PerfectStar Goes Bipolar

In this case “bipolar” does not refer to a disorder, but to a type of motor.  The current version of PerfectStar, like most focus controllers, uses “unipolar” stepper motors.  The indirect reason for this is that I (and probably most of my competitors) designed the controller to be compatible with existing motors (and for the motor to be compatible with other controllers) despite some significant advantages of bipolar motors over unipolar.  I’ll provide more details on that later, but first, I want to point out some other changes.

In the above photo we see the “front” end of the controller.  It looks very similar to the current PerfectStar, with jacks for power, the handset connection, and the USB connection.  The big difference here is that this is an anodized aluminum enclosure rather than plastic.  And because it’s aluminum, I can engrave the name and other text directly into the surface – no more adhesive labels (in this prototype I have only engraved the name).  Those labels also need to have holes punched in them to show the LEDs.  In the new version you can see the 3 gray dots in the top.  Those are “light pipes” that transmit light from the LEDs inside.

A bigger change becomes apparent when we look at the back:


Instead of the old DB-9 connector there is now a modular connector.  In fact, it is the same type of connector used for wired ethernet, and a standard ethernet cable (or patch cord) is used to connect the controller to the motor.  A DB-9 connector is actually a pretty good choice for this application, except for one thing:  The corresponding cable pretty much always has a molded plastic cover over the mating connector, and that cover is bulky.  In particular, the length of the cover sometimes makes it difficult to use in tight situations.  Worse, the length provides a lot of leverage, so a good snag on the cable might rip the connector apart.  That is why I (and my competitors) typically use ribbon cable for the motor connection.  Ribbon cable is really not good for this application, but allows the use of a very low profile connector.  The modular connector and cables provide a reliable connection by simply snapping the plug into the jack, and the plug is quite small.  In fact, if necessary to fit a tight space, a right-angle plug can be used.

The handset has also changed:

Although functionally identical to the current handset, this one fits better in the hand and is heavier so that it isn’t so easily pulled by the cord when you set it down.  The knob shown here is not the final version.  In fact, this is an actual focus knob from one of my telescopes!  But it will be larger than the current design, so that it is easier to use when you are wearing mittens (or when your hands are frozen because you’re NOT wearing mittens).  In this prototype handset the base is a heavy type of plastic called “Delrin” and the top is an aluminum plate.  I rather like the contrast of silver on black, but this piece also needs to have some text engraved in it, and the best/easiest way to do that is to start with a plate that is painted black and form the text by cutting through the paint.  I can do this with my CNC router, but I could also have them laser engraved using the same concept.  But if you have any comments on whether the top should be black or silver (natural aluminum color), please let me know.

The new motor housing will also be aluminum:

Again, I haven’t decided whether this will be “natural” color or black, and again, part of the issue is how to engrave text in it.  You can see the ethernet cable connected to the back of the motor housing in this photo.  The jack just above that is for a temperature sensor – just like the current PerfectStar focus motors.  The round black part on the other side of the housing is the coupling to the telescope.  This is the part that is different for each type of telescope.

I have prototyped all of this design and finished most of the firmware changes already.  Still, there is much to be done before this can ship to end users.  In fact, at this point I can’t really say when it will first ship.

So why did I decide to make these changes?  As I previously noted, the original design was driven by the need for compatibility with other focus control systems.  There are a couple of high-end manufacturers already selling systems based on bipolar stepper motors, but they are really in a different class and not competitive with PerfectStar.  But there does seem to be a diminishing requirement for compatibility with older systems.  At NEAF, one manufacturer told me that he has  already begun transitioning his focus control systems to bipolar.  Given the many advantages (and almost no disadvantage) to making such a switch, I think this is the right time.

I will continue offering the unipolar version of PerfectStar, at the current pricing.  This makes it appropriate to use the transition to bipolar motors to also go “upscale” in the new version, and that is why I am going to the metal enclosures.  I haven’t yet set the list price for “PerfectStar B”, but it will probably be 25 to 40% higher than the current PerfectStar controller and motors.

The rest of this post covers some of the technical advantages (or just differences) of bipolar motors versus unipolar, so I invite you to continue reading if that interests you!

