, Budapest, September 25–27, 2017

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Fig. 2 F/T sensor fixed to the wheel axis.

Fig. 3 F/T sensor fixed to the wheel axis.

outputs point cloud data of the depth from the sensor origin to the target object. The ToF camera can also capture 8bit/16-bit monochrome images. Hence, wheel slippage estimation was performed based on the optical flow technique(Higa et al., 2016a). The wheel system can measure forces/torques in real time via a 6-axis F/T sensor. The test wheel ismounted on the wheel driving shaft via the F/T sensor. Thus, the F/T sensor can detect all reaction forces/torques thatact from the traveling terrain. Accurate estimation of the wheel performance and terrain conditions can be provided by
combining the F/T sensor data with the rover attitude data obtained from an internal measurement unit (IMU).

3. Real-Time Sensing of 6-axis Force/Torque of Proposed Wheel System
In order to maintain mobility control of lunar/planetary exploration rovers, the rover should be able to calculate thereaction forces/torques of the terrain it is traversing in real time. The stress measurement technique described earlier
was accurate; however, it is not well suited to real-time operation, since several F/T sensors would be needed inside thewheel. Moreover, the update interval of the stress distribution is less than that required for mobility control, and thewheel maintains its traveling velocity during the stress measurements. Although accurate stress measurements areneeded for effective estimates of wheel performance on loose soil and the target terrain/soil characteristics, the traveling
control of the rover does not require the stresses distributed beneath the wheel, but rather requires the forces/torques
acting on the wheel.
Therefore, in this section, we describe a real-time measurement method for the 6-axis force/torque acting on thecenter of a traveling wheel.

3.1 Wheel configuration for real-time sensing of reaction forces/torques
To sense the 6-axis reaction forces/torques of the wheel on loose soil, we used a low-profile F/T sensor(FFS080F151M300R6; Leptrino Co., Ltd.) shown in Fig. 2. The sensor can sense 3-axis forces up to ±150 N and 3-axis
torques up to ±30 Nm.
The 6-axis F/T sensor was attached to the wheel shaft to measure the reaction forces/torques acting on the wheel inreal time. The sensor rotates with the wheel, so the output fluctuates with the angle of wheel rotation. Thus, as shown in
Fig. 3, the forces/torques acting on the wheel are projected as follows:
𝐹𝑥𝑤 = 𝐹𝑥𝑠 sin 𝛼𝑤 − 𝐹𝑦𝑠 cos 𝛼𝑤 ,𝐹𝑦𝑤 = −𝐹𝑧𝑠 ,𝐹𝑧𝑤 = 𝐹𝑥𝑠 cos 𝛼𝑤 − 𝐹𝑦𝑠 sin 𝛼𝑤 ,𝑀𝑥𝑤 = 𝑀𝑥𝑠 sin 𝛼𝑤 − 𝑀𝑦𝑠 cos 𝛼𝑤 ,𝑀𝑦𝑤 = −𝑀𝑧𝑠 ,𝑀𝑧𝑤 = 𝑀𝑥𝑠 sin 𝛼𝑤 − 𝑀𝑦𝑠 cos 𝛼𝑤 ,(1)where the first index denotes the axis that each force or torque is acting on, the second index denotes the wheel
coordinates or sensor coordinates, and 𝛼𝑤 is the angle of wheel rotation (counter-clockwise around the Y-axis of thewheel coordinates is positive).
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The reaction forces/torques, which are projected by the above equations, require a precise angle of wheel rotation.
In this system, the angle of wheel rotation is obtained from the wheel motor encoder at 50 Hz.

3.2 Calibration of real-time force/torque estimation
Single wheel tests were performed to validate the estimation of real-time force/torque acquisition. Before testing,
the wheel sensor was calibrated. The calibration method was as follows:
1.
2.
3.

Rotate the wheel several times in the air while obtaining sensor data.
Calculate the offset errors using the maximum and minimum values of each axis.
Subtract the offset errors in each axis.

4. Experimental Setup
Traction tests, using the single wheel testbed, were performed to confirm the proposed real-time force/torquesensing method. This section describes the single wheel testbed and experimental conditions.

