Engineers and scientists expect in the coming years that robots will be an integral part of our lives with a presence in everything including farming, hospitals, maintenance, construction and even in our homes. They will potentially replace humans in many industries, especially those where precision is necessary. Robots will complete tasks that are difficult to perform correctly by human labor or will replace humans in conditions that are deemed to be hazardous. In many of these industries, applications will be strongly focused on wheel driven robots.
Wheeled robots navigate on the ground using motorized wheels to propel themselves. This design is simpler than using treads or legs, and by using wheels they are easier to design, build and program for movement in flat, not-so-rugged terrain. Wheeled robots are popular among the consumer market because their differential steering provides low cost and simplicity. Robots can have any number of wheels, but three wheels are sufficient for static and dynamic balance. Additional wheels can add balance, however, additional mechanisms will be required to keep all the wheels on the ground, especially when the terrain is not flat. The motion solution is comprised of motors coupled with gearboxes driving the wheels which increases the torque capacity for better drivability.
This article describes the market need and demand, application requirement, selection criteria, technological advantages and future advancements in robotics.
MARKET NEED AND DEMAND
There is a tremendous demand for robots to be used in hospitals for infection control, medical services, medical waste delivery, biochemical specimen delivery and general medical tasks. The demand is increased multifold with the current COVID-19 pandemic worldwide. Another growing market is in the aerospace and defense (A&D) industry, where robots play a crucial role in surveillance and military operations. An upcoming market is pipeline inspection (e.g. in underwater systems) where the pipelines are inspected by driving robots where images are taken to help identify cracks or faults in the infrastructure. The key requirements faced with the products used in these applications are:
|•||Compact and lightweight|
|•||High durability (Long life)|
|•||Low noise (Hospital and A&D)|
|•||High efficiency and low current consumption|
(Figure 1)Typical products used in robots are brushed or brushless DC motors, coupled with compact planetary gearboxes. The requirement may differ slightly from application to application, but typical specifications of motion solutions are:
|•||Motor: Coreless Brushed DC / Brushless DC Motor|
|•||Gearbox Configuration: Planetary – 2/3 Stage, 30:1 to 120:1 Ratio|
|•||Packaging Size: < 40mm diameter|
|•||Gearbox Output Torque: 4 to 8 Nm|
|•||Gearbox Output Speed – 50 to 150 rpm|
Selecting a motor and gearing is a critical task when designing a robot for applications. The key aspects to focus on are illustrated in Figure 2.
The first step in selecting a motor and gearbox is to determine the operating and maximum conditions that the product will see. The most critical factor in the design and selection of a motor + gearbox is to confirm the required speed and torque experienced at the wheel output.
It is easier to first determine the output torque required and then work backwards to find your motor and gearing. The torque on the wheel should be determined based on robot acceleration, wheel diameter, carrying capacity (should be enough to pull the entire robot if some actuators fail or wheels are slipping), climbing the minimum slope, or overcoming obstacles. The friction and efficiency also should be considered for arriving at the final torque.
After determining how much torque you need, the next step is to determine the speed that the wheel needs to rotate. First, determine the desired speed of the wheel (i.e. final output) and then you are ready to choose the motors and gearing. The robot manufacturer generally finalizes the speed at which the robot is supposed to drive and the wheel diameter determines the required speed at the wheel output.
After you know the basic performance characteristics of the motor you need, the next step is to make sure the motor stack (encoder + brake + motor + gearing) fits in your robot and can be packaged cleanly. The encoder allows you to measure how much the motor shaft turns, and the brake system helps to hold the torque and provides dynamic stopping in the event of an emergency. There are different types of encoders and brakes used in robotics.
The operating voltage is used for powering the motor. Typically, the higher the voltage, the higher the speed capability of the motor. You can look at the voltage constant (back EMF constant) from the motor datasheet to determine how fast the motor will rotate per volt.
This is often not an issue, but if your motor stack is enclosed you want to make sure it does not overheat. The temperature range for the gearbox is of concern which can affect lubrication life and deteriorates the performance over time.
You need to know the mass of the load to determine the torque for motor selection. A mass estimate (or even better, an actual mass) is critical for choosing a motor. If you are designing based on a mass estimate, you should apply a safety margin of approximately 25%. You can look at the torque constant from the motor datasheet to figure out how much torque output you will get per amp.
It can sometimes be tempting to build gear boxes from scratch since they may be less expensive. However, if you take the time to design, assemble, and test the new gear, it is often more economical to get the gearbox from a standard catalog.
Precision / Accuracy / Efficiency
How much lag can you afford in your gearing? Often in wheel motors you can handle a little less precision and accuracy. These gear motors are used in various terrains and torque profiles and since all applications do not demand high performance (like lower noise / vibration), low precision can be tolerated. However, in a robotic arm or instrument you often need low backlash systems that are more precise and accurate.
Reliability and Noise
In most applications, higher reliability is the driving factor, and the motor stack must survive the required working points. In some critical applications like surveillance robots, low noise is a major driving factor, in addition to the higher reliability, so the motor and gearbox needs to meet both requirements.
Let’s look at an example. For a wheeled robotic application, a product specification for the motion solution is designed by Portescap with the following details:
|•||Motor Details: - Brushed DC 35 GLT|
|•||Main Gearbox: - Planetary Gearbox, 3 Stage, Spur, 99.8 Total Gearbox Ratio|
Many wheeled robots use differential steering which uses separately driven wheels for movement. A better balanced design is with a 4-wheel drive robot which comes with 2 pairs of powered wheels. Each pair can turn in the same direction. If the pairs do not run at the same speed, the robot will move slowly and cannot drive straight. An optimum design has a differential steering mechanism similar to those used in a car which allows the robot to turn left or right’ this only needs one motor. Another common configuration of robots uses motors that drive wheels independently instead of differential steering, in this case, separate motors are required to drive each wheel.
The overall specifications of the solution are defined as:
|•||Packaging: - 32 mm dia x 115 mm length|
|•||Gearbox Output Torque Capacity: 8 Nm|
|•||Gearbox Output speed: 80 rpm|
|•||Life Expectancy: - 1000 Hrs|
|•||Maximum Temperature: - 125 deg C|
A Portescap product has the advantage in terms of smaller packaging, higher torque-carrying capacity, and higher durability which can be adapted for multiple wheeled robotic applications.(Figure 3)
FUTURE ADVANCEMENTS IN WHEELED ROBOTICS
The major drawback of wheeled robots is that they cannot navigate well over rocky terrain, sharp declines or areas with low friction. Demand for development of a single robot which can negotiate these limitations is increasing day-by-day. This requires changes in the mechanism of robots such as tracks (differential drive), skid steer 4-wheel, differential drive 2 wheels + passive caster(s). This adds more complexity and requires a detailed study with an impact on cost.
Without a major change in the overall architecture, focused needs are given to optimizing the motion solution. This article highlights the improvements in motion solutions that can benefit the wheeled robot’s performance in terms of durability, efficiency and low noise:
|•||Newer Bearing Solutions - Needle roller bearing (Figure 4)–avoids scuffing failures and provides smooth rotation of planet gears on planet pins|
|•||Optimum Gearbox - Gear teeth combination with lower torsional forces to have low noise (Figure 5)|
|•||Advanced FEA based analysis – This helps to identify potential failures and eliminate them in early stage of design (Figure 6)|
|•||Advanced Acoustic Simulation - to predict noise and allow the optimization of the design for critical applications where noise is the primary focus (Figure 7).|