Modified Design of a Precision Planter For a Robotic Assistant Farmer

3-3-3- Spring Design: 
There are two springs used in the design of the planter. They provide down force, to push 
planter downward, to keep the disc into the soil and push the press wheel down to pack the soil. 

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Although a dead weight can provide the down force for the planter and its press wheel, but it 
means higher inertia for the system. Higher inertia for the system will result in higher power 
consumption and also out-of-control movements and huge dynamic loads when encountering 
external forces. Springs will increase system stability. On the other hand, springs help the planter 
to be flexible. It can go easily up and down on the field and be flexible when it hits a rock. 
Based on the dimensions and forces, two proper springs were designed for the planter. 
Figure 3-16 shows the position and orientation of the main spring and the press wheel spring. 
The main spring which is installed on the front of the planter provides the down force on the disc 
and gauge wheel. One end of this spring is attached to a fixed plate, which is attached to the 
robot. The other end pushes the parallelogram linkage downward to provide the down force. The 
second spring provides the down force for the press wheel. It is an extension spring. The feature 
of the extension springs that makes it a good choice for this purpose is the preload. Extension 
springs have the ability to be fabricated with the pre-tension or pre-stress. This can help to have 
smaller spring constant with higher forces.

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Figure 3- 16- Position and orientation of the springs 
A) Main spring design: 
The free body diagram of the parallelogram linkage is shown in Figure 3-17 with the 
dimensions. Some lines are removed to make this figure simpler for analysis. Using this free 
body diagram, the spring force can be found.
(Eq. 3-31) 
 
(Eq. 3-32) 
This load is applied to the link shown in figure 3-17 all the time in static and steady 
working condition. 
Main spring 
Press wheel 
spring 

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Figure 3- 17- Free body diagram of the parallelogram link and the spring 
Assuming ±100 mm displacement for the planter (on point B) as it goes on the farm field 
will result in ±27 mm displacement on the spring (on point sm). In the static condition the spring 
displacement from its free length is 27 mm. So it can be said that, 
(Eq. 3-33) 
In which k is the spring constant and Δl is the displacement of the spring from its free 
length. Also in the steady working condition, the length of the spring should be S=150 mm. So 
with 27 mm displacement, the free length of the spring should be 150+27=177 mm.
The spring index, C, can be defined as,
(Eq. 3-34) 
in which D
s
is the mean coil diameter of the spring and d
s
is the diameter of the wire of 
the spring. It is recommended that spring index be between 4 and 12 [21]. Thus, C=6 was chosen 
and d
s
changes from 5mm to 10 mm, a wire diameter can be found that matches the dimensions 
and also gives a safety factor of more than one.