# Coupled System of Dual-Axis Clinostat and Helmholtz Cage for Simulated Microgravity Experiments
## 2 Design and Construction  
### Principle of Clinostat Operation  
The two-axis clinostat developed in a recent project, also called a random positioning machine [32], consists of an aluminum frame (Figure 1) that rotates relative to the clinostat base that is also made of aluminum. Inside the rotating frame, a spherical region rotates independently, where experiments in simulated microgravity can be carried out. In the sphere, several physical parameters, such as acceleration, angular velocity, temperature, humidity, and light intensity, can be measured and sent remotely to an external computer. Moreover, the cultivation area can be observed by a camera, providing online visual information about the growth progress.  
The two rotations--one of the frame and one of the cultivation sphere--create a situation in which the average effective value of gravity is reduced to zero. Because of this, plants are not able to sense gravity's direction and are thus forced to grow in a state similar to weightlessness in cosmic space. The ratio of angular velocities--of the frame and the sphere--define the number of turnovers for which the optimum conditions of average zero-gravity will be achieved. Our calculations and tests showed that the ratio of 1:2.2 (frame to sphere velocities) also gave the lowest mean gravity fluctuations, leading to values between 0.0657 g and 0.0072 g. Since the axes operate at constant speeds, the devices belong to a 3D clinostat class. Figure 2 depicts the results of time-dependent measurements of the acceleration components using built-in accelerometers. Additionally, which is worth noting, the angular velocities have to be kept at low values to obtain satisfactory conditions for plant growth, especially if taking into account the spatial distribution of humidity in the cultivation area [33]. Thus, we usually used a velocity of 0.31 rad/s for the frame and 0.68 rad/s for the sphere.  
Figure 1: Designed and constructed clinostat (left panel) with the centrally mounted, optically isolated, spherical area (diameter of 300 mm) in which the micro-gravity experiment can be carried out. The system is powered by two stepper motors (one visible in front at the vertical edge and the second symmetrically mounted at the vertical edge in the back) and a system of rubber stripes along with transmission gears. The axes of rotations are marked by red arrows. In the bottom-right corner, the control unit for the stepper motors is visible. The box on the left, mounted at the upper horizontal edge, is the main control computer. On top of the sphere, the system of LEDs (light emitting diodes) illuminating the cultivation process is visible. The right panel presents the spatial orientation of the three-axis magnetic-field sensor (LIS3 DH, Adafruit® (Brooklyn, NY, USA)) located in the cultivation sphere.  
Figure 2: Results of measured gravitational acceleration components (x, y, z) in the rotating clinostat with an angular velocities ratio (frame to sphere) equal to 1.2.2. In the top panel, the temporary values are given. In the bottom-left panel, the averaged values of gravitational acceleration components (x, y, z) are provided, while in the bottom-right panel, the absolute (square root of the squares of three acceleration-component instantaneous values), the averaged value of g is presented. To measure the gravity, the three-axis LIS3 DH (Adafruit®) acceleration sensor was used and mounted in the center of the cultivation sphere (comp. Figure 1). The normal, static g acceleration is parallel to the vertical direction in the laboratory room.