Techno Linear Motion Catalog25Technical Information6) Motors and EncodersElectric motors are by far the most common component to supply mechanical input to a linear motionsystem. Stepper motors and servo motors are the popular choices in linear motion machinery due to theiraccuracy and controllability. They exhibit favorable torque-speed characteristics and are relativelyinexpensive. The term NEMA refers to the physical size of the motor, and has become an industry standard.All motors of the same NEMA frame size should exhibit the same dimensions. This section will discuss thefeatures associated with each of these types of motors.Stepper motorsconvert digital pulse and direction signals into rotary motion and are easily controlled.Although stepper motors can be used in combination with analog or digital feedback signals, they areusually used without feedback (open loop). Stepper motors require motor driving voltage and controlelectronics.The rotor of a typical hybrid stepper motor has two soft iron cups that surround a permanent magnet whichis axially magnetized. The rotor cups have 50 teeth on their surfaces and guide the flux through the rotor-stator air gap. In most cases, the teeth of one set are offset from the teeth of the other by one-half toothpitch for a two phase stepper motor.The stator generally has the same number of teeth as the rotor, but can have two fewer depending upon themotors design. When the teeth on the stator pole are energized with North polarity, the correspondingteeth on the rotor with South polarity align with them. Similarly, teeth on the stator pole energized withSouth polarity attract corresponding teeth on the rotor that are energized with North polarity. By changingthe polarity of neighboring stator teeth one after the other in a rotating sequence, the rotor begins to turncorrespondingly as its teeth try to align themselves with the stator teeth. The strength of the magnetic fieldscan be precisely controlled by the amount of current through the windings, thus the position of the rotor canbe precisely controlled by these attractive and repulsive forces.There are many advantages to using stepper motors. Since maximum dynamic torque occurs at low pulserates (low speeds), stepper motors can easily accelerate a load. Stepper motors have large holding torqueand stiffness, so there is usually no need for clutches and brakes (unless a large external load is acting,such as gravity). Stepper motors are inherently digital. The number of pulses determines position while thepulse frequency determines velocity. Additional advantages are that they are inexpensive, easily andaccurately controlled, and there are no brushes to maintain. Also, they offer excellent heat dissipation, andthey are very stiff motors with high holding torques for their size. The digital nature of stepper motors alsoeliminates tuning parameters.There are disadvantages associated with stepper motors. One of the largest disadvantages is that thetorque decreases as velocity is increased. Because most stepper motors operate open loop with no positionsensing devices, the motor can stall or lose position if the load torque exceeds the motors available torque.Open loop stepper motor systems should not be used for high-performance or high-load applications, unlessthey are significantly derated. Another drawback is that damping may be required when load inertia is veryStatorWindingsRotorPermanentMagnet