FEATURE Drives, Controls & Motors
Specifying motion solutions for a wheeled robot
Pradeep Deshmane, Principal Engineer – Gearbox, R&D, Portescap, and Manoj Pujari, Lead Engineer – Product & Development, Portescap, discuss the motion requirements of wheeled robots
A
major benefi t of robots is their ability to complete tasks that may be diffi cult or hazardous for humans to perform.
Many such applications rely on wheel- driven robots. From a manufacturers’ perspective they’re easier to design, build and program. For the user, providing the terrain is not heavily obstacle-strewn, wheeled robots can be faster, more effi cient and more resilient. Uses for wheeled robots are wide, from pipeline inspection to healthcare environments and even domestic use.
Integral to a wheeled robot is its motor and gearbox system. At a fundamental level, the system has to be compact and lightweight and be able to generate the required torque for movement across the specifi ed environment. Frequently used in tough environments, they’ve also got to be highly durable. For many applications, whether military surveillance or hospitals, noise emissions must also be minimal. Critically, wheeled robots also require high energy effi ciency with a low current consumption. For these reasons, coreless brushed or brushless DC motors coupled with compact planetary gearboxes are frequently specifi ed. As the motion system is required to fi t within a footprint smaller than 40mm diameter, mini motors and gearboxes are required. Despite the compact size, the gearbox output torque must typically handle 4-8Nm at speeds between 50 to 150rpm.
Specifying the motion solution To determine the specifi cation, start by calculating the output torque required. This must factor 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, fi nally, friction and effi ciency of travel should also be taken into account. Following calculation of the torque
requirement, the wheels’ rotational speed can be determined; appropriate selection
42 March 2022 | Automation
speed will be reduced and steering won’t be aligned. Unlike diff erential steering that uses separately-driven wheels, this four-wheel drive solution has a diff erential steering mechanism that requires only a single motor, which reduces footprint, energy use and cost.
The solution comprises Portescap
Motor and gearbox selection criteria
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 suffi cient packaging space must
be provided for the encoder, brake, motor and gearing, an important factor if the motor stack is enclosed is operating temperature and avoidance of overheating. Motor capacity and heat dissipation techniques may need to be considered in such circumstances to ensure robot reliability.
Wheeled robot solution Portescap recently provided the motion solution for a four-wheel drive robot. Two pairs power the wheels, each turning in the same direction. It’s vital that the motion system controls both pairs to run at the same speed, otherwise overall robot
Needle roller
bearings for smooth rotation of planet gears
brushed DC 35 GLT motor and a planetary gearbox with a 99.8 total gearbox ratio. In a packaging of 32mm diameter and 115mm 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 to 125°C. These results provide reliable, durable motion to a variety of wheeled robot applications.
Specialist application engineering Terrain can be a challenge for wheeled robots, particularly among rocky environments, sharp declines or areas with low friction. While technology such as tracks with a diff erential drive and skid steer four-wheel drive are increasingly used, techniques to optimise existing motion solutions can also be deployed to increase durability, effi ciency and reduce noise.
For example, needle roller bearings can
be introduced to increase durability and effi ciency by avoiding scuffi ng 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. To achieve the optimum motion system for a wheeled robot design, it’s crucial that it is specifi ed according to analysis of the application requirements and the environment in which the robot will operate.
CONTACT:
Portescap
www.portescap.com
automationmagazine.co.uk
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