Objective: This series of articles presents the design of a mobile satellite ground station. The basic idea is to build a custom low-cost and mobile rotator for satellite tracking and pointing a parabolic dish for C-band communication with a satellite. Part 2 will cover component selection and assembly.
Recommended Prerequisite articles to read
As presented in the previous article, the rotor system will be based on a simple motor+chain+sprocket mechanism. I will hereby describe briefly some of the major design components.
In the previous article, we identified the turntable as the starting point for the design. The next major item is the motor. For a satellite tracking system, two motors are required for azimuth and elevation tracking. In these kinds of designs, there are two major motor candidates: DC brushless motors and stepper motors. The stepper motors have an advantage over general DC brushless motors in that their angular position can be specified with a great degree of accuracy (with no feedback). This formed one of the basis for selecting stepper motor.
The Stepper-online NEMA-23 motor was selected. This is quite a powerful motor with a holding torque of 2.4Nm. At the moment, I think it’s an overdesign. Nevertheless, it is better to overdesign than under-design in this project. More so, this motor is bipolar and has a rated current of 1.8A per phase. The torque will further be increased proportional to the ratio between the motor sprocket and the axle/shaft sprocket. Hence, I can proceed in optimism that the torque requirements for the parabolic dish movement will be adequately met.
Stepper motors require a driver for optimum control, including micro-stepping using pulse width modulation (PWM). I chose the StepperOnline Digital Stepper Driver DM556T. It has an input supply voltage range of between 20V – 50V. Hence in this particular design I will use a 24V DC power supply for all the electrical power requirements.
Chain, Sprockets and chain breaker
Number 25 chain from Misumi is selected for this project. For assembling the chain, a chain breaker is required. This is cheaply available on Amazon at 2000 Yen. The motor shaft is 8mm in diameter. Hence a 8mm sprocket from Misumi with 11 teeth is suitable.
For the azimuth control, I decided on using a 15mm shaft, hence I got a 15mm in diameter sprocket from Misumi with 30 teeth. For elevation control, a larger shaft of 30mm in diameter is chosen. Hence, a 30mm in diameter sprocket from Misumi with 42 teeth is picked.
Azimuth and Elevation shafts
As noted above, 15 mm and 30 mm in diameter shafts were chosen for azimuth and elevation control. It is critical to ensure that the shafts are grooved so that the sprockets can be firmly attached. Luckily, Misumi allows one to specify where the grooves can be placed during the shaft ordering phase.
Azimuth and Elevation Bearings
To ensure smooth rotation, the shafts needed to be held in place using bearings. Self-aligning bearings were chosen since they allow some allowance for slight mis-alignment in the shaft positions. These self-aligning thrust bearings from Misumi were chosen for the azimuth shaft whilst these pillow block bearings from Misumi were picked for the Elevation shaft.
Bolts, nuts and screws
I tried to standardize on the hole sizes at M6. Hence most of the bolts and screws are M6 in size. These are readily available online and locally.
I started by assembling the base of the structure as shown below:
The next is the attachment of the upper part of the rotor. The bottom of the turntable is fixed on the rotator base. The top of the turntable will be fixed on the upper part of the rotator. To ensure that this joint is rigid, two shaft holders are used: on both sides of the plate as shown in the image slider below
This plate is then attached to the top plate of the turntable as shown below
Once this plate has been firmly fixed both to the shaft and the turntable, the top part of the structure can be assembled as shown in the following images
We can now work on completing the bottom part for the azimuthal rotation. The 15mm 30 teeth sprocket is attached onto the azimuth shaft as shown here
The next step is to attach the stepper motor for azimuth rotation
The motor is first attached to the its holder which is then attached to the bottom part of the structure
Once the motor is fixed, the chain can now be measured and fixed. The chain breaker comes in handy here. It’s crucial to get the right chain length so as to avoid wasting the chain away. I made an allowance of an extra link hence the chain was loose enough to attach smoothly. This is tightened later by moving the motor holder further towards the end of the structure.
The elevation bearings are now attached to the top side of the rotor
Once the azimuth part of the structure has been assembled, we can test the azimuth rotation. The power supply, motor and motor driver are first tested. The code is based on the Arduino Mini Pro and the SatNOGS open source resources.
An optical endstop is incorporated on the structure as a form of feedback to mark the origin or “home” point for the rotational control as shown below
At this point the control is a simple test for the motor rotation, speed control, micro-stepping and endstop functionality. This has been successfully verified as shown in the video below (You may need to refresh the page for the video to load). The rotor begins at a random position and first identifies the “home” position. It then rotates 355 degrees and stops.
The project now moves to the elevation part and parabolic dish integration. Once this is complete, part 3 of this article series will be available.