Customised motors aid product differentiation

John McKay, European Sales Director for PennEngineering Motion Technologies, looks at some of the customisation options on offer with Pittman and MAE branded DC brushed, brushless and stepper motors.

In today's increasingly competitive markets, OEMs are reducing production costs and increasing profitability by sourcing customised subassemblies in preference to purchasing individual components.

This value-added approach allows many benefits including design optimisation, reduced production timescales, simplified purchasing and potential improvements in quality control.

This is particularly true of mechatronic based products where motors, gearboxes, encoders and power transmission components can be purchased as combined subassemblies rather than individually purchased from several suppliers.

Incremental optical encoders for precision motion control applications supply accurate feedback of position, velocity, acceleration, and direction.

Encoders can be added to any motor or gearmotor with leadwires or with side-exiting power terminals or with connector assemblies.

They can be integrated into the motor housing, be self contained within their own housing of can be simply mounted on a frame in open air.

Encoders are often factory-mounted but may be prepared for mounting by the customer in-situ, in the final stages of his end-product assembly.

Some manufacturers (such as Pittman) are even equipped to provide end users with complete encoder kits which can reduce the cost of adding feedback for servo applications.

Encoders are usually specified with either two or three-channels of TTL-compatible quadrature outputs.

The line count or number of pulses per revolution will depend on the application requirements and can be from a few hundred counts per revolution up to several thousand counts per revolution.

In a three-channel unit, the third channel normally provides an index signal or a pulse once per revolution and may be used as an accurate datum reference in motor-leadscrew assemblies in conjunction with a second sensor on the linear travel part of the assembly.

Another encoder option, the rotary pulse indicator (RPI), is a single-channel unit with open-collector or TTL-compatible outputs.

RPIs are low-cost solutions for simple appliance applications that need a few counts per revolution or less and can also be used as stall detection devices in open loop stepper motor or nonfeedback DC motor applications.

The shaft of any motor may be modified with a flat, journal, cross-hole, keyway, slot or groove, or any combination of these.

Additionally the shaft may be fitted with customised cam, pinions, gears, or pulleys for belt driven systems.

Special shaft materials, diameters, and lengths can be customised to suit performance and operational requirements.

In some applications the motor shaft may also act as a leadscrew with a driven nut arrangement in a linear actuator assembly.

In these cases the nut may move linearly on rotation of the motor or the nut can be fixed allowing the motor to move.

Gearheads are used to increase the output torque available to drive the load (with a consequent reduction in speed) and can also be specified to increase the available angular positioning resolution in a position control application.

For many applications a spur gearbox will be flexible enough to satisfy accuracy, torque, noise, and cost requirements.

Standard spur gearboxes feature sintered nickel-steel gears, which provide moderate power handling with average audible noise.

The sintering process allows for close tolerances at a relatively low cost, and the sintered gear functions as a lubrication holder and helps dampen sound.

When more strength is required, a hybrid cluster (an assembly of a cut-steel pinion and a sintered gear) or precision-cut steel gears can be chosen.

Delrin (mouldable polymer) gears are also available for applications where lower noise is required.

Where a right-angled drive is desirable, bevel gearboxes or worm-wheel gearboxes can be supplied.

Other options include planetary gearboxes for lower backlash, improved accuracy and much higher torque but with addition cost to the system.

Custom wire and cable assembly options are often specified to accelerate the process of installation on the customer's machine and can include flying leads, screw terminal connectors, special connectors with mating pieces if required.

As they are factory fitted and tested, custom wire and cable assemblies help to boost component reliability.

Almost any connector style and wire type can be specified for motors, gearmotors and encoders.

Component solutions have evolved to reduce the amount of electrical noise generated by a motor and it is normally much simpler and cost effective for the motor manufacturer to take care of noise suppression.

For low-frequency RFI (typically below 30MHz) capacitors are generally effective, and there is an inverse relationship between the value of the capacitor and the attenuated noise frequency.

Capacitors installed by the motor manufacturer enable strategic placement inside the motor frame for optimum filtering as close to the noise source as possible.

Pittman has designed a unique cartridge brush assembly, known as the "SuperBrush" which allows RFI components to be mounted in very close proximity to the brushes; such designs provide optimal noise suppression.

For high-frequency noise (generally above 30MHz) ferrite beads can help reduce RFI.

A combination of ferrite beads and capacitors provides the most effective suppression by creating a lowpass LC filter that is inductive-capacitive at low frequencies and dissipative at high frequencies.

Mounting for each component may vary from slipping ferrite beads over wires to soldering chokes near the motor terminals.

Developed as a safety and energy-saving feature, rear-mounted power-off or power-on electromagnetic brakes prevent a motor or gearmotor from rotating freely when required.

A power-off brake stops a motor when power is removed and releases the motor when power is reapplied.

In low-duty applications, the brake saves energy by maintaining a known motor position without power.

An added safety feature is, that should power be lost while the motor is lifting an object by pulley or lead screw, the brake will lock the motor and prevent the object from falling.

Power-off brakes are especially useful in applications where the load is moved vertically against gravity and would be in danger of backdriving the mechanical system if not braked in position on power off.

A power-on brake holds the motor in place when electrically powered and releases the motor when power is removed.

Power-on brakes are therefore not considered as a true safety device and are consequently not used as commonly as their power-off counterpart.

Applications involving high side or axial loads can benefit from special bearings to prolong the life of the assembly.

In some applications where axial movement of the motor or gearmotor shaft needs to be minimised, special bearing arrangements can be supplied.

Motors and gearmotors can be fitted with special bearings ranging from larger diameter sintered materials to ABEC grade roller open or sealed bearings.

Mechanical interfacing of motors and gearheads to the power transmission system or interfacing complete actuator assemblies can be accommodated with specially designed mounting plates in cast form, premachined metal or in moulded plastic.

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