Screwjack Solution

Charlie VK3NX's elevation screwjack in action

Charlie VK3NX's azimuth screwjack

Charlie's azimuth and elevation readout

Firstly it should be noted that screwjacks use a pulsing system to keep track. They are not absolute systems (generally). Whenever the power is lost from the AZ/EL unit the direction will be lost. When this happens, the antenna will have to be taken back to 0 heading and the re-calibrate switch triggered.

Screwjacks provide a mechanism for moving a small dish. They often contain microswitches or magnetic reed switches that operate when the dish moves. It is also necessary to provide information about the direction of travel. So in essence there are three switches.
1. Pulse count of movement
2. Up switch
3. Down switch

My solution makes use of the AZ/EL board, part of the set of 5 boards provided for my beam indicator project. Essentially, the following connections are made on the encoder board connector on the AZ/EL unit – J1
1. "Count in" connects to pin 7 of the connector
2. Direction switch “Up” connects to pin 5
3. Direction switch “Down” connects to pin 6

Either the old or new AZ/EL boards can be used for this purpose. The circuit of the AZ/EL board remains substantially the same except for the interconnections described in this document. The correct software must be loaded. This is called Screw.hex

The 16F628 (or A) is again used, although a 16F648A would also be suitable.

The program adjusts to the range of the screwjack but in order for this to happen some values must be placed in eedata of the PIC.

Because each screwjack arrangement is different the user will have to develop their own corrections table as described here, so programming facilities will be required.

Charlie, VK3NX has developed an interface from the screwjack to the AZ/EL Board running the screwjack program. Normally, two interfaces will be required for a complete azimuth and elevation system. The board requires three inputs. One from the switch on the screwjack and the other two inputs sense the direction of the motors. In Charlie's diagram it is shown as 24V from the motor drive but voltage is not critical and any reasonable value from 5 volts DC up should be fine as Charlie has a 5V zener across the input.

Click for a larger image

Downloads

Download the manual here.

Download the hex file for the 16F628 here