Linear Encoders For Linear Motors, Part I

Linear-motor-based drives have enjoyed continuous development in recent years, and have now reached the point at which it is possible to build powerful control systems that can provide a significant improvement on results and machining, as well as on productivity, in comparison to electromechanical drive systems. This is especially true in machine applications specially designed for linear motors.

Possible applications for linear motors in the metal-cutting industry are characterized basically by the following three requirements:

	High contouring accuracy at high traversing speeds (typical for milling of gray cast iron and aluminum; 10m/min to 15 m/min) i.e., wide frequency range of the position control loop and therefore, high feed forward control (typically 15m/min*mm to 30m/min*mm and greater).
	High acceleration capability in the machine axes (typically; 10 m/s2 to 40 m/s2, occasionally even much higher).
	High permissible rapid traverse velocities (typically: 60 m/min to 90 m/min, occasionally 120 m/min).

These capabilities are usually found in those machines widely known as High Speed Cutting (HSC) machines. For linear motors on other machines such as handling machines, textile machines, printed-circuit-board drilling machines, measuring and test equipment, etc., other requirements are of primary significance.

Now that several manufacturers of controls and drives are offering a variety of different linear motors, machine builders are beginning to show an increasingly strong interest in linear motors. For years, HEIDENHAIN has been developing exposed and sealed linear encoders for applications on linear motors in parallel to the developmental work of motor and machine manufacturers, so that in many areas, experience has been gained on prototype machines as well as on machines in series production. The most important points regarding linear encoders are summarized in the following paragraphs.

Exposed or Sealed Linear Encoders?
The engineer designing a machine with linear motors must decide very early whether to incorporate an exposed or sealed linear encoder. Table 1 shows the advantages of both types. An exposed system is recommended if the environment at the machine is clean enough to ensure that there is no threat of contaminating the optical system. If however, the machine makes use of coolants and lubricants, or if the machine is completely encapsulated, then sealed linear encoders should be used. Attainable traversing speeds generally present no problems for sealed linear encoders in typical applications. The primary advantage of sealed linear encoders is that they reduce requirements on the mounting surfaces for the encoder and on the guideway accuracy of the machines, which results in faster mounting procedures and lower costs.

Exposed Linear Encoders	Sealed Linear Encoders
Higher accuracies	Simple mounting
Higher traversing speeds	Higher protection rating
No friction

Table 1 - Relative advantages of exposed and sealed linear encoders

Exposed linear encoders are therefore found most often in precision machines, measuring systems, and in production equipment in the semiconductor industry. Sealed linear encoders are used widely on metal-cutting machine tools (Photo 1).

Photo 1: HEIDENHAIN offers a variety of linear scales

Transmission of Position Information to the Subsequent Electronics
In addition to the actual position value, linear encoders must also supply values of the speed control loop and for commutation. In order to attain good dynamic performance in the drive for digital speed control, the sampling time should be kept down to 500 µs or less. To minimize dead time in the closed loop, the actual values for position and speed must therefore be available in the control system with the least possible delay within a few ms. These stringent time requirements on the transmission of position information from the encoder to the subsequent electronics can be fulfilled by incremental encoder signals with 90° phase shift (Figure 1) either in sinusoidal or square-wave form. In order to attain good speed constancy with linear motors as feed drives on machine tools, one needs a resolution of 0.1 µm and finer.

Figure 1: Output Signals For Incremental Rotary Encoders

If one wishes to traverse, for example, at a minimum velocity of 0.01 m/min with a sampling time of 250 µs, and if one assumes that the measuring step should change by at least one step per sampling cycle, then one needs a resolution of approximately 0.04 µm.

Feed drives for machine tools are expected to attain 90 m/min, sometimes even 120 m/min. Handling equipment achieves velocities between 5 m/s and 10 m/s. Input frequencies of less than 1 MHz are desirable in order to be able to transmit output signals over large cable distances (>50 m) to the subsequent electronics. Interpolation and digitizing circuits, which up to now have been either integrated in the encoder or connected as a separate unit between the encoder and subsequent electronics, are therefore unsuited for this purpose. A velocity of 60 m/min and a measuring step of 0.1 µm (after 4-fold evaluation in the subsequent electronics) results with these circuits in an input frequency of 2.5 MHz.

For high traversing speed and small measuring steps, linear encoders with sinusoidal output signals particularly with levels of 1 Vpp at a -3dB cutoff frequency of approximately 200 kHz and more, and a permissible cable length of up to 150m are best suited for linear motors. Figure 2 illustrates the input frequency to the subsequent electronics as a function of the signal period and of the traversing speed for linear encoders with sinusoidal output signals. Even with signal periods of 4 µm and a traversing velocity of 120 m/min, the frequency reaches only 500 kHz.

Figure 2

Encoders are highly subdivided in the subsequent electronics. The subdivision factor determines the number of steps into which one signal period is divided. This subdivision factor must, however, remain in a reasonable proportion to the accuracy of the linear encoder. A subdivision factor of 1024 and a signal period 20 µm or 4 µm results in resolutions of approximately 20 nm and 40 nm, respectively.