Vista in sezione dell’unità dimostrativa con posizione dei singoli sistemi di misura angolari.

The Right Angle Encoder for your Application

Heidenhain developed a demo unit equipped with four different angle encoders to help designers choose the best possible measuring method for specific applications. The demo unit is controlled by Heidenhain TNC 640 control which enables positioning tasks to be simulated with each single encoder.

Why are there so many different angle encoders on the market? Why do they use different scanning and measuring methods? And which solution should a designer choose? A Heidenhain demo unit equipped with four different angle encoders provides clear answers to these questions.

An extremely simple configuration

The demo unit has a simple design: four different angle encoders are mounted to a TMB+ torque motor from ETEL:
a) Heidenhain RCN 8311 absolute encoder, representing a typical sealed angle encoder for rotary tables and swivel heads in high-accuracy machine tools
b) Heidenhain ECA 4410 absolute encoder, demonstrating a typical modular optical angle encoder with a steel scale drum for wide-axis rotary tables and swivel heads in high-accuracy machine tools.
c) Heidenhain ECM 2410 absolute magnetic encoder, representing a particularly contamination-tolerant modular angle encoder
d) WMxA 1010 absolute inductive encoder from AMO, demonstrating a typical scale-tape version for applications requiring a highly compact, contamination-tolerant solution with versatile installation options.
Commanded by a Heidenhain TNC 640 control, the demo unit can simulate positioning tasks for each angle encoder, analyzing how signal quality affects dynamic performance and how the measuring principle affects accuracy. The unit also reveals the potential for greater process reliability through the intelligent use of full-system data from the motor, angle encoder, and sensor box.

Signal quality: a critical factor for surface quality

In direct drive motors, the signal quality of an encoder plays a critical role in the amount of electrical current noise, thus affecting the attainable dynamic performance and the possible power dissipation of the motor. As a retroactive effect of interpolation errors, noise affects the attainable dynamic performance of an axis. Interpolation errors are rapid changes in the position value causing errors in the speed calculation. These speed calculation errors, in turn, lead to higher current noise. To avoid instability in the drive system, an increase in noise must be counteracted by reducing the loop gain at the cost of dynamic performance.
Noise also affects the thermal behavior of the motor. Low noise translates into lower power dissipation and thus a lower motor temperature, whereas high noise increases the motor’s power dissipation, thus significantly raising its temperature (Figure 1).
A noise comparison of the different encoders clearly reveals the difference in behavior. Optical encoders cause low, steady noise, whereas magnetic and inductive encoders cause higher, much more heterogeneous noise, even with a low-pass filter. Optical encoders are therefore the best choice for getting the most out of a motor’s performance potential and for obtaining the best-possible surface quality (Figure 2).

Actual and desired position

Whether or not the actual position of a rotary table coincides with the desired position can be evaluated based on a statistical measurement of the positioning accuracy in accordance with ISO 230-2. This calls for five clockwise and five counterclockwise revolutions of the rotary table, with twelve measurements taken at 30° steps over each revolution.
The key metrics for evaluating an encoder are Parameter A, the bidirectional accuracy of positioning, and Parameter M, the range of the mean bidirectional positional deviation. Parameter A is comparable to the system accuracy of an angle encoder, and Parameter M to the graduation accuracy, each taking into account the error from the application (Figure3).
To evaluate the achievable contouring accuracy at a given maximum traversing speed, Heidenhain went beyond the ISO 230-2 criteria, adding the dynamic accuracy of positioning (designated by the letter D). As in ISO 230-2, measurements are once again performed over five clockwise and five counterclockwise revolutions of the rotary table. This time, however, the measurements are taken at a continuous scanning rate of 5 kHz and a traversing speed of 20 rpm.
Measuring the dynamic positioning accuracy at the achievable contouring accuracy uncovers very high deviation for the WMxA inductive angle encoder. This deviation is a by-product of the inductive scanning method, causing the accuracy to vary with speed. In contrast, the RCN and ECA optical encoders demonstrate hardly any deviation between the desired and actual position. The ECM magnetic encoder exhibits mid-level performance without extreme deviation (Figures 4, 5).

Intelligent motor protection for reliable machining processes

Torque motors such as the ETEL motor used in the demo unit boast particularly high performance in a compact design. But overheating is a risk if the distribution of current in the windings becomes asymmetric during certain machining situations, suddenly causing the temperature of a single winding to surge. Digitizing the sensor data for thermal behavior close to the application and transmitting it to the control offers intelligent motor protection for high process reliability. The optimized usability of this information, in particular, increases the reliability and efficiency of machining processes.
The EIB 5200 sensor box from Heidenhain monitors all three windings of the motor, providing the temperature data for immediate use. The box is installed in direct proximity to the motor, interposed between the angle encoder and the machine control. If the motor’s thermal model is already known and saved, as is the case with the ETEL torque motor, then the sensor box will rapidly detect any sudden rise in temperature, thereby preventing damage to the motor windings and protecting the motor from overheating (Figure 6).

Figure 6 - The EIB 5200 sensor box from HEIDENHAIN offers intelligent motor protection for the machining process. With minimal cabling, additional data from the motor system are immediately usable.
Figure 6 – The EIB 5200 sensor box from HEIDENHAIN offers intelligent motor protection for the machining process. With minimal cabling, additional data from the motor system are immediately usable.

Choosing the right encoder and intelligently using the various data available from the machining process are therefore vital to the reliability, stability, and accuracy of machining processes. Knowing the specific characteristics of different encoder models helps designers and developers choose the most suitable angle encoder for their application. After all, choosing the right encoder isn’t just about dynamic performance and accuracy; designers and developers must also take design-related factors such as the shaft diameter and mounting scenario into account, not to mention cost-effectiveness.