3-D Lidar for mobile machinery

Autonomous mobile work machines need the capability of sensing and mapping the surrounding area. Finnish researchers developed 3-D Lidar, based on a 2-D laser scanner and electric motor drive that rotates the scanner.

05/23/2017


Figure 1: Control system architecture of autonomous mobile machine [3]. Courtesy: CAN in Automation/TUTAutonomous mobile work machines require accurate maps of their surroundings in order to be able to perform the required tasks efficiently and especially safely. The machines need first of all rough information of their position at the work site, where an accuracy of a few meters is often acceptable. This can be provided to the machine with GPS-based navigation. However, in addition they need high-resolution information—accurate to a few centimeters—of their position in relation to the nearby objects and environment. As the machines are constantly moving, this information needs to be constantly updated.

 

 

Some commercial solutions of 3-D Lidars can be found on the market, for example from manufacturers such as Sick [6] and Velodyne [10], but in many cases these are quite expensive, especially for outdoor applications. Therefore, there is a need for a robust low cost 3-D Lidar, especially for use in research. This paper presents a solution for a low cost 3-D Lidar based on a 2-D laser scanner, an electric motor drive rotating the 2-D scanner, and data fusion with the navigation system of the mobile machine. The 2-D laser scanner provides range and intensity data from the measured plane. The controller of the electric motor drive provides the rotation angle and rotation speed of the laser scanner.

By combining these measurement data with the navigation data one can create point clouds that can be used for sensing and mapping the environment of an autonomous mobile machine. In this paper, the mechanics and control system structure of the servo are presented in detail to make building similar types of systems easier for the interested reader. The developed hardware described in this paper was made for the laser mapping of the autonomous mobile machine used in the GIM project [2]. The rotating laser scanner scans the fore field of the machine. The laser map is used among other types of maps for path planning and obstacle avoidance purposes.

Control system of an autonomous mobile machine

The work machine, for which the mapping system was developed, is a modified version of a multi-purpose wheel loader. The frame of the machine is original, but the control system, electronics, and hydraulics have been changed to enable researching autonomous operations. The control system architecture of this autonomous machine is illustrated in Figure 1.

The hardware devices are located physically in two different locations. Visualization and operator computers are off-board computers and those are connected through a network switch and over WLAN to an on-board network switch. On-board computers and peripheral devices are also connected to the on-board switch. The on-board computers and peripheral devices follow the device architecture illustrated in Figure 1.

The low-level control device architecture consists of a control of the actuators, such as hydrostatic drive pump, diesel engine, and hydraulic valves of the machine. It also takes care of the data logging of the inertial measurement unit (IMU), central joint resolver, pressure, and some other sensors. The low-level control is based on six Epec embedded vehicle computers, which communicate with the middle level control through four CAN networks. The implementation of CAN followed the CANopen standardization, which enables fluent CAN network management and consistency handling.

The middle level control, including navigation, path planning, and network interfaces with real-time kinematic global navigation satellite system (RTK-GNSS) was implemented on an industrial embedded PC using Matlab/xPC Target as an operating system. The control of the servo—including the electric motor drive and the encoder measurement—were also implemented in this level via CAN. The high-level control of device architecture is also implemented on an industrial embedded PC. This PC communicates with the middle level using the UDP protocol. This protocol is also used in data transfer from the 2D laser scanner to the middle level control to ensure a real-time data transfer of the laser scanner data.

Electric motor drives and 2-D laser scanners

In general, two different types of DC motor exist: Brushed direct current (DC) motors and brushless direct current (BLDC) motors are the most typical motors. BLDC motors are typically driven by three phase conductors and phase voltages are generated by the braking voltage of the intermediate circuit. In accurate position and speed electric motor drives, BLDC motors are more common. [4] Thus, a BLDC motor was also used in this case. A ceramic planetary gear was used for the reduction of the output rotation speed and to increase the output torque.

Feedback sensors

The accurate and not (or in practice minimum and constantly) delayed measurement of the rotation angle is essential to later enable data fusion with laser scanner range data for creating a 3D point cloud data. This is more important than for example an accuracy of the position or speed control of the servo because in these cases a small error or delay is not reflected to in the 3-D point cloud calculation.

DC motors typically use a built-in brushed commutator for commutation but the selected BLDC motor requires an external sensor to sense the angle and speed of the rotor for commutation. The commutation type depends on the sensor type and it can be sensor-less commutation, six-step commutation or sinusoidal commutation. Sensor-less commutation doesn't use sensors at all, six-step commutation uses Hall sensors, and sine commutation uses for example incremental encoders for commutation. Sine commutation is the most recommended commutation type if the application requires a constant torque generation over the whole rotation speed range [7].

Thus, an external incremental encoder was selected for measuring the speed of the electric motor for speed control purposes. The measurement of the incremental encoder is a relative measurement, referenced to a certain reference rotation angle of rotor. It can't be directly used as a rotation angle measurement because every time the sensor is started it has to be calibrated, i.e. by finding the zero position.

Therefore, an external absolute encoder was selected for measuring the rotation angle despite the fact that absolute encoders are typically more expensive than incremental encoders because the internal structure of absolute encoders is more complex. Because data fusion with laser scanner range data requires the accurate measurement of the laser scanner rotation angle, the rotation angle measurement is made directly from the load side. Thus, inaccuracies resulting from the backlash of the planetary gear can be avoided.

Control system of the electric motor

In this paper, the control system of the electric motor controller is based on two control loops as illustrated in Figure 2. Two control loops were used because the system includes a planetary gear that brings backlashes to the system under control. Furthermore, backlashes cause delays in control systems and it might have an effect on the system stability. The planetary gear is needed because it is hard to precisely estimate the torque that the load requires. The purpose of an auxiliary control loop is to stabilize, define damping and the dynamic behavior of the system. The sensor in the auxiliary control loop is an incremental encoder that is located at the backend of the electric motor. The primary control loop uses an absolute encoder as a feedback sensor and controls the load angle of the rotation.

Figure 2: Control system with two control loops. Courtesy: CAN in Automation/TUT

All controllers in the control system are implemented to the electric motor controller as discrete time controllers. The current regulator is a PI controller with 10-kHz sampling time; the speed controller is implemented as a PI controller with speed and acceleration feed forward. A position controller is implemented as a PID controller with speed and acceleration feed forward.

The sampling times for both controllers are 1 kHz. In the control system, the speed feed forward can compensate speed-dependent friction that is caused by bearing among other things. The acceleration feed forward provides more current in cases when one needs a high acceleration or the load inertia is high. The electric motor controller is delivered with ready-made software that can be used for controller tuning.

Table 1: Properties of a Sick LMS111-10100 laser scanner. Courtesy: CAN in Automation/TUT2-D laser scanners

The purpose of the electric motor drive is to continuously rotate a 2-D laser scanner. The scanner type—a Sick LMS111—was predefined for this hardware by the end user. Table 1 presents the most relevant properties of the scanner. The laser scanner and its final installation position on the roof of the autonomous machine are presented in Figure 3. The measurements in Figure 6 illustrate how the position (left) and angular speed (right) behave over one movement cycle. One can clearly see that profiles are quite smooth over time and only a small angular speed ripple exists. The operating principle of the 2-D laser scanner is based on the time-of-flight (TOF). The 2-D laser scanner gives range and intensity values as measurement values from the measured plane. Measured range values are relative to the rotating mirror of the 2-D laser scanner and for every measured point one can also get the intensity value [6].

Figure 3: 2-D laser scanner (Sick LMS111-10100) installed on an autonomous mobile machine. Courtesy: CAN in Automation/TUT


<< First < Previous Page 1 Page 2 Next > Last >>

The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by Control Engineering subscribers. Vote now (if qualified)!
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
HMI effectiveness; Distributed I/O; Engineers' Choice Award finalists; System Integrator advice; Inside Machines
Women in engineering; Engineering Leaders Under 40; PID benefits and drawbacks; Ladder logic; Cloud computing
Robotic integration and cloud connections; SCADA and cybersecurity; Motor efficiency standards; Open- and closed-loop control; Augmented reality
Programmable logic controllers (PLCs) represent the logic (decision) part of the control loop of sense, decide, and actuate. As we know, PLCs aren’t the only option for making decisions in a control loop, but they are likely why you’re here.
This digital report explains how motion control advances and solutions can help with machine control, automated control on assembly lines, integration of robotics and automation, and machine safety.
This article collection contains several articles on how advancements in vision system designs, computing power, algorithms, optics, and communications are making machine vision more cost effective than ever before.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Control room technology innovation; Practical approaches to corrosion protection; Pipeline regulator revises quality programs
Cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by Control Engineering subscribers. Vote now (if qualified)!
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
HMI effectiveness; Distributed I/O; Engineers' Choice Award finalists; System Integrator advice; Inside Machines
Women in engineering; Engineering Leaders Under 40; PID benefits and drawbacks; Ladder logic; Cloud computing
Robotic integration and cloud connections; SCADA and cybersecurity; Motor efficiency standards; Open- and closed-loop control; Augmented reality
Programmable logic controllers (PLCs) represent the logic (decision) part of the control loop of sense, decide, and actuate. As we know, PLCs aren’t the only option for making decisions in a control loop, but they are likely why you’re here.
This digital report explains how motion control advances and solutions can help with machine control, automated control on assembly lines, integration of robotics and automation, and machine safety.
This article collection contains several articles on how advancements in vision system designs, computing power, algorithms, optics, and communications are making machine vision more cost effective than ever before.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Control room technology innovation; Practical approaches to corrosion protection; Pipeline regulator revises quality programs
Cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by Control Engineering subscribers. Vote now (if qualified)!
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
HMI effectiveness; Distributed I/O; Engineers' Choice Award finalists; System Integrator advice; Inside Machines
Women in engineering; Engineering Leaders Under 40; PID benefits and drawbacks; Ladder logic; Cloud computing
Robotic integration and cloud connections; SCADA and cybersecurity; Motor efficiency standards; Open- and closed-loop control; Augmented reality
Programmable logic controllers (PLCs) represent the logic (decision) part of the control loop of sense, decide, and actuate. As we know, PLCs aren’t the only option for making decisions in a control loop, but they are likely why you’re here.
This digital report explains how motion control advances and solutions can help with machine control, automated control on assembly lines, integration of robotics and automation, and machine safety.
This article collection contains several articles on how advancements in vision system designs, computing power, algorithms, optics, and communications are making machine vision more cost effective than ever before.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Control room technology innovation; Practical approaches to corrosion protection; Pipeline regulator revises quality programs
Cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me