STEPPER MOTOR SERVO MOTOR
How to identify whether a motor is Servo or Stepper ?
Simple. A servo motor has only 3 wires and generally in a plastic container.A stepper has more wires 4,6 or 8 and constructed in a metal body.
Both are technically called Permanent Magnet Synchronous Motors (PMSMs).
Stepper motors are Open Loop ,meaning, it cannot achieve desired location under load. This result in Poor position accuracy while micro stepping. Steppers have very poor torque characteristics at higher speeds.
In Stepper systems the controller only tells the motor ,how many steps to move & how fast to move.But there are no feedback sensors to know the exact position of motor.
Stepper motors are widely used in Printers & Plotters(Printer carriage drive,paper feed,ribbon wind,print head position matrix).In Industry it is used for x-y-z table positioning of Milling machines,Laser cutting machines & Lathes.
As Servos are Closed Loop they can achieve positional accuracy. The position of the motor is tracked by a feedback signal.
Servos can produce speeds & powers 2 to 4 times that of similarly sized steppers. This is achieved because of Closed Loop & constant position feedback. Any discrepancy between a desired & an actual position is corrected from the constant positional feedback. Also ‘stalling’ cannot occur because of closed loop, unless there is an immovable object in the path.Servo motor resolution depends upon the encoder used , encoders produce 2000 to 4000 pulses per revolution
Stepper motors are cheaper .Small applications are limited to steppers. It can operate with or without feedback (encoders), but the exact position will not be known by the controller.
Applications requiring significant torque at higher speeds are generally best solved with servos. Servos will use only what current is needed to produce a required torque.
Steppers are driven in constant current mode, creating heat in motor & the drive electronics. Stepper motors are less expensive. They are brushless & experience little or no wear.
Typically brushless servo motors have 2, 4, 6 & 8 magnetic poles on the rotor, whereas the most common motor design used in factory automation has 100 poles. So Steppers have higher torque capability but lower speed range.
What is a Torque ?
Torque is a force that tends to rotate or turn things .A good example is tightening the nuts on your car wheels by a wrench. When you use a wrench, you apply a force to the handle. This force creates a torque on the lug nut which tends to turn the nut. The SI unit (International system of units) for measuring torque is Newton-meter.
Torque is moment of force. To calculate torque you just have to multiply the force by the distance from the center.
Servo assembly & functioning
A servo motor is an assembly of a DC motor, a gear reduction unit, a position sensing device (a potentiometer) & a control circuit.
The function of a servo is to receive a control signal that represents a desired output position of the servo shaft and apply power to its DC motor until its shaft turns to that position. The shaft does not rotate freely like a normal motor, but can turn only 200 degrees back & forth.
A Servo motor can be either DC or AC. AC servos can handle much higher current surges than a DC (as DC has brushes).Only entry level machines have DC servos , CNC machines use only AC servos. Brush type servo motors require change of brushes every 5000 hours. But brushless ones have no servicing requirements.
a) SERVO WIRING b) STEPPER WITH 6 LEADS
c) A UNI POLAR STEPPER d) A BIPOLAR STEPPER
Servo has 3 wire connection – Power(Red) , Ground(Black) and Control (Yellow).
The servo control signal is a PWM (pulse width modulation). A Servo has its own internal drive electronics for running its built in motors.So as a control input ,a timing waveform is given.
Stepper assembly & functioning
Stepper motor wiring is complex.Inside a stepper motor there are 4 coils of wire located 90 deg, away from each other & in the middle is the Rotor.The rotor has permanent magnets fitted around its circumference.Most steppers have 6 wires (there are also 4,5 or 8 wire motors).Each of the 4 coil is made up of one length of wire with 2 ends, a live & a common.In a 6 wire stepper ,as seen in fig.c above,there are 2 pairs of common wires joined together (wires 2 and 5).In a 8 wire stepper all 4 coils are separate and none of the 4 common wires are joined together.
General color coding for a 6 wire stepper is as follows .: (see fig.c)
–1st winding end colors are Black-1/Brown-3 and Green-2 is center tap for this.
–2nd winding end colors are Yellow-4/Orange-6 and Red-5 is the center tap.
But this coding of color may vary from one make to other.Simply by measuring the impedence(resistance value) of coils by a multimeter reveals the coils .All coils have similar resistance.By making note of this we can easily sort out the coil connections.
Further, steppers are of 2 types, Unipolar and Bipolar.
As seen in above fig.c a unipolar stepper has 2 coils ,each has a center tap.
A bipolar stepper has 2 coils per phase (see fig.d above) .No center tap is used & it uses entire winding.But a unipolar stepper uses only half the winding at one time.A unipolar stepper can be used as a bipolar one by simply not using the center tap.
Drivers to control Servo
A great advantage of servo motors is that they are very easy to control compared to other types of motors . The servo motor has all this control circuitry internally.
The power & ground wires are hooked directly up to power supply . The Signal wire will be hooked up to the microcontroller used to control the servo.The signal that we need to create in order to control the servos is called a Pulse With Modulation signal or PWM for short. The best part of the servo drive design is that it only requires 1 pin from the microcontroller like PIC you are using to generate PWM signal.
Drivers to control Stepper
Unlike servos , steppers do not have inbuilt control circuits.So the control driver circuit has to be assembled separately & it is nearly 4 times costlier than the stepper motor itself.
ICs L297 &L298 pair together perfectly for controlling a stepper motor
The L297 has 4 inputs supplied by the user, a ground, step input (clock), direction and enable. From these inputs the L297 decides what outputs should be given to the L298 in order to control the stepper motor properly.
The L298 will constantly be supplying current to the stepper motor unless you add additional sensing circuitry, so make sure you don’t forget the 10Ω resistors off of pins 1 & 15, the sense pins. This limits the current to 300mA and means out L298 won’t get heated up.
L298 IC DRIVE BOARD USING L297 &L298
For further details contact :
Saravana Electronics, 91422 2398827