Machine-tool Energy Efficiency: Current Issues

Industrial Energy Management: A range of energy consumers, such as pneumatics, hydraulics and lubricant processing, influence a work center’s power profile.

04/02/2012



Discussions about the efficient use of energy are becoming pervasive in many industrial sectors, including more efficient deployment of machine tools. Courtesy: Heidenhain Machine Tool DivisionMachine tools include numerous motors and auxiliary components. Energy consumption varies significantly during operations. The main spindle drive and the coolant system, for example, work near rated power while roughing at a high stock-removal rate, but power consumption during finishing is significantly lower. Close interdependence exists between individual components and subassemblies on the one hand and productivity and quality measures on the other. From the process itself to individual component power consumption, savings potential can be evaluated and measures defined for more efficient energy use.

One area of potential savings comes from the machine tool base load, which consumes energy even in nonproductive phases. The base load is determined substantially by the machine’s auxiliary components. Besides use of energy-efficient motors in these components, many opportunities for reducing the base load can be found. Some energy consumers, for example, can be switched off by the machine control during nonproductive phases.

Scrap inevitably increases energy consumption per good part. Manufacturing with accuracy from the very first part can therefore be decisive for energy efficiency. Machine designs with balanced thermal behavior and precise position measuring technology have a distinct advantage here.

Energy demand during milling

Power requirements of a milling process fall into the following consumer groups:

  • Cooling lubricant processing
  • Compressed air generation
  • Electrically powered milling-machine auxiliary components
  • CNC control package with main spindle and feed-axis motors

Proportionally calculated energy for lighting, ventilation, and air conditioning must be added to these groups. Milling process energy demand depends primarily on the size of the milling machine and the machining task.

Dry machining has great potential for improved energy and resource efficiency. In many milling applications, however, doing without cooling lubricant increases scrap rate and, therefore, raises mean energy consumption as well.

Compressed air is required for minimum spindle lubrication, tool changing, and work piece cleaning. Small quantities are required as sealing air. Mean compressed air power changes only slightly across production readiness, roughing, and finishing.

Machine electricity consumers include the CNC control with main spindle and feed-axis motors, as well as numerous auxiliary components, including the pallet changer and cooling, hydraulics and automation systems.

Drive component efficiency

Spindle and feed-axis motors are among the central components of a machine tool. Drive-component energy efficiency depends on the ratio of delivered power to consumed power. The network of drives converts consumed electrical energy to delivered mechanical power. Drive network components include a power supply module, drive modules, motors and mechanical components. Data on efficiency typically refer to the rated power. For other rated values, individual component efficiency can vary significantly. Supply modules and drive modules can attain efficiency values of more than 95%.

Comparing power consumption during rough-face and circular-pocket milling reveals that feed drives consume only a small share of the CNC’s total power usage. On the other hand, spindle selection can significantly affect energy consumption. If a spindle drive operates far below its rated power, the drive’s intrinsic losses increase in proportion, with negative effects on the energy balance. If the spindle limits the maximum possible metal removal rate, the milling process inevitably takes longer. The result: energy efficiency decreases due to the base load generated by the auxiliary components. Potential also exists for more efficient design of milling processes through consideration of spindle-motor efficiency, for example by using synchronous instead of asynchronous motors.

Regenerative supply modules

Every drive’s acceleration requires a braking process in return. Energy from the drives’ moving masses is largely reconverted to electrical energy. In a non-regenerative supply module, kinetic energy released by braking is converted to heat by the braking resistors. A regenerative supply module returns this energy to the power grid. However, the path required for returning the energy and the necessary components for smoothing the grid power generate losses even when the drives have no power requirement. Power loss increases slightly even when power is not being regenerated. Thus, a regenerative supply module operates more efficiently than a non-regenerative module when the regenerated energy more than compensates the higher power loss. Machine operation therefore determines what type supply module to employ.

Tool change frequency also impacts this decision. In one example, a milling operation at 15 kW is interrupted cyclically by a tool change. Starting the spindle requires peak power of approximately 60 kW. A regenerative supply module returns 48 kW to grid power. High metal-cutting power requirements mean that the mean-input power sinks the more frequently the milling process is interrupted by tool changes.

A regenerative supply module works more efficiently as soon as the time interval between two tool changes is less than 100 seconds (equals 0.6 tool changes per minute). In processes with many tool changes per minute, a regenerative supply module often proves to be the better choice. During contour milling with infrequent tool changes, the advantages are on the side of the non-regenerative system.

