Stepper motors for extremely hostile conditions
31 October 2018
Stepper motors are designed to give graduated and precise motion control and are used extensively in a wide range of everyday industrial applications, some which might be quite surprising.
For example, the aircon systems in motor vehicles today with sophisticated automatically regulated temperature settings may be fitted with up to 30 or more BUS controller stepper motors. These are used to precisely control the opening of the aircon flaps situated all around the vehicle to ensure the correct cabin temperature is delivered. Now, consider for a moment the number of vehicles out there and you get an idea of the huge volumes involved.
A different approach is required when designing and manufacturing motors for hostile conditions
The challenge in this sort of application is that high volume manufactured components have to find the trade-off between delivery of the specification required and the unit cost of production which will be acceptable to both manufacturer and client.
This means that, when considering a motor design for really hostile environments, it is not a good idea to start the process by taking the basic design of a standard product and trying to “design it up” to meet the demands of the new specification. Because the operating conditions can have such extreme demands on material, lubricants, sealing and the windings of a stepper motor the only sensible way to approach the design is to start with a blank sheet of paper and carefully assess all aspects of the operational criteria from scratch.
LG Motion has expertise in supplying stepper motors from the US based manufacturer Empire Magnetics for the most difficult applications imaginable.
Motors in outer space and under water – that’s extreme!
Motors operating in a vacuum or radiation sensitive environments, stepper motors required to operate under extreme temperature conditions in outer space for example or in cryogenic applications and last of all, stepper motors which need to be waterproof to withstand the presence of water, moisture and solvents.
Waterproof stepper motors
Such motors need to perform at a minimum depth of 30 feet and are truly waterproof designed motors using shaft seals, O rings, hermetic cable feedthrough, pressure equalisation and other features. The main problem is that a standard stepper motor will be wound with a solid copper wire coated with enamels or a varnish. Even though the coating process is very good, the varnish will always have small pin holes in it and, even if only a small amount of moisture gets through the holes and between the wires of the coil it will promote conductivity from pin hole to pin hole.
A waterproof motor is designed to accommodate O-ring seals, allowing the watertight sealing that is not practical in a standard motor. Further, the wiring that leads to the motor coils in a waterproof motor requires a hermetically-sealed feed-through device to prevent water ingress into the motor via the cable conductors.
Other seals are required. For instance, a threaded pipe plug at the rear of the motor is designed to allow the housing to be sealed after the motor connections have been made; this plug is also fitted with an O-ring seal. The motor shaft itself is provided with redundant shaft seals. Additional life-extending features include double insulation, coated laminations, and fittings for pressurisation.
Outer space – the final frontier for stepper motors as well
Motors fitted to satellites rotating the panels, for example, are exposed to incredible temperature swings from +200°C in blazing sun to -200°C freezing shadow in a matter of seconds. Contraction of metallic parts and hardening of non-metallic parts are the two primary factors that can cause failure of the stepper motor operating under these conditions. In the instance of contraction, if motor components with critical dimensions contract at different rates, the motor may lock. The resultant stress can crack metal parts made brittle by super-cooling. To combat these effects, special alloys must be selected for a low temperature motor, and all metal components must have comparable coefficients of thermal expansion.
Both cable insulation and bearing grease are susceptible to hardening at cryogenic temperatures. Dry lubrication may be required and insulation polymers must be carefully selected to retain molecular integrity through low temperature cycling.
LG Motion UK based motor and control specialist
Basingstoke based LG Motion design and manufacture a wide range of motion control products complemented by close partnerships with internationally recognised specialists in diverse motion control solutions such as US based Empire Magnetics.
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