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FEATURE MOTION CONTROL The motion system


that powers a wheeled robot must be carefully specified according to the application requirements and


Robot measuring pipeline parameters and analysing potential defects for automatic pipe quality control


W


Powering robotics


heel-driven robots are often used to complete tasks that may be difficult or hazardous for humans to perform. For


the user (providing the terrain is not heavily obstacle-strewn), wheeled robots can be faster, more efficient and more resilient, meaning they are used in applications ranging from pipeline inspection to healthcare environments and even domestic use. For the manufacturer, these are easier to design, build and program. Integral to a wheeled robot is its motor


and gearbox system.The system has to be compact, light in weight and able to generate the required torque for movement across the specified environment. Often used in tough environments, it’s also got to be highly durable. In addition, for many applications, whether military surveillance or in a hospital, noise emissions must be minimal. Critically, wheeled robots also require high energy efficiency with a low current consumption. For these reasons, coreless brushed or


brushless DC motors coupled with compact planetary gearboxes are frequently specified. However, as the motion system is required to fit within a footprint of less than 40mm diameter, mini motors and gearboxes are required. Despite this, the gearbox output torque must typically handle 4-8Nm at speeds between 50 and 150rpm.


sPecifying the motion solution


To determine the specification, start by calculating the output torque required, factoring in the desired acceleration, as well as wheel diameter and mass. Torque requirements necessary to ascend gradients or overcome obstacles must also be added to the equation and, finally, friction and efficiency of travel should also be taken into account. Following this, the wheels’ rotational


speed can be determined, and appropriate selection of the motor and gearbox can then be made based on the output frequency. Typically, the higher the operating voltage, the higher the motor’s speed capability. While sufficient packaging space must be provided for the encoder, brake, motor and


4 DESIGN SOLUTIONS MAY 2022 0


gearing, operating temperature and avoidance of overheating is an important factor if the motor stack is enclosed. Motor capacity and heat dissipation techniques may need to be considered in such circumstances to ensure reliability when the robot is deployed in the field.


in motion


Portescap recently provided the motion solution for a four-wheel drive robot which featured two pairs of powered wheels. With each pair turning in the same direction, it was vital that the motion system controlled both pairs to run at the same speed, otherwise overall robot speed would be reduced and steering wouldn’t be aligned. Unlike differential steering that uses separately driven wheels, this four-wheel drive solution has a differential steering mechanism that requires only a single motor, which reduces footprint, energy use and cost. The solution was achieved using a Portescap


environment. Pradeep Deshmane, principal engineer – gearbox, R&D, Portescap, and Manoj Pujari, lead


engineer – product & development,


Portescap, discuss the requirements


Brushed DC 35 GLT motor and a planetary gearbox with a 99.8 total gearbox ratio. In a packaging of just 32mm diameter and 115mm in length, the gearbox output torque capacity reaches 8Nm with an output speed of 80rpm. With a life expectancy of 1,000 hours, the motion system can withstand temperatures up to 125˚C. These results provide reliable, durable, motion for a variety of wheeled robot applications.


imProving Performance


Rocky environments, sharp declines or areas with low friction can be a challenge for wheeled robots. While technology such as tracks with a differential drive and skid steer four-wheel drive are increasingly used, techniques to optimise existing motion solutions can also be deployed to increase durability and efficiency, and reduce noise. For example, needle roller bearings can be introduced to increase durability and efficiency by avoiding scuffing failures and providing smooth rotation of planet gears. Ensuring the gearbox has the optimum teeth combination with lower torsional forces can also reduce noise, which can be improved with an advanced acoustic simulation to predict noise emission. So, to achieve the optimum motion system for a wheeled robot design, it’s crucial that it is specified according to analysis of the application requirements and also the environment in which the


robot will operate. Engineers at Portescap can provide a range of solutions for robotic motion, and customised designs for precise needs can be developed if required.


Motor and gearbox selection criteria


Portescap www.portescap.com


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