The original decision to use unipolar motors for focus systems – I don’t know who made it – was probably driven by the fact that the driver circuit for a bipolar motor is much more complicated.  And at that time it was probably also much more expensive.  And most of the advantages of a bipolar motor are not critical to the function of a focus motor.

But times have changed.  Driver circuits have become far more integrated and less expensive, as is the usual case for electronics.  And the advantages of bipolar motors have been recognized and exploited in so many other applications, such as robotics, that they and the corresponding driver chips have become far more common and inexpensive.

The primary advantage of a bipolar motor is greater torque.  For a given motor size, a bipolar motor will produce more torque than a unipolar motor.  It is also true, to a lesser degree, that a bipolar motor produces more torque for a given input power.

And yet, perhaps the biggest advantages of moving to bipolar motors are indirect.  For example, the primitive drivers for unipolar motors feed whatever voltage you provide directly to the motor – they just control when each individual winding of the motor is energized.  So the motor’s physical drive characteristics are totally dependent on the source voltage, which can vary a lot if you are using batteries to power your gear, as astronomers often do.  The controller for a bipolar motor acts as a regulator, providing just the right amount of current for the motor, regardless of the supply voltage.  And this current can be controlled.  In the current version of PerfectStar I give the user the option of using current to “brake” the motor (to hold its position while not moving) or not.  Most of my competitors simply rely on friction to hold the position, but that can lead to small errors in the position, which accumulate over time.  But using the “braking” option uses a lot of power – and generates heat.  With the bipolar driver you can simply reduce the drive current to provide just enough braking to hold the position – far less than the current needed to move the motor.

Controlling current to the motor also reduces the effect of inductance.  In stepper motors, separate coils are switched on and off to create a step movement.  But each on/off switch produces secondary voltage swings due to the inductance of the coils, and the next switch (the next step movement) has to be delayed until the voltage settles down.  Controlling the current to minimize this effect can be done with either bipolar or unipolar motors, but it is NOT done by the simple driver circuits used in most focus controllers.  Furthermore, PerfectStar B will use motors designed for a lower voltage (and rely on the controller to regulate the current accordingly), which means that the coils have lower resistance – and lower inductance.  The end result is that the new version will be able to “step” a motor much faster.  With the unipolar controller I sometimes have to select a different gear ratio in the motor to optimize the trade-off between speed and torque for each telescope design.  But with PerfectStar B I am hoping to use just one model of motor (one gear ratio) for all types of telescopes.  It will have enough torque to handle very heavy loads, but still have enough speed to be practical with telescopes that need a lot of turns to adjust the focus.

The motors I now offer come in 2 different sizes, although the smaller one can only be used in very limited circumstances.  The new motor housing is just slightly larger than the current small motor, but can be used with almost any telescope.  The new controller is very similar in size to the current model, as is the handset, although it is a rectangular shape rather than square.

The transition to metal enclosures is mostly driven by a desire to make the product more rugged and attractive.  But there is also another benefit:  In some cases, electrical noise generated by the controller could find its way into the very sensitive circuitry of a camera.  Using a metal enclosure can greatly reduce the chances of this happening.  Ultimately, you would want the entire system (and especially the parts of it physically near the camera) to be enclosed in what is called a Faraday cage.  Just having the controller and motor enclosed in metal does not do that – you also need to enclose the cable between them.  Fortunately, this is very easy to do because you can buy shielded ethernet cables and that shielding connects directly to the metal enclosure at both ends.  Shielded cables will not be standard with PerfectStar B, since they are usually not needed, but it is an available and relatively inexpensive solution should the problem ever arise.  Doing the same thing with the current PerfectStar would be quite complicated and expensive.

There will be some small changes to the ASCOM driver to support the hardware changes, and I might add one or two new features there as well, but nothing very complicated or dramatic.

PerfectStar B will appear in the “products” tab of this website when I am ready to start accepting orders, and I’ll announce it here in the blog as well.


About Greg Marshall

I am a retired electronics engineer and after a few months of enjoying my leisure I began to miss doing product development. My astronomy hobby always needed new solutions to unique problems, so I decided that whenever I came up with a good solution I would try to make it available to others.

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