The intrinsic friction of the Nanomotion motor offers an interesting solution for Z applications, where gravity functions as a permanent load. Nevertheless, some considerations must be taken into account to assure safe and reliable system operation. This report highlights these considerations in order to assure successful implementation.
This application is very unsymmetrical, where much higher forces are required to move upwards as compared to downwards.
Special tuning of the control set up with different parameters for each direction are required. This application note concentrates on the intrinsic friction and the static hold force.
Figure 1 above highlights the correct installation of the Nanomotion motor as stipulated in the motor manuals.
While operating Nanomotion motors in the Z direction, the following points should be considered:
The static holding force is typically 4N per motor fingertip in the HR series of motors. The intrinsic friction may be slightly reduced with time due to humidity and absorption of other contaminants on the alumina surface.
With a safety margin, we shall assume then a static holding force of v 2 N/µ after very long periods of non-use (this is an exaggerated precaution).
The shearing of the double sided tape may be very noticeable for a small contact area, nevertheless, the shear of the tape is overcome by the epoxy.
The shear of the epoxy is as follows:
Let shear modulus G=800 µPa
Epoxy drop diameter 2r=2mm
Shear section (thickness of the double sided tape) d=0.1mm
Number of drops = n
F=((G • n πr2)/d)• ∆X
Where : △ X is the shear , F is the applied force
= 12.5 106 n ∆X
For n=2 drops we get
F=25 ● 106 ∆X or stiffness of 25 N/µ
This value is far beyond the motor stiffness. However, in applications with multiple HR-8 motors, obviously more epoxy drops should be used.
Assuming a vibration spectra Az (W) at the mass m in the gravitational direction, the vibration force Fv is given by:
Fv = m∫ w2Az(w)dw
The static holding force is typically 4N per motor fingertip in the HR series of motors. The intrinsic friction may be slightly reduced with time due to humidity and absorption of other contaminants on the alumina surface.
As the vibration is symmetrical and the gravity is not, the system will be stable when:
Fv + mg < F H
Lets take a numerical example:
One) let m be 200 grams and an HR-1 motor, we get:
Fv + 2 = 2
Fv = 0
Any vibration will cause the stage to slip.
Two) Let m be 100 grams and an HR-1 motor:
Here Fv ≤ 1N
At 100 Hz, the maximum vibration amplitude is:
A ● (2π ● 100)2 = 10
A = 10-3 / 36
3 ●10-5 = 30 µ
The attached table indicates vibration levels in various environments. The ISO workshop value is a typical industrial standard.
We see a value of 32000 µ inch/sec @ 50 – 100 Hz.
The vibration amplitude A is given by:
A=(32•103•10-6)/40∆f)m
As ∆f = 50Hz
A = 16 µ
Taking a center frequency of 75 Hz, the acceleration a is given by:
a = Aw2 = 16 ● 10-6 ● (2π ● 75) = 3.55 m/sec
Bearing in mind that the vibration is a statistical effect, an additional factor 2 is suggested to overcome local fluctuations.
Thus, av = 7 m/sec
Going back to Fv + mg < FH
Where Fv = m av
We get for a single HR-1 motor
m=(FH/a+7)=2/17=117gr
An HR-1 motor will safely hold position, overcoming gravity and vibration, for a mass no larger than 117 grams. This statement can be generalized as follows: in an industrial environment, a Nanomotion motor can maintain vertical position for a mass which is 3.5 times less than the static holding force.
Considerations for the motor operational in the Z direction overcoming gravity is described. It is clear that with proper system design, the unique feature of intrinsic stall force can be implemented.
This report did not consider closed loop functions, but of course position can be maintained under closed loop, also overcoming vibration.