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Change motors and drive

Change motors and transmission system

 

 

         

It is as if the wheel radius gradually goingfrom L1 radius to L2 radius, by varying the proportion ratio of up to a minimum of 1.1 to 0.9 for example, and does so periodically since the phenomenon is repeated for each rotation of the worm, a period of 10 minutes in the case of the worm of a mount type EQ5/SPDX. This will induce a periodic advances and delays in the speed of rotation transmitted, in astronomy this problem is known as the periodic error, it is created by the system gear / worm. In a system without friction and without unprecision other than the lack of spherical aberration of gears, we could measure an error periodic of sinusoidal form:


   

This error is quite regular and therefore predictable. This means that a system of periodic error compensation (PEC) would be quite able to catch up fresh variation of the output speed of the two gears in mixing the speed of the motor in input. To make this more realistic modeling of periodic error, it must be added various phenomena that make it more random, and therefore less predictable and periodic. In fact any element of the chain of transmission of movement provides a more or less strong contribution to the deformation thereof. The main distortions are, however, due mainly to blocks moto-reducers (gearboxes) with different speeds, plays, especially friction and vibration induce additional components to the movement of initial rotation. Here is an example of periodic error measured on a mount of mid-range, a Vixen GP-DX with mechanics not changed:



Improve mount As we can see, the key to approaching monitored more perfect despite the inevitable periodic error is the reproducibility of these errors. A particularly effective system can provide results quite exceptional for this type of problem: replace the original drive by precise motors coupled to the worm by a system of pulleys / belts:


  

Motors and pulleys / belts of relatively good quality are expensive, but keep in mind that a complete system of this type is less expensive in lot of case than the original motors of the mount. The main advantages are:

• Elimination of high frequency harmonics of the periodic error caused by motor-reducers
• High Reduction of plays, opening the possibility of precisely offset by Pic-Astro, offering the precision necessary for auto-guiding, Goto...
• Obtain an error actually periodic very close to a theoretical model and very sweet. The periodic error correction becomes frighteningly effective.
  

Here is the result with my mount Vixen SP-DX (not so distant ancestor of the GP-DX, the same design):

We can see from this example that the tracking error of the mount is periodic even if it occurs every 600 seconds (time for a round worm), its magnitude is not predictable and PEC system would not effectively correct it. Any problems in the monitoring of a German equatorial mount is summarized thus in the following points:

• The drive by worm / gears tangent induces a periodic error more or less soft depending on the quality of mechanical components 
• If this error is reproducible, then it is perfectly possible to cancel by a PEC
• The gear plays that can occur at all levels are additional factors of deterioration of monitoring but may also be a serious obstacle to the guiding, and let autoguiding

The key to compensating for these errors by mechanical electronics is reproducibility. Example of periodic error totally unpredictable:


   

Another important source of improvement and smoothing monitoring is the use of Micro-steps allowed by quality motors. The motor of any German equatorial mount worthy of the name is through stepper motors. These motors have the characteristic of rotating the axis by "steps", giving the rotational movement jerky dynamics similar to the second hand of a clock. The jerky movement is sufficient in most cases, but when used photographic example it becomes crucial to follow up as best as possible. It is interesting to try to soften these jerks and achieve the ideal of making the shaft of the engine regularly. In a stepper motor, the magnetic field which rotates around the rotor (rotating part of the motor), and it does precisely by steps. The solution would be to make these steps smaller and smaller by cutting them in half-step and even in micro-step. That's what Pic-Astro superbly do by adding the icing on the cake: the linearization of these micro-steps, ie the amount of travel between each micro-step (1 / 16 steps) is adjustable so as to be always the same! This contributes strongly to the smoothing of the curve of error and therefore brings quality monitoring at a level which is exceptional. The motors of course play a crucial role in quality monitoring, their main characteristics are:

• Number of steps per revolution. For example 48 for the motors of the SPDX, 400  for Nanotec motors that were used for the transmission pulley / belt
• Torque: the engine power. The more it is powerful, the less it will be slow, disrupt, or have small vibrations from mechanical defects. 
• Voltage / Power Intensity
• Size: very important for adaptation on a mount where space is limited! 