Deactivation of auxiliary components

In the ready condition, energy use of several consumer groups is only slightly reduced. Therefore, these nonproductive phases must be kept as brief as possible. With machining centers for smaller production batches, energy consumption can be significantly reduced by the selective deactivation of auxiliary components. Beyond this, potential savings result from the use of energy efficient pumps in the coolant and lubricant circuit.

However, consistent switch-off of auxiliary components -- such as hydraulics and spindle cooling -- or of the compressed-air supply can also have a deleterious effect. If sudden removal of waste heat from auxiliary components, or of temperature-stabilizing media, leads to thermal displacement in the machine frame, scrap can result. Selective auxiliary component switch-off therefore functions best on machines with little inclination to thermal displacement.

CNCs can be the central control unit for machine tool energy management, taking advantage of special PLC functions for linking events in the production process (such as NC stop) with outputs for controlling auxiliary components. Delay times can be assigned to events so that, for example, motors can be locked and disconnected from current after standstill. Functions for deactivating various auxiliary devices, axes, light in the working space, etc., can be generated on this basis. These basic functions are the responsibility of the machine tool builder. For users, it is helpful to adapt energy management to specific usage habits.

Measuring servo-controlled motors

In the control loops of spindle motors and direct-drive feed axes, even the smallest feedback signal disturbance can result in large motor current fluctuations.

Control loop with rotational direct drive (torque motors) Courtesy: Heidenhain Machine Tool Division

Signal interpolation of a position encoder includes short-range deviations within one signal period (interpolation error), typically of approximately 0.5% of the signal period. If interpolation error frequency increases, the feed drive can no longer follow the error curve. However, additional current components are generated by the interpolation error. Therefore, if torque remains constant, energy consumption and motor efficiency get worse. Additional energy required is converted to heat that must typically be dissipated by a motor cooling system, which itself consumes energy.

Motor current of a direct drive with position encoder: A) with low interpolation error (optical encoder); and B) with high interpolation error (modular magnetic encoder). Courtesy: Heidenhain Machine Tool Division

Comparing the effects of an optical and a modular magnetic encoder on a direct-drive rotary table drive illustrates the point. An angle encoder with 16384 lines generates only barely noticeable motor-current disturbances and generates little heat. Because of the magnetic scanning principle, the encoder has notably fewer signal periods. Significant disturbances occur in the motor current with the same controller settings. For example, at a shaft speed of approximately 30 min–1 the effective value of the current is 15 A greater than for an optical encoder. The result is greater motor heat generation.

An encoder with low signal quality can lead to greater motor energy loss. The additional energy requirement for active cooling also has to be included in the energy balance. To increase the energy efficiency of the motor, the encoders need high-quality signals.

Minimizing scrap with a closed loop

Bad parts reduce process productivity and therefore contribute to energy costs per manufactured part. A primary cause of bad part generation is thermal drift of feed axes running on recirculating ball screws.

Temperature variations of a direct drive with optical (A) and magnetic (B) position encoder. Courtesy: Heidenhain Machine Tool Division

The temperature distribution along ball screws can be changed very rapidly by feed rates and moving forces. On machine tools in a semi-closed loop the resulting changes in length -- typically: 100 μm/m within 20 minutes -- can cause significant workpiece flaws.

If a linear encoder is used for slide position measurement, an increase in the ball screw’s temperature has no influence on accuracy. This is referred to as “operation in a closed loop” because drive mechanical errors are measured in the position control loop and are therefore compensated.

- Information in this article was provided by Dr. Johannes Heidenhain GmbH and edited by the Control Engineering staff www.heidenhain.us 

Sidebar notes: Components of machine tool energy efficiency

Power consumed by a CNC control with feed-axis and spindle motors frequently comprises as much as 30% of the total required for a metal-cutting process. Auxiliary components play a dominant role in the energy balance.

Whether a CNC control operation benefits from energy recovery to line power depends on the frequency of tool changing during milling operations or of workpiece changes during turning. 

Further energy savings are possible depending on whether spindles can be closely adapted in their speed and torque to the machine’s range of operations. If a universal spindle design is required, its motor may have to run at low efficiency rates—with the expected consequences for energy consumption.

Position encoder selection can have a decisive impact on spindle-motor and direct-drive efficiency. Insufficient resolution and accuracy generate high current values in the position control loop. Position encoders with high line counts are essential for servo-controlled drive efficiency.

Linear encoders increase accuracy and therefore contribute to precise and reproducible machining. This makes it possible to reduce production waste and energy requirement per good part.

Processing cooling lubricants, supplying compressed air and hydraulics, and accomplishing cooling command a dominant share of total power used. Efficient pump motors save significant amounts of energy. With the relatively high base load of machine tools, minimizing nonproductive phases is a high priority. 



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