8.1 Angle Measurement

The sat-nms ACU provides exchangeable interfaces for several types of position sensors. Position sensor interfaces may be selected individually for each axis. This gives a maximum of flexibility for application where the sat-nms ACU replaces an existing antenna controller.

Position sensor types

Actually there are three types of position sensor interfaces available for the ACU. The interfaces principally are field-replaceable, however changing interface boards inside the ACU should be done by skilled personnel only. ESD protection must be followed when handling the ACU boards. The ACU is capable to interface to the following types of positional sensors:

• SSI --- SSI type digital position encoders are the first choice for antennas which are setup from scratch. They are precise, reliable and provide a standardized interface. The ACU supports types from 13 to 24 bit resolution, both, gray coded and binary variants. The ACU provides 5V and 24V supply voltages (200mA max) for the encoders, so external power supplies may be omitted in most cases.
• RESOLVER --- Many existing antennas are equipped with resolvers. The resolver interface module permits to re-use these ready mounted and cabled sensors when an old antenna controller is to be replaced by a sat-nms ACU. The resolver interface board is optimized for the use with 2V type resolvers which are very common. The interface board drives the resolver with 4Veff / 2000Hz and expects 2Veff at the sin/cos inputs. The resolver interface has 16 bit binary resolution.
• ANALOG --- For small antennas with reduced accuracy requirements using analog angle to voltage sensors (in fact precision potentiometers) is an inexpensive alternative. The analog sensor interface board may be set up for several voltages to connect to the commonly used sensor types. The analog interface board uses a 16 bit precision ADC with a temperature compensated voltage reference.

Angle calibration

Analogous to the modular position interface hardware concept, the ACU software uses configurable drivers to read the different types of position interfaces. From the user's point of view, the ACU accepts and displays pointing angles as floating point numbers with 0.001° resolution. Internally the software treats angles as 32 bit integer numbers where the full 32 bit range corresponds to 360°. This is equivalent to a resolution of 0.000000084°.

When the software calculates the pointing angles from the sensor readings, it includes some calibration parameters configurable at the Setup page. The steps of calculating a pointing angle are as follows:

1. Get the raw value.
2. Extend (left shift) the value to 32 bits.
3. Reverse the sign if this option is set in the setup.
4. Apply (add) the pre-scale offset.
5. Convert to degrees.
6. If the calibration scale is nonzero, multiply by the calibration scale.
7. Add the post scale calibration offset.
8. For the azimuth axis add the antenna course, too.

The angle calculated this way may exceed a full circle of 360° if the scaling ensures that there is no overflow of the encoder reading itself. The value displayed as raw reading at the test page, is the result of step 2.