Dimensions of the system Let us now how to modify your mount. The choice of motors main criteria to consider in the choice of motors are those mentioned above, but it is very important to ask the following questions:

• What is the maximum size that can support the mount to accommodate the motors? In the case of motors originally built as in the case of a mount HEQ5/EQ6 it is possible to modify the motor housing and remove the electronics that will be replaced (favorably!) By Pic-Astro.
• What is the torque required for your mount? Ideally, the more the motor will have torque the better it is.
• What is the maximum power deliverable by the battery or the power supply that you have during the time of your observations? ?

Nanotec motors ST4209-L1704 were selected and tested with great success by many users of Pic-Astro . They have helped in testing conditions to move mount to 1500x sidereal speed which confirms that their torque. These motors associated with pulleys / belts were used to obtain the curve of periodic error cited above. In practice they are used at 100-200x sidereal speed, which really kick-37x compared to the max allowed by the original motors. Here are their features:

Type	Connexion	Intensity/Phase	Maintening torque	Résistance /phase	Weight	Length A
ST4209L1704	Bipolar	1.68A	44Ncm	1,65 Ohms	350g	47mm

These motors can be ordered directly from the site of Nanotec: Choice http://www.nanotec.de .Pulleys / belts choice : regarding the choice of pulleys / belts, the following questions to ask are:

• What is the maximum size that can support the mount to accommodate the pulleys, while maintaining maximum freedom of movement and without risking a clash that could damage the pulleys / hide the worm.
• What is the maximum acceptable distance to position both the motors and pulleys?
• What is the ideal gear ratio to use periodic error correction with Pic-Astro ?
  

The Excel file Courroies_V2-1.xls in the "Docs. Mounts" of the site will make you a good part of the calculations for you, let you guide.

Adjustement of the worm

The Problem worm is the key to the transmission of movement on an equatorial mount. From it and its settings will depend plays and tracking errors, key factors in astrophotography, but the softness of their rotation will also be a key element when using Goto and motors in general . Some mounts come with manufacturer's defects of various adjustment and lubrication, or just your mount goes wrong with the time and it is necessary to re-adjust it. This operation is quite delicate, so the golden rule is that if everything works, do not touch anything! Once the worm of the mount cleaned and properly lubricated with suitable lubricants such as lithium grease, silicone grease etc, all the subtlety of adjustment is a problem simple to apply sufficient force on the worm to ensure good contact with the gears without the crush. The consequences of poor adjustment are: Worms tight

• Sometimes irreversible deformation of the worm
• Appearance of high-frequency vibrations on the periodic error
• Increases in periodic error double up as a result of the deformation of the worm
• Increasing the hardness of the movement of the worm up to prevent a motor even with a couple of very hard to turn

Worm loose

• Plays gears (backlash) that can be impressive in training
• Play on the routes that would move the OTA by hand over several tenths of degrees
• Item 3

Usually setting your mount has the elements for the following settings: 

• 2 screws for tightening the worm on the gears: A1 and A2 
• 1 screw for tightening the previous two: B
  
• A set screw / nut against lateral movement adjustment: C

The adjustment procedure is:

• Tighten the screws gently A1 and A2 in order to bring the worm into contact with the gears
• The B screw is loose, you will quickly feel a hardness in the rotation of the worm, then tighten the screw B to get to the point where the fluid movement back
• Now repeat the first two steps until you find the equilibrium setting where the movement of the screw remains very fluid whilst being slightly beyond the point where the movement was hard (screws too tight).
• Tighten the screw and against C-lock nut so as to eliminate the lateral movement of the screw. You can "feel" the play by touching the lateral axis of the worm and by turning it one way then the other with the other.
• Check the play resulting from this transaction blocked by manipulating the brake lines RA and DEC, normally no play should be perceptible.
• Check the play now by observing a fixed point at high magnification (200x) and turning the worm alternately in both directions, you will be able to report on whether or not to refine the setting.
  

Important note: plays are needed in any mechanical connection to the axes of freedom (rotation, translation), do not eliminate them altogether, you risk damaging your mount. Examples of wheel and worm mounted on a Vixen SPDX: