The sat-nms ACU performs the pointing / motor control as a closed control loop, independently for each axis. If the measured position value differs from the target value, the motor is activated to compensate this difference. It makes no difference if a new target value has been commanded or if the antenna has moved a little bit due to a squall.

Hysteresis

To avoid that the motor is switched forth and back all the time, the ACU tolerates small differences between measured and target value within a hysteresis value. This hysteresis is individually configurable for each axis at the Setup page. To ensure that the motor stands still when the target position is reached, twice the resolution (step size) of the position sensor / encoder must be set. Common values are:

If the motor control loop still oscillates with the recommended hysteresis values, this is due to the off-carriage of the antenna drive. Either turn down the motor speed at the motor driver unit or enlarge the hysteresis value in this case.

Motor drive signals

The sat-nms ACU-ODM, the core module, provides a number of output signals to control a motor driver unit. These signals (they are available for all three axes) are:

• FWD — Depending on the motor driver type configured, this signal generally switches the motor on for both directions or for the forward direction only.
• REV — Depending on the motor driver type configured, this signal reverses the motor direction or it activates the motor in reverse direction.
• SPD1 — This signal is active while the ACU wants to run the motor slowly.
• SPD2 — This signal is active while the ACU wants to run the motor fast.
• RESET — The ACU activates this signal for 800 msecs if the operator clicks RESET. The signal may be cabled to an input of the motor driver which resets latching faults.
• FAULT — The ACU monitors this signal all the time. The signal is low-active, i.e. the ACU expects current flowing through the optocoupler while the driver is OK, if the circuit is opened, the ACU signals a fault and stops the motor. If the motor driver does not provide a fault signal, the clamps of the FAULT input must be wired to 0V/24V otherwise the ACU will not move the motor!

The ACU knows two different configuration modes to control a motor driver. They are called ‘DIR-START’ and ‘DUAL-START’. In ‘DIR-START’ mode, the ‘FWD’ signal switches the motor on/off, the ‘REV’ signal controls the motor direction. This is the configuration many frequency inverters use. In ‘DUAL-START’ mode, the ‘FWD’ signal switches the motor on in forward direction, ‘REV’ activates the motor in reverse direction. This configuration mode is convenient to control a motor with relays.

The speed select signal SP1 and SPD2 actually are the same signal, but with different logical polarity. For most motor drivers it is sufficient to connect one of these two signals, select the one which matches the polarity the motor driver expects.

Remark for operation in the northern/southern hemisphere:

Look ‘through the antenna’ to the satellite for the correct orientation of left/right, up/down and clock/counterclockwise.