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ROS Röbe Oltmanns Systeme

FAQ

FAQ: Frequently asked questions

Please understand that the questions are answered only in certain contexts and just in a short way. Questions and answers are for users, who may interested in this topic the fist time. Your will reveive further informations by contacting us or in the technical literature. We would be glad to consult you.
1. What is a stepping motor drive?
A stepping motor drive is a combination of a stepping motor and a stepping motor controller. Depending on the respective application reference switches, light barriers, clutches etc. may be included. The stepping motor converts electrical impulses into a rotating motion. With each step the rotor keeps turning itself around a continuous angle. Usual motors have a stepping angle of e.g. 1.8 degrees (full step). Therefore they need 200 steps for a complete turn. This is the spatial conception of the motor. Temporarily a certain angle of rotation be done successively with one or more steps. The average rotating speed results from the frequency of the steps and the size of the step angle.

The stepping motor controller produces the step sequence for the stepping motor. Depending on the motor and on the operation mode there are e.g. sequences of 4 steps (full step) up to 128 mini-steps (1/32-step). After the last step of a sequence was done, it starts once again from the beginning. The controller manages the temporal succession of the steps. E.g. it makes available: A starting frequency, an acceleration curve, a driving frequency, a brake curve and a stop frequency. Higher speeds can only be achieved by a defined pull-out curve. It can be left by a selected brake ramp only. The controller determines the spatial and temporally course of the movement. By counting the number of steps the controller is able to persue the current position.

Step 1

Step 2

Step 3

Step 4

2. What is a stepping motor drive for?
You can use this drive for the positioning and the movement of constant loads very well. The rotating motion can be converted by mechanical elements e.g. also in linear motion. Usual applications: Positioning of a print head, dosage of liquids by dosing pumps, filter changes and lens adjustment in optical devices, positioning of test tubes, sample transport in an analyzer and so on.
3. What are the benefits of a stepping motor drive?
It is an economical solution, because a current feedback of the current position is usually void. The continuous digital control simplifies the drive.

The durable stepping motor drive can be used under adverse environmental conditions: e.g. ideally suitable for high operating temperatures, vibrations, shocks and so on.

There is no restriction of the angle of rotation. In STOP-position a retaining moment is possible. If necessary a retaining torque up to the nominal torque is possible.

The only substantional wearing parts which are worth to be mentioned are the bearings of the motor.

4. What are the disadvantages of a stepping motor drive?
An overloading leads immediately to step losses, and/or to the stop. This is the reason why we care very much for the correct dimensioning of controller and motor. If necessary we check the important points of course, in order to determine step losses in time and take appropriate measures. Overloading can result e.g. from too strongly varying loads, unexpected external influences (like wind), as well as to contamination, wear and voltage drops.

Resonant frequencies, which can occur particularly in the full step operation, cause noise and step-losses may occure. The solution are purposeful measures in the mechanics (e.g. reinforcement, absorption) and/or the choice of another motor and/or the choice of another resolution (half step, mini step).

5. What kinds of stepping motors are available?
The mainly motors are claw pole, hybrid and disk magnet stepping motors. The claw pole -, (also known as tin-can motor) has a relatively simple structure and is particularly suitable for the FMCG (fast moving consumer goods) industry because of the low costs. This industry usually works with sliding bearings, the resolution is in the range between 24 and 48 steps/turn (15 degrees, and/or 7.5 degrees of step angles).

The hybrid motors have finer structures and are more complex in construction and manufacturing. In this case ball-bearings are used. Typical step angles are 1.8 degrees for the 2-phase motor and 0.72 degrees for the 5-phase motor.

The benefit of the disk magnet stepping motor is (among other things) a very small rotor moment of inertia. Therefore it is inserted in high-dynamic drives very often. Some motors have very good micro stepping characteristics. Typical step angles are 3.6 degrees and 1.8 degrees.

The windings of the 2-phase motor in many cases are alternatively available in unipolar (6 connections, for each coil beginning, center, end) or bipolar (4 connections, for each coil beginning and end). In the bipolar mode the full copper volume is always used. In the unipolar mode only half of the capacity is used. Some motors are available with 4 coils (8 connections), which can be switched in row or parallel .

6. What are the differences between the controllers?
On the one hand there are active and passive controllers. Active controllers implement all functions independently. Passive controls mostly need a step signal and a direction signal. These signals must be generated from external, additional components. Active controllers mostly have their own processor and can execute driving orders autonomously.

On the other hand there are important differences in electrical handling of the motor coils: The simple constant voltage mode needs only few elements. However, thereby the motors do not achieve their full performance. The constant current mode is more complex in the construction, but it improves the motor performance very much and micro stepping becomes possible.

7. What is a reference drive for?
The reference drive is for finding a point of reference. For the reproduction of positions or angles it is usually enough to register only one certain point of the course as a reference point. Then all other points of the course depend on it relatively. The controller is adding the steps with consideration of the polarity.

After switching the device on, a reference drive is normally made first. It ends with the finding of the point of reference. This point is defined in the simpliest case by a mechanical barrier but better by an electrical contact.