Linear Encoders For Linear Motors, Part III

Linear Encoders Recommended for Linear Motors
The sealed linear encoders LS 186 and LS 486 (signal period: 20 µm) are provided for standard applications with linear motors at measuring lengths up to approximately 3 m or 2 m, respectively (See Table 2). The LB 381 (signal period: 100 µm) is suitable for measuring lengths up to approximately 30 meters. The sealed interferential linear encoder LF 481 with a signal period of 4 µm is especially attractive for metal-cutting machine tools for which very high demands are placed on distance-coded reference marks (LS 186 C, LS 486 C, LF 481 C and LB 381 C). This type of referencing makes it possible to establish an absolute position value after moving a distance of no more than 20 mm, or on some encoders, 100 mm. All linear encoders supply sinusoidal output signals with levels of 1 Vpp and with high signal quality.

Machine tools presently use both synchronous and asynchronous linear motors, since both principles offer specific benefits. With synchronous linear motors, the absolute position value must be immediately available upon switch-on or after a power interruption. The absolute linear encoder LC 181 is therefore especially well suited for synchronous linear motors. They eliminate the need for additional commutation through Hall-effect devices. The absolute position value is transmitted through the EnDat interface. Due to its bi-directional transmission capability, the EnDat interface:

	Permits transmissions of all relevant adjusting parameters of the linear encoder for the subsequent electronics: this permits automatic commissioning
	Supports the monitoring and diagnostic function of the subsequent electronics

These benefits are attained without any separate data lines. In addition, the LC 181 supplies additional delay-free incremental signals with a period of 16 µm and signal levels of 1 Vpp for the position and velocity controller, resulting in good control quality.

The incremental linear encoder LT 181 is intended primarily for use on short-travel linear motors with high stroke rates, for example double-chamber motors, solenoid motors or electrodynamic motors (plunger-coil motors). The maximum travel is 60 mm. The integrated coupling of the linear encoder permits relatively generous mounting tolerances.

The exposed incremental linear encoders LIP 382 and LIP 481 with signal periods of 0.128 µm and 2 µm, respectively, are available for applications with very high requirements on accuracy. In contrast to laser interferometers, accuracy is not affected by the refractive index of the air, by temperature fluctuations, or by changes in the pressure or chemical composition of the air. In addition, there are no sudden changes in the measuring position due to fluctuations in the wave length, as is the case with semiconductor lasers. The LIP 382 permits measuring steps as small as 1 nm and is available in measuring lengths up to 70 mm in the accuracy range of ±0.1 µm (See Photo 1). The LIP 481 is supplied in measuring lengths up to 420 mm on graduation carriers with defined thermal expansion coefficients. This makes it possible to ensure a thermal behavior that closely resembles that of the measured object. The LIP 481 is produced in measuring lengths up to 100 mm in the accuracy grade ±0.1 µm.

The incremental linear encoder LIF 181 with a signal period of 4 µm is intended for measuring lengths up to approximately 3 m, for which position deviation within the signal period must be as small as possible. Its PRECIMET adhesive mounting film allows fast mounting, and in its accuracy grade over larger measuring lengths (±5 µm), the LIF 181 is comparable to sealed linear encoders.

Photo 1: HEIDENHAIN'S Exposed Incremental Linear Encoder For
Extremely High Resolution Applications

The new incremental linear encoder LIDA 185 has been developed for measuring lengths up to approximately 30 m and relatively small position deviations within its 40 µm signal period. Its special scanning method permits relatively large mounting tolerances and, thanks to its "quasi singlefield scanning", it is very resistant to contamination.

Both the LIF 181 and the LIDA 185 are available in versions with distance-coded reference marks (LIF 181 C and LIDA 185 C). Here the absolute position value can be determined after a traverse of no more than 20 mm or 40 mm, respectively.

The magnetic linear encoder LIM 102 supplies sinusoidal current signals with a signal period of 12.8 mm. It is intended for linear motors on handling equipment with traversing speed up to 10 m/s.

Summary
HEIDENHAIN provides linear encoders to suit various applications. These encoders are characterized by high rigidity in the measuring direction and a high natural frequency.

Sealed linear encoders with a natural frequency of 2 Khz and more, traversing speed of up to 120 m/min and high quality output signals are ideal for installation in machine tools with linear motors.

For high-precision positioning tasks with measuring steps within the nanometer range, HEIDENHAIN offers exposed linear encoders that operate on the principle of the diffraction and interference of light.

Exposed linear encoders for traversing speeds up to 10 m/s are available for use in handling equipment and linear motors.

Linear Encoders Recommended for Linear Motors
Tables 2 and 3 show the models of linear encoders recommended by HEIDENHAIN for use with linear motors, along with the most important values for this application.

Table 2: Sealed linear encoders recommended for linear motors

	Incremental					Absolute
	LS 486	LS 186	LB 381	LF 481	LT 181	LC 181
Signal Period	20 µm		100 µm	4 µm	20 µm	16 µm1
Position deviation within a signal period (worst case)	±0.4 µm		±2 µm	±0.1 µm	±0.4 µm	±0.3 µm
Position deviation at acceleration in measuring direction	0.07 µm/g2	0.1 µm/g2	0.6 µm/g	0.03 µm/g	0.04 µm/g	0.05 µm/g
Natural frequency in measuring direction	2kHz/g2	1.6kHz2	650 Hz	2kHz
Max. permissible acceleration in measuring direction	50 m/s2	100 m/s2	100 m/s2	80 m/s2	100 m/s2	50 m/s2
Max. permissible3 traversing speed	120 m/min			60 m/min	120 m/min
Vibration (55 to 2000 Hz) Shock (11 ms)	<30 m/s2 <100 m/s2	<200 m/s2 <400 m/s2	<300 m/s2 <300 m/s2	<80 m/s2 <200 m/s2	<100 m/s2 <100 m/s2	<30 m/s2 <100 m/s2
Output signals/ Data output	~1Vpp					~1Vss4 EnDat
Max. measuring length	2040 mm	3040 mm	30,040 mm	1220 mm	60 mm	3040 mm

1 Finest incremental track
2 Higher values in development
3 Also depends on the input frequency of subsequent electronics
4 Absolute value applies up to 3 m/min

Table 3: Exposed linear encoders recommended for linear motors

	LIP 382	LIP 481R	LIF 181	LIDA 185	LIM 102
Signal Period	0.128 µm	2 µm	4 µm	40 µm	12.8 mm
Position deviation within a signal period (worst case)	±0.001 µm	±0.02 µm	±0.1 µm	±0.4 µm	±0.1 mm
Max. permissible acceleration in measuring direction	200 m/s2
Max. permissible1 traversing speed	15 m/min	30 m/min	120 m/min	480 m/min	600 m/min
Vibration (55 to 2000 Hz) Shock (11 ms)	<4 m/s2 <50 m/s2	<10 m/s2 <50 m/s2	<10 m/s2 <100 m/s2	<100 m/s2 <200 m/s2	<200 m/s2 <500 m/s2
Output signals/ Data output	~1Vpp				~11µAss
Measuring length	70 mm	420 mm	3040 mm	30,040 mm	28,040 mm

1 Depends on the input frequency of the subsequent electronics

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