Technology Update: Leg-wheeled hybrid mobile robot

Flexible programming helps in developing an energy-efficient leg-wheeled hybrid mobile robot that can drive quickly and smoothly on flat terrain and stably negotiate natural or artificial uneven terrain.


Quattroped, a leg-wheel hybrid mobile platform design from National Taiwan University; image courtesy of National InstrumentsIntegrated hardware and software helped with a mobile robotic design that uses the best attributes of legs and wheels, two widely adopted methodologies for ground locomotion.

Legs of most ground animals are agile, powerful, and capable of moving animals smoothly and rapidly on uneven, natural terrains. Humans invented wheels for specialized locomotion on flat ground, providing excellent power efficiency and smooth travel at high speeds on flat ground, exceeding capabilities of legged locomotion.

Combining wheeled mobility on flat ground and legs on rough terrain, the Bio-Inspired Robotic Laboratory (BioRoLa) team at National Taiwan University aimed to design a leg-wheeled hybrid robot to handle flat or rough terrain.

Mechanism design

Compared to most hybrid platforms, which have separate mechanisms and actuators for wheels and legs, the Quattroped leg-wheeled hybrid mobile robot uses a “transformation mechanism” that deforms a specific portion of the body to act as a wheel or a leg.

Quattroped platform architecture; diagram courtesy of National Instruments

From a geometrical point of view, a wheel usually has a circular rim and a rotational axis located at the center of the rim. The rim contacts the ground, and the rotational axis connects to the robot body at a point hereafter referred to as the “hip joint.” With most wheeled locomotion on flat ground, the wheel rotates continuously and the ground-contact point of the wheel is located directly below the hip joint with a fixed distance. In contrast, in legged locomotion the leg moves in a periodic manner without specific geometrical configuration between the hip joint and the ground-contact point. The relative position of the legs varies frequently and periodically during locomotion.

Shifting the hip joint out of the center of the circular rim and changing the continuous rotation motion to other motion patterns implies the locomotion switches from wheeled mode to legged mode. This provided design motivation for a mechanism that directly controls the relative position of the circular rim with respect to the hip joint so it can generate wheeled and legged motions. Because the circular rim is a two-dimensional object, adding a second degree of freedom (DOF) can adjust the relative position of the hip joint to the center of the circular rim along the radial direction. Two DOF motions are orthogonal to each other. The same set of actuation power can be efficiently used in wheeled and legged modes.


Robot computation is mainly powered by a 400 MHz real-time embedded control system operating at a loop rate of 1 kHz with a gate field-programmable gate array (FPGA) embedded chassis running at 10 kHz. The real-time control system microprocessor communicates to a remote-control (RC) PC laptop via standard IEEE 802.11 wireless protocols. The FPGA connects to digital and analog I/O modules, which connect to robot sensors and actuators.

Robot sensors include temperature sensors on the motors and power amplifier chips for health monitoring; voltage and current measurement sensors for power management; Hall-effect sensors for leg-wheel configuration calibration; a 6-axis inertial measurement unit (IMU) and a 2-axis inclinometer for body state measurement; and three infrared (IR) distance sensors to measure ground clearance. Various sensors, such as GPS, vision, and laser ranger, are also used to improve the robot’s perception ability. Actuators on the robot include eight dc brushed motors for driving the robot, two high-torque remote-controlled servos for front leg-wheel turning, and four small remote controlled servos and four small dc brushed motors for leg-wheel switching.

Software, three cores

Three computation cores (PC, real time, and FPGA) running object-oriented control software are responsible for different tasks. A remote-control PC operated by the user exchanges only essential information with the microprocessor of the real-time controller, such as high-level commands to drive the robot in different modes, passing back crucial motor and electronic status for health monitoring and logging state data.

Quattroped Control FPGA Block Diagram, courtesy of National Instruments

Most computation is executed within the onboard real-time processor. Some algorithms that require high-speed signal exchanges are compiled within the FPGA, such as proportional-integral-derivative (PID) control for the dc motors, encoder readings, and pulse-width-modulation (PWM)-based RC servo commands. The robot is programmed with various state machines, and each state represents one of the robot’s particular operating behaviors.

After the robot is powered on, motors are calibrated to define the absolute geometric configurations of the two active DOFs on each leg-wheel. Calibration is achieved by matching the relative position between Hall-effect sensors installed on the body and magnets mounted within the leg-wheel. The calibrated robot can be operated in legged mode or in wheeled mode, depending on the current rim configurations (that is, wheel or half-circle leg). Leg-wheel switching transforms the leg-wheel configuration.

