Accuracy Is Required When Designing The Drive

2023 Author: Hannah Pearcy | [email protected]. Last modified: 2023-08-25 03:36

Designing a drive means adapting the torque-speed characteristic of the motor to the characteristic of the driven machine. When designing a new machine, the first calculations are usually made at a very early stage. It is often necessary to calculate different variants and to estimate whether the performance is still sufficient if a smaller drive is used or to what extent this can be increased with the next larger drive

If the drive is undersized, the machine will not achieve the desired performance and the drive must be replaced by a larger one. This entails effort and possibly even design changes. If the motor was designed too large with the controller, the acquisition and maintenance costs increase in later operation because the drive works with poor efficiency and consumes too much energy.

User meeting mechatronic drive technology

The focus of the user meeting mechatronic drive technology is on the mechanical components of gears, clutches and brakes as well as their design, dimensioning and interaction in the overall mechatronic system.

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Drive design step by step: From mechanics to the network

The drive design takes two aspects into account: dynamic adaptation in order to comply with the specified travel-time conditions and thermal adaptation so that the motor does not overheat.

The adjustments can be made in different ways. On the one hand, the natural characteristic course of electric motors can be used. The classic example is or was the series connection motor for rail drives: high torque at low speed for starting and low torque at high speed after the acceleration phase for constant travel. A more elegant method is to manipulate the motor characteristic using actuators. This releases the link to the rigid characteristics of the various engine types. In many cases, the simple and robust asynchronous machine can be used in this way, or, if more efficiency and dynamism is required, the synchronous machine with permanent magnets.

When designing the drive, it is important to analyze the mechanical conditions and to calculate the required torques and speeds, taking into account the travel-time conditions. This is the basis for the selection of the motor, a coordinated controller and the mains connection components.

Determine the speed and torque-time curve

As a first step, the required speed-time curve and the mean speed (arithmetic mean) are calculated based on the desired motion profile (Figure 1). If the exact sequence of movements is not known, it helps to carry out the calculation based on the relative duty cycle. All movement cycles are referenced to 100% and the individual shares for acceleration, braking etc. are estimated as a percentage.

Next, all acting forces such as friction, process or weight forces are converted into torques and the masses into moments of inertia. Furthermore, "soft" factors such as additional friction due to cable drag must also be taken into account.

If the drivetrain contains transmission elements such as gears or belt counters, the previously mentioned values are converted to the engine side. Using these processed parameters, it is then possible to calculate the required torques on the engine. Whether the motor has to deliver or take up torque, i.e. acts as a motor or generator, depends on the direction of action of the torques. To check the thermal load on the drive, the effective value of the torque curve (root mean square) must also be determined (Figure 2).

Define the motor, inverter or converter

The individual torques with the associated speeds are thus known and are shown in a diagram. The effective torque at medium speed is also entered here (Figure 3).

On this basis, a motor is selected that can provide the individual torques. Now that the moment of inertia of the motor is known, the calculation of the torques must be carried out again. The individual torques rise a little and the suitability of the motor has to be checked again. Possibly. it may be necessary to take the next largest motor.

Once the motor and thus its power requirements have been determined, a suitable inverter or converter is selected. It is sufficient if this covers the actual power requirement from the torque requirements. To make the selection easier, Rexroth provides torque-speed diagrams for complete motor-controller combinations.

Select supply, additional components, line choke and filter

If several inverters are operated on one DC voltage supply in a modular system, the required power requirement per drive is determined and added up. Continuous feed-in and regenerative power, peak power and regenerative energy are included in the calculation. The same sizes are necessary to include required additional components such. B. to determine braking resistors or additional capacities.

The last step in the drive design is to select the line choke and filter that may be required. This depends on the mains current, the number of all drives operated on an infeed, the motor cable lengths and the parasitic capacitances of the cables and motors.

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Software tools help

The drive design requires a certain basic understanding and formula knowledge. Software tools such as the IndraSize design program from Rexroth offer support for the procedure and the individual calculations. This simplifies the optimal dimensioning of the drive controller, motor and mechanical transmission. Users can define all common drive mechanisms such as ball screw drives, toothed racks, toothed belts, linear direct drives, rotary tables, rollers or winders via the convenient user interface within a few minutes. In addition, IndraSize offers the possibility of describing typical applications such as concurrent machining in a very simple form by entering parameters. Movement profiles can also be freely configured. Specially optimized motor-controller combinations for servo drives (rotary or linear), S1 drives (pressure applications) and main drives are also stored in a database. This means that users always get the most economical drive and avoid expensive oversizing.

* Dipl.-Ing. Reinhard Mansius is Product Manager Drive Systems at Bosch Rexroth AG, Lohr.