Stepper motor control servo

Discussion in 'The Projects Forum' started by Trackmann, Sep 7, 2013.

  1. Trackmann

    Thread Starter New Member

    Sep 7, 2013
    6
    1
    Hello Members,

    I have developed and built a straightline tracking tone arm for record players. It is driven by a stepper motor which receives it's step rate from the amplified output of an analog Hall Effect device. I have a circuit diagram that a friend designed for me in the nineties. I had no time to work on this project until recently when I retired. Over the years some of the information my friend provided got corrupted so that when I breadboarded the design recenly, it didn't work. I always thought my friend a genius but I've lost contact so that I can't ask him.
    I've been around electronics all my professional life but I've never learned to design circuits myself. I used to tape PCB artwork in the seventies but most of my work was mechanical design.

    I don't know if it is appropriate to offer compensation for help in designing the circuit I need, but I can offer precision machining and 3D solid modelling in return.

    Sincerely,

    Ralf
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,509
    2,369
    It seems a simple design on the surface?
    It sounds as though you want a stepper motor to index at the rate of a Hall effect device, is this one to one or is the step rate magnified in any way?
    I guess you do know a stepper motor does not have as fine a resolution as the PID servo?
    This is due to the resolution of a stepper being defined by the 1.8°/step rather than the relatively infinite motion of the servo.
    Did you intend building the stepper drive as well, or using an off the shelf drive?
    Max.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,509
    2,369
    Was the original design tested and performed OK?
    Is there a diagram that you can post of the original?
    Max.
     
  4. Trackmann

    Thread Starter New Member

    Sep 7, 2013
    6
    1
    The original diagrams were sketches which I recently converted to a CAD drawing. I think that is where the mistakes crept in. I'll get that circuit diagram ready and post it later tonight. At that time I'll answer your questions too.

    Sincerely,

    Ralf
     
  5. Trackmann

    Thread Starter New Member

    Sep 7, 2013
    6
    1
    Answering your questions in order:

    I agree. I think it would be a simple design for a knowledgeable designer.

    A magnet on the tone arm moves across the analog Hall Effect device, causing an output from 0V to 5V. this output is amlified and applied to a 555 timer, wired as a VCO. The output of the 555 timer should vary from .27Hz to 27Hz. That square wave is then applied to a Portescap stepping motor via an allegromicrosystems stepper motor driver. The stepper motor in turn drives a 6-32 lead screw via an O-ring drive belt. The lead screw then drives the tone arm carriage along its track.

    The reason for using a stepper motor is as follows: There have been many motor driven tangential tone arms in the past and they all used fast running DC motors with gear reductions to achieve the ultra low RPMs required to move a tone arm across an LP record. So I chose a stepping motor. In addition, a stepping motor has no brushes to wear. I actually held a US patent on the use of a stepping motor for this type of application. It was a 1984 patent and has long since expired.

    I built everything except the electrical components.

    In the nineties when the circuit was first designed, I breadboarded it.
    I used an Airpax stepping motor then and I had it ticking away. but I feared, the breadboard got old and I was worried of tarnished contacts which caused me to take it apart and to buy a new breadboard.

    I have an old dual beam Philips oscilloscope and a Fluke multimeter. The oscilloscope hasn't been calibrated in years but it is good enough to measure the stepping frequencies and to see the square waves I need.
    I also built a test fixture that allows me to move a magnet past the Hall Effect device in a controlled fashion. The design of the tone arm is such, that the magnet can move + or - .090" across the Hall Effect device. In that distance it must produce the square wave from .27Hz to 27Hz.

    On the schematic you will see a dual 556 timer. The lower half is used to produce the stepper motor frequencies requred to actually play an LP record. The upper half of the 556 timer is used in a seperate circuit which can be ignored for the time being. That circuit is supposed to produce frequencies for rapid traversing the tone arm across the LP record.

    I have all the data sheets for the various electrical components that I intend to use.

    Originally I used many components from Sprague Electric but they went out of business and their Hall Effect devices and stepper motor drivers were taken over by Allegro Microsystems.

    I also have a block diagram which I drew some time ago. I'll find it tomorrow and post it.

    If you want to see an actual picture of the tone arm, I can post that too.

    I am actually playing LPs with the tone arm but not having a working servo, I nudge the tone arm carriage along its track manually with a finger. That is a relatively clumsy way of doing things but it works. I have to get up every 20 to 30 seconds to move the carriage.

    If you need more information, please ask.

    Sincerely,

    Ralf
     
  6. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Does the hall effect sensor circuit detect a change in the tone arm angle and then correct for it ?
    So, you actually built the turntable ?
    Could you clarify, what drives the platter ?
     
    Last edited: Sep 8, 2013
  7. killivolt

    Active Member

    Jan 10, 2010
    341
    310
    I think it's more like supports the change, advancing the screw prevents it from skipping backward. If that's what your thinking I would agree.

    It would be cool if he built the hole thing, but from the sound of it. Probably not.

    This is a cool project. I'm learning a lot.
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,509
    2,369
    I would imagine the platter is driven in the traditional way with a AC mains driven synchronous motor, taking advantage of the 60Hz supply?
    The tone arm transport is then moved across the record exponentially as the dia tracked decreases.
    If correct, this is slightly opposite to the systems I have worked on where constant surface feed is initiated, IOW, the revolving object speed increases with a linear feed of a tool into a work piece towards centre to obtain the constant surface