In wheeled mode, the robot can stand, drive, and sit. Standing and sitting are two transient states to bridge the initial on-the-ground configuration and the driving behavior. When driving, the forward speed and turning rate are continuously adjustable. In legged-mode operation, standing and sitting behaviors also are included for transient states. After the robot stands, it can perform various behaviors, including walking, trotting, step crossing, bar crossing, and stair climbing.

Hardware, software benefits

Robots, in general, are high-DOF complex systems. Successful robot development requires time and effort to properly integrate various mechanical, electrical, and computer systems. For the BioRoLa team at National Taiwan University (mainly students with mechanical engineering backgrounds), a reliable, modular, easy-to-use, and well-integrated mechatronic system suitable for rapid prototyping helped robot development.

The graphical programming interface makes it easy for students to construct logic flow followed by coding and to understand the programs created by other developers. A compact, durable, and modular controller helps mobile robot development where the size, weight, performance, and learning time are important factors. Software and hardware compatibility reduces the time and efforts of developers in performing system integration.

Sensors, perception

On the hardware side, various sensors are being integrated into the current mechatronic system to improve the perception capabilities of the robot. On the behavioral side, legged behaviors are being refined with a closed-loop feature to improve robot mobility on various challenging terrains and to explore dynamic legged gaits.

At a glance: Leg-wheeled hybrid mobile robot application, software and hardware

NI LabVIEW software, CompactRIO hardware, and I/O modules are used to rapidly integrate mechanical design, mechatronics, and programming for a functional robot prototype that combines the best attributes of wheels and legs for locomotion. The system includes the following NI hardware and software.

  • Computation is mainly powered by a 400 MHz NI cRIO-9014 real-time embedded control system operating at a loop rate of 1 kHz with an NI cRIO-9104 3M gate field-programmable gate array (FPGA) embedded chassis running at 10 kHz.
  • FPGA connects to NI 9205 and NI 9264 analog I/O modules and NI 9401 and NI 9403 digital I/O modules, which connect to robot sensors and actuators.
  • Three computation cores (PC, real time, and FPGA) run LabVIEW 8.6. A remote-control PC operated by the user exchanges essential information (high-level commands to drive the robot in different modes) with the microprocessor of the real-time CompactRIO controller. It passes back crucial motor and electronic status for health monitoring and logging state data.
  • NI LabVIEW graphical programming interface makes it easy for students to construct logic flow followed by coding and to understand the programs created by other developers.
  • Compact, durable, and modular CompactRIO system is extremely suitable for mobile robot development where the size, weight, performance, and learning time are important factors.
  • Well-defined integration between LabVIEW and NI hardware significantly reduces the time and effort of developers in performing system integration.


The authors thank NI Taiwan for kind support in equipment provision and technical consulting.

- Authors, from Department of Mechanical Engineering at National Taiwan University, are Pei-Chun Lin, Shen-Chiang Chen, Ke Jung Huang, Shuan-Yu Shen, and Cheng-Hsin Li. Edited by Mark T. Hoske, Control Engineering,

AIMing for Automated Vehicles blog

No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Leaders Under 40 program features outstanding young people who are making a difference in manufacturing. View the 2013 Leaders here.
The new control room: It's got all the bells and whistles - and alarms, too; Remote maintenance; Specifying VFDs
2014 forecast issue: To serve and to manufacture - Veterans will bring skill and discipline to the plant floor if we can find a way to get them there.
2013 Top Plant: Lincoln Electric Company, Cleveland, Ohio
Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Plant Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.

Bring focus to PLC programming: 5 things to avoid in putting your system together; Managing the DCS upgrade; PLM upgrade: a step-by-step approach
Balancing the bagging triangle; PID tuning improves process efficiency; Standardizing control room HMIs
Commissioning electrical systems in mission critical facilities; Anticipating the Smart Grid; Mitigating arc flash hazards in medium-voltage switchgear; Comparing generator sizing software

Annual Salary Survey

Participate in the 2013 Salary Survey

In a year when manufacturing continued to lead the economic rebound, it makes sense that plant manager bonuses rebounded. Plant Engineering’s annual Salary Survey shows both wages and bonuses rose in 2012 after a retreat the year before.

Average salary across all job titles for plant floor management rose 3.5% to $95,446, and bonus compensation jumped to $15,162, a 4.2% increase from the 2010 level and double the 2011 total, which showed a sharp drop in bonus.

2012 Salary Survey Analysis

2012 Salary Survey Results

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.