Unternehmensportrait. High Pole Servo. Stepper Motor basics vs. High Pole Servo

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Adelia Eaton
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1 High Pole Servo Stepper Motor basics vs High Pole Servo

2 Stepper Motor types Hybrid-Stepper Motor Principal Construction like a BLDC (brushless DC Motor), but higher pole count Rotor and Stator silicon steel laminations, Rotor with embedded permanent magnets Step angle typ. 1,8 or 0,9 Most 2 phase

3 Stepper Motor construction

4 Stepper motor control (open loop) The traditional method to control a stepper motor is control with a fixed current, no feedback/regulation (open loop) Advantage: no encoder necessary To move the rotor, the coils are activated after each other, either digitally (Full-/halfstep) or with a sine formed current (microstepping) As long as the torque of the motor is sufficient, the rotor will follow the electric field, and thus the external speed signal exactly

5 Stepper motor control (open loop) Fullstep and Microstep generate the same torque (and the same amount of heat) with the same effective current In Microstep the peak current of the sine if higher by the square root of two than in full step, to reach the same effective current (integral of the current curve) All values we give in our documentation are always effective current! Microstepping will reduce resonances (see next slide), and noise. The positioning accuracy is not increased reliably, especially under load!

6 Resonances With the rotor inertia, and the magnetic holding forces, every stepper motor is a oscillatory system with small self-damping (imagine a spring between field and rotor) If the control frequency is the same as the resonance frequency of the system, osciallations in the system are excited. These oscillations can lead to the rotor loosing the electrical field External load will add damping -> more unused torque means more resonances Resonances in the low speed range (40-60 rpm) result from the fourth harmonic of the detent torque, and reappear with lessened intensity at the double and quadruple frequency A second resonance range is in falling range of the torque curve ( mid-band resonance ). Since here, the load angle is already more than 90, small additional osciallations can result in the rotor loosing the synchronization Besides the natural resonances, all other external influences, like errors in the driver, can add resonances, or heighten the natural ones

7 dspdrive vs. standard open loop dspdrive means that the current control is done by a PI controller in the DSP microcontroller Basic PWM bipolar current control (standard way): winding is turned on with full operating voltage. After the desired current is reached, the winding is turned on/off by PWM. When turning off, the current in the winding has to be decayed Slow decay: good for slow speed, energy is destroyed in the control circuit Fast decay: necessary for high speed, energy is fed back to the supply =>current ripple, noise Mixed decay: at the peak of the sine, first fast decay is used, then after a fixed time, slow decay => good compromise, but not for low speeds, because of the fixed time dspdrive: the PI controller will switch between slow and fast decay based on the reaction of the winding (like an adaptable mixed decay )

8 Identical Torque at low speed More torque at high speed through > voltage, and parallel connection

9 Closed Loop Technology What is a High Pole Servo? Field Oriented Control through an encoder Advantages: smooth, quiet and resonance free Position control and correction No step loss, high load tolerance Faster acceleration

Closed Loop Technology What is Field Oriented Control? The control algorithm controls the lead angle between rotor and stator field to be 90 (electrically) So, in torque mode, every 0.

10 Closed Loop Technology What is Field Oriented Control? The control algorithm controls the lead angle between rotor and stator field to be 90 (electrically) So, in torque mode, every 0.02 ms, the stator field angle is moved according to the current rotor position, to keep it 90 advanced. Since the current is fixed, the motor will accelerate to a max. frequency In speed mode, the target current for dspdrive is not coming from a fixed sine table like in open loop It is calculated by a PI controller based on the measured deviation from the target speed

11 Closed Loop Technology Lead angle, or: why do we need a calibration run? 1. Rotor/Encoder angle determination For FOC, we need to know the angle of the rotor magnets. But we don t know the angle between the encoder lines and the magnets. We have to find it through the calibration run The calibration run will try different angles, and run the motor with it. The angle that leads to the highest speed is the right one This angle is saved with reference to the index 2. Angle determination In the area where the torque curve is falling the lead angle needs to be higher than 90, to induce a voltage to counter the Back-EMF The open loop motor does this automatically The angle change is dependent on the motor, so it is also calibrated

12 Closed Loop Technology Life time Standard-Stepper motor (open loop): high enviromental temperature is shortening the bearing lifetime High Pole Servo Stepper: less heat, meaning longer lifetime of the bearings.

13 Closed Loop Technology Overload Standard-Stepper (open loop): overload will stop the motor High Pole Servo: Overload tolerant because of current/torque/position control, the motor does not stop: if current/torque can be increased, it will compensate the overload.

14 Closed Loop Technology Resonances Standard-Stepper (open loop): Resonances stop the motor Closed Loop Stepper: no resonances, because: torque is controlled to the load => no spare torque, less resonances if the motor starts to oscillate, the field oriented control will counter the osc.

15 Closed Loop Technology Positioning accuracy Standard-Stepper (open loop): position differences because of different load/load angle are not detected High Pole Servo (Stepper): Precise, load independent positioning up to the resolution of the encoder

16 Torque and Power (ST5918L4204 at 48V) Torque Efficiency Output Power

Torque and Power compared with DB59L 4500 180 4000 3500 3000 2500 2000 1500 1000 500

17 Torque and Power compared with DB59L Speed Efficiency Output Power

High Pole Servo Technology/replacing BLDC & AC Servo Size For the same torque traditional servo motors require a gearbox High Pole Servo (Stepper): If you

18 High Pole Servo Technology/replacing BLDC & AC Servo Size For the same torque traditional servo motors require a gearbox High Pole Servo (Stepper): If you have a BLDC with a low ratio gear (1:3-1:10), and the output speed is below 1500 rpm, you have a good chance to be able to replace it with a High Pole Servo.

19 Closed Loop Technology Thanks for your attention! Nanotec Electronic GmbH & Co. KG Kapellenstr. 6 D-8522 Feldkirchen b. München Tel: +49 (0) Fax: +49 (0) info@nanotec.de

MANTECH ELECTRONICS. Stepper Motors. Basics on Stepper Motors I. STEPPER MOTOR SYSTEMS OVERVIEW 2. STEPPING MOTORS

MANTECH ELECTRONICS. Stepper Motors. Basics on Stepper Motors I. STEPPER MOTOR SYSTEMS OVERVIEW 2. STEPPING MOTORS MANTECH ELECTRONICS Stepper Motors Basics on Stepper Motors I. STEPPER MOTOR SYSTEMS OVERVIEW 2. STEPPING MOTORS TYPES OF STEPPING MOTORS 1. VARIABLE RELUCTANCE 2. PERMANENT MAGNET 3. HYBRID MOTOR WINDINGS