4.1 Single wheel testbed
In this study, the single wheel testbed shown in Fig. 4 was used for evaluating the proposed sensing wheel system.
The single wheel testbed, which includes the sandbox, is 2.50 m long, 1.05 m wide, and 1.45 m high. The testbed allows
the test wheel to weigh up to 100 N and allows the wheel diameter of 250 mm to travel with a maximum sinkage of
85 mm on test soil. Furthermore, the wheel load can be adjusted by a load-canceling mechanism. Two types ofexperiments can be conducted using this testbed. One is a forced slip experiment in which a constant velocity is
imposed on both the wheel motor and the conveying motor. The second is a traction load experiment in which the wheel
motor maintains a constant velocity while pulling the wheel using several traction loads via pulleys and a rope. The
traction load experiment was selected in this study.
The testbed consists of a wheel unit, a conveying unit, and a sandbox. The wheel unit consists of two vertical slide
shafts with bushes, a wheel with a driving motor (speed-controlled by a microcontroller), a wheel load cancellation
mechanism, and a linear encoder to count the wheel sinkage. The wheel can naturally sink into the soil in the verticaldirection, along with the vertical slide shaft. The wheel is turned by a DC motor with a harmonic drive gear with a final
gear ratio of 800:1. A microcontroller maintains a constant rotational velocity via a motor driver under proportional
integral (PI) control at 10 ms intervals and measures the wheel sinkage by counting the linear encoder signals at thesame intervals. The linear encoder resolution for the wheel sinkage measurement is 25 μm. Measurements are calibrated
by assigning a zero level to the soil surface.
The conveying unit consists of two linear guides to restrict motion in the horizontal direction, a conveying motor,
and a linear encoder to count the wheel traveling distance measurement. The conveying motor and timing pulley arecoupled, and the timing pulley is connected to the wheel unit via a timing belt. One of the two test conditions (forced
constant-slip test or traction test using several traction loads) can be selected by connecting a rope to the timing belt.
The linear encoder for the wheel traveling distance measurement is connected to the wheel unit. The conveying motor isalso PI-controlled using a microcontroller via a motor driver at 10 ms intervals. The microcontroller counts the wheel
traveling distance at the same intervals. The resolution of the linear encoder for the wheel traveling distance is 50 μm.
The testbed is mounted on a sandbox with a length of 1.6 m, width of 0.3 m, and depth of 0.2 m. One of two
isometric sandboxes can be selected to perform experiments, each using a different type of soil. One sandbox is filledwith Toyoura sand, and the other is filled with lunar regolith simulant (FJS-1), manufactured by Shimizu Corporation.
FJS-1 provides a good approximation for the soil characteristics on the moon’s surface (Kanamori et al., 1998). Its grain
size distribution is wider than that of Toyoura sand. The grain size of Toyoura sand is known to be nearly homogeneous,and its cohesion is almost exactly zero. In this study, we selected the Toyoura sand to validate the proposed system.

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Fig. 4 Single wheel testbed.

Fig. 5 A configuration of test wheel. (Grouser length is 25 mm.)

4.2 Experimental condition
Single-wheel tests under free slip conditions and with different traction loads (𝑇𝐿) were performed using Toyoura
sand as a test soil. The test wheel was 𝜙250 mm in diameter and 100 mm in width and weighed approximately 50 N.
We tested the wheel with 25 mm grousers, 12.5 mm grousers, and no grousers. The angular velocity of the wheel wascontrolled to be 0.16 rad/s. Traction loads of 0, 10, and 20 N were applied to the wheels with grousers, and traction
loads of 0, 5, and 10 N were applied to the smooth wheel. The slip ratio, 𝑠, of the test wheel is defined as follows:𝑠 = (1 −𝑣𝑥(𝑟 + 𝑙𝑔 )𝜔

) × 100,(2)where 𝑣𝑥 is the traveling velocity of the wheel, 𝑟 is the wheel radius, 𝑙𝑔 is the length of the grousers, and 𝜔 is theangular velocity of the wheel.The slip ratio is expressed as the ratio of the traveling speed of the wheel with grousers to
the circumferential speed at the edge of a grouser; in general, the value of the slip ratio is between 0% and 100%. For
instance, the case s = 0% indicates that the rover can travel without slippage, and the case s = 100% indicates that thewheel slips completely, and the rover cannot travel at all. The traveling velocity is counted by the horizontal linear
encoder of the single wheel testbed. The angular velocity is acquired by the rotary encoder of the wheel motor.

