A cross pipe networks involved in construction projects, as well as already installed builds and infrastructure, survey teams and maintenance contractors are increasingly turning to robots to achieve more in-depth reports.
To achieve a survey, mobile robots integrate a variety of sensors, such as high-resolution cameras to provide visual inspection, laser profiling to confirm pipe geometry, acoustic sensors to detect leaks, and ultrasonic sensors to measure pipe wall thickness. These mini vehicles can also integrate sonar to survey into the ground beyond the pipes.
A variety of mobile robots can be used within pipes, depending on the intended environment of operation and pipe architecture, with common designs including rail-mounted robots, crawlers that operate with tracks, as well as wheeled robots. To propel the robots through a pipe network, control the movement of their sensor apparatus, and actuate their tools, a capable motion system is an essential requirement across all robot designs.
Central to the coordination of these motion requirements is a multi-axis motion controller. Mobile robots can have independently driven wheels or a differential system to calculate wheel speed based on turning radius, so real-time coordination of independent motor axes is crucial. Tools might also need to coordinate with the motion of the robot, while robotic arms hosting end effectors integrate multiple joints that need to operate in concert.
These requirements mean that in addition to a multi-axis capability, a motion controller must integrate a real-time, deterministic communications protocol, such as CANopen or EtherCAT, to achieve the necessary speed and precision in motion coordination.
Yet a key challenge for designers of these mobile robots is the confined environment of a pipe network. The space constraints demand a robot with a compact footprint that must also reach the desired level of operational performance and reliability.
The need to meet both these requirements was presented during a recent project to develop a tunnel-based inspection and maintenance robot. The initial prototype integrated an industry-scale motion controller to coordinate the wheels, which were driven by four, standard DC motors. The design also featured two further DC motors to actuate its robot arm that would host various tools.
The footprint of this set-up, which was sufficiently compact for a factory cabinet yet oversize for a mobile robot, limited the size of battery the robot could accommodate. This meant relatively frequent battery recharge replacement, increasing the overall time the robot would spend on a task.
While aiming to minimise space, lower torque, smaller motors were also used on the prototype. However, the hard-run motors were liable to overheat within just minutes of operation, forcing the robot to cease operation for an interval – or risk failure.
The robot development team engaged maxon for a solution, which would be based around the maxon MiniMACS6 multi-axis motion controller. At 140.5mm wide and 109.5mm long, the freely programmable master motion controller instantly achieved a significant size reduction, as well as reducing mass down to just 370g.
As the robot arm was only used when the robot was stationary, maxon’s engineers proposed that the wheel-driving motors could also be switched to control the robot arm. As a result, the six, standard DC motors were replaced by just four, high-torque versions of maxon’s ECX flat brushless DC (BLDC) motors, paired with maxon GPX gearheads.
With a 32mm diameter, the ECX flat motor design reduced dimensions and mass while achieving the required torque output. The developer also revealed that in real-world conditions, the robot could encounter pipes set a gradient, which would increase the load faced by the drive system. This demand would be handled by the motor’s 34.1 mNm maximum continuous torque, achieved thanks to its winding design and high-performance magnets which would help to optimise torque density.
Crucially for the robot designer, the total reduction in size and mass enabled them to integrate a larger battery. Moreover, upgrading the original motors with a high efficiency BLDC design reduced the demand on the power source. Drive system efficiency was further enhanced by the GPX gearhead, designed to optimise power transfer and minimise losses. These advantages will enable the robot to operate for longer intervals within a pipe before requiring battery recharge, increasing the productivity rate of inspection and maintenance.
The motor is also highly thermally efficient, and the open design of the ECX flat range allows it to operate at high speed while ensuring heat dissipation, resulting in normal running temperatures, even within the robot’s compact housing. This minimises the potential of overheating and has significantly improved the reliability of the robot’s prototype – as well as removing the need for pauses to allow cooling.
Programming to coordinate the motion of the robot has been supported by maxon’s multi-axis motion control engineers. While offering expertise in motion development, the team is also able to draw on a vast library of applications. This portfolio of motion programming can be quickly adapted to achieve the specific coordination required for each project.
MOTION CONTROL FOR BRUSHLESS DC MOTORS
Recently launched by Portescap, the global leader in miniature motion solutions, and available from Mclennan, the new PCR 56/06 EC SD is an intelligent drive for single-axis control of brushless DC motors that is offered as an integrated hardware and software developer kit package for rapid product development-to-production timescales.
Tailored for use with Portescap’s BLDC series miniature motors – also fully supported and available from Mclennan – the new motion controller features a powerful 350W drive stage in a very compact package and includes Windows-based motion and diagnostic software with auto-tuning as well as C-based command sets.
In terms of performance the PCR 56/06 EC SD includes S-curve, trapezoidal, velocity contouring, and electronic gearing profiles with position, velocity and torque control modes. Powered from 12 to 56V DC with a continuous output of up to 5.5A, the drive stage features an advanced PID filter with velocity and acceleration feed forward providing smooth precision motion. Hall sensor and encoder feedback options are available.
