pole count of the motor determines the fundamental frequency and the design of the stator core and permanent magnets set the flux density levels. In particular, the eddy current iron loss is a function of both the fundamental frequency squared and flux density squared. The operating speeds of the robot’s application are a key factor in the electromagnetic design of the framelessmotor as the selection of the motor pole count and design of the stator core determine the balance of I2R and iron losses. An optimized electromagnetic design results in the best motor efficiency across the robots operating speed range. Robot sensors sensitive to heat Robots use many sensors — including encoders, resolvers, and torque sensors. A key characteristic of these sensors is their sensitivity to environmental temperature. For optical encoders, the increase in temperature causes a decrease in the LED’s light output. Thermal expansion also impacts optical encoder performance. In fact, thermal expansion can narrow the air gap between the disk and the source (detector) by 0.020 in. (0.51 mm) in some situations. Extreme instances of such thermal expansion can bring the subcomponents into contact and cause encoder damage or even catastrophic failure. For magnetic encoders, thermal expansion and contraction of the magnetic wheel can alter the pitch of the magnetic poles and thus alter the output.
Servo drives and other electronic components Servo drives are key in a variety of robots including cobots, industrial articulated robots, and medical robots. But electronic component failure rates skyrocket at high temperatures. The Arrhenius law states that for every 10° C increase in operating temperature above the rated temperature, life expectancy will be halved. As one of the major sources of electric noise, thermal noise arises from the thermal fluctuations in the electron density within a conductor and thus is always present in electronic circuits. It highly depends upon the temperature — in other words, the higher the temperature, the higher the thermal noise level. The only way to reduce the thermal noise content is to reduce the temperature of operation. Bearing lubrication in robot joints Lubrication has significant influence on the performance of both motors and strain-wave gearing. Several bearings types are used in robots — cross-roller bearings, wave generator bearings, and regular deep-groove ball bearings. These bearings are lubricated by mineral oilbase greases. Of course, bearing friction is affected by several factors — including temperature, velocity, load, lubricant properties, and environmental conditions. Temperature changes causes significant grease-viscosity and molecular-friction changes. Cold can significantly reduce the oil-release
characteristics of grease and lead to insufficient lubrication — with the potential for wear and system failure. As a comparison, high temperatures can sheer or crack the oil molecules into smaller molecules for decreased viscosity. This may induce oil leakage from the bearing. In fact, high temperatures can also trigger two other grease failure mechanisms: Oil oxidation can lead to increased oil viscosity, deposits, and the loss of the ability to form a protective lubricant film. The second failure mechanism unique to grease is a waning ability of the thickener to retain the oil phase. Under extreme conditions, the latter can induce permanent loss of lubricating oil. As a rule of thumb, the rate of chemical reactions (which includes oxidative and thermal degradation) changes by a factor of two for every 10° C change in temperature. That means increasing temperature by 10° C doubles the rate of reaction — halving expected life. Elevated temperatures drive grease failure modes quickly as they increase. Harsh environmental effects on robots Some types of industrial robots are designed to operate in harsh environmental conditions such as spray-painting robots, welding robots, and polishing and grinding robots. Such extreme conditions like wet or muddy terrain, dust, humidity, vibration and shock, corrosion, toxic conditions (such as radiation) and so on can significantly influence the robot’s performance and life.
Military and other field robots can be subject to harsh environmental conditions. | Source: Kollmorgen
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