5. Results and Discussions
Figures 6a, 7a, 8a, and 9a show the estimated drawbar pull, estimated normal force, estimated wheel torque, andangle of the resultant force of a wheel with no grousers, respectively. The median values of slip ratios under a steadystate of the wheel with no grousers under traction loads of 0, 5, and 10 N were approximately 12.6%, 89.7%, and 95.5%,
respectively. Figures 6b, 7b, 8b, and 9b show the estimated drawbar pull, estimated normal force, estimated wheel
torque, and angle of the resultant force of a wheel with 12.5 mm grousers, respectively. The median values of slip ratios
under a steady state of the wheel with 12.5 mm grousers under traction loads of 0, 10, and 20 N were approximately
15.2%, 59.6%, and 90.1%, respectively. Figures 6c, 7c, 8c, and 9c show the estimated drawbar pull, estimated normal
force, estimated wheel torque, and angle of the resultant force of a wheel with 25 mm grousers, respectively. The
median values of slip ratios under a steady state of the wheel with 25 mm grousers under traction loads of 0, 10, and
20 N were approximately 0%, 37.4%, and 79.9%, respectively.

5.1 Drawbar pull
As shown in Fig. 6, the estimated drawbar pull increases with the traction load. As wheel slippage is introduced via
the traction load, each estimated drawbar pull converges under steady state conditions. If the experiments were
performed under forced slip conditions, which forcibly maintain the constant traveling velocity of the wheel, theestimated drawbar pull would vary periodically because of the grousers (Ding et al., 2011). Although the estimateddrawbar pull includes the viscosity resistance of the guide rail and the tension of the linear encoder, the difference inProceedings of the 19th International & 14th European-African Regional Conference of the ISTVS, Budapest, September 25–27, 2017

6(a) No grouser.

(b) 12.5 mm grouser.

(c) 25 mm grouser.

Fig. 6 Drawbar pull estimated by the F/T sensor at wheel axle.

(a) No grouser.

(b) 12.5 mm grouser.

(c) 25 mm grouser.

Fig. 7 Normal force estimated by the F/T sensor at wheel axle.

each drawbar pull corresponds to the difference in traction load. Therefore, it can be concluded that the drawbar pull canbe well estimated using the projection approach.

5.2 Normal force
As shown in Fig. 7, the estimated normal force fluctuates significantly. This phenomenon implies that each grouser
interacts with the terrain beneath the test wheel. When the grousers exert additional tractive force, the normal force
becomes larger than the wheel load. Thus, wheels with grousers are better than wheels without grousers. Moreover,although the traction loads are different, the maximum normal forces under each condition are similar. Thisphenomenon indicates that the maximum normal forces depend on the grouser length.
Adversely, the grousers induce soil failure by digging into the soil when they first make contact with the ground.
This phenomenon is reflected in the estimated normal force. When the soil collapses under a grouser, it loses its bearing
capacity. Thus, the normal force becomes less than the wheel load. This phenomenon becomes notable when the wheelslippage increases. The minimum normal forces under each traction load condition decrease as the traction loadincreases. Furthermore, when 𝑇𝐿 = 20 N, most of the normal force is less than the wheel load until the angle of wheel
rotation reaches 540°. After reaching 540°, the normal force periodically switches between its maximum and minimum.
The wheel condition can be regarded as being in the steady state condition. This implies that the wheel condition canalso be estimated from the normal force estimates.

5.3 Wheel torqueAs shown in Fig. 8, the estimated wheel torque increases with the traction load. Moreover, the wheel torquefluctuates greatly, similar to the normal force. However, the amplitude of the wheel torque when 𝑇𝐿 = 20 N decreases
in comparison to the case for 𝑇𝐿 = 0 N. This indicates that the wheel torque reaches a certain maximum and becomessaturated.
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7(a) No grouser.

(b) 12.5 mm grouser.

(c) 25 mm grouser.

Fig. 8 Wheel torque estimated by the F/T sensor at wheel axle.

(a) No grouser.

(b) 12.5 mm grouser.

(c) 25 mm grouser.

Fig. 9 Angle of resultant force estimated by the F/T sensor at wheel axle.

It is known that wheel torque increases as the wheel sinks. Increasing the wheel sinkage widens the contact patch.
Therefore, two or more grousers always exert traction. Thus, the amplitude of wheel torque under large traction loads issmaller than under low-traction conditions.

5.4 Angle of resultant force
The angle of resultant force is proposed as an evaluation index. This is calculated from the ratio of drawbar pull tonormal force as follows:𝐷𝑃(3)𝛼𝑟𝑒𝑠 = tan−1 ( ),𝑊
where 𝐷𝑃 and 𝑊 denote the drawbar pull and normal force of a wheel, respectively.
As shown in Fig. 9, the estimated angle of resultant force increases with the traction load. Because the angle of theresultant force is calculated from the normal force, it fluctuates when the wheel has grousers. The estimated angles
under each condition converge after the wheel reaches the steady state condition. The converged angle is close to the
maximum stress angle of the stress distributions of a wheel without grousers under similar slip conditions. Althoughadditional discussion regarding a relationship between the normal stress and the angle of resultant force is needed, theangle of resultant force may be used to estimate the maximum stress angle.

6. Conclusion
In this study, we proposed a real-time reaction force/torque sensing wheel system on loose soil for in situ
monitoring. Real-time sensing of force/torque was realized by fixing a 6-axis F/T sensor to the wheel axle. The drawbar
pull, normal force, and wheel torque were projected using the angle of wheel rotation. In particular, the drawbar pulland normal force revealed a correlation with wheel sinkage and slippage. This information is useful for traversal on
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unknown loose terrain because the force information will detect whether the ground support is sufficient for the rover.
Moreover, the angle of resultant force was also derived. Because this angle is calculated based on drawbar pull andnormal force, the angle effectively indicates the direction of the dominant force. This implies that the angle of resultant
force may support estimates of the maximum stress angle of a wheel.

Nomenclature
𝐹𝑥𝑤
𝐹𝑦𝑤
𝐹𝑧𝑤
𝑀𝑥𝑤
𝑀𝑦𝑤
𝑀𝑧𝑤
𝐹𝑥𝑠
𝐹𝑦𝑠
𝐹𝑧𝑠
𝑀𝑥𝑠
𝑀𝑦𝑠
𝑀𝑧𝑠
𝛼𝑤
𝑠
𝑣𝑥
𝑟
𝜔
𝑏
𝑙𝑔
𝑡𝑔
𝑇𝐿
𝐷𝑃
𝑊
𝑇
𝛼𝑟𝑒𝑠

drawbar pull of a wheelside force of a wheel
normal force of a wheelroll moment of a wheel
pitch moment of a wheel (wheel torque)yaw moment of a wheel
force in the x-axis of the F/T sensor coordinate
force in the y-axis of the F/T sensor coordinate
force in the z-axis of the F/T sensor coordinate
moment in the x-axis of the F/T sensor coordinate
moment in the y-axis of the F/T sensor coordinate
moment in the z-axis of the F/T sensor coordinate
angle of wheel rotation
slip ratio
traveling velocity
wheel radiusangular velocity of the wheel
wheel width
length of a grouser
thickness of a grouser
traction load
drawbar pullnormal force (wheel load)
wheel torqueangle of the resultant force

[N]
[N]
[N]
[Nm]
[Nm]
[Nm]
[N]
[N]
[N]
[Nm]
[Nm]
[Nm]
[rad]
[%]
[m/s]
[m]
[rad/s]
[m]
[m]
[m]
[N]
[N]
[N]
[Nm]
[rad]

Acknowledgements
This work was partially supported by JSPS KAKENHI, Grant-in-Aide for JSPS Research Fellow, Grant Number
16J02425.

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Proceedings of the 19th International & 14th European-African Regional Conference of the ISTVS, Budapest, September 25–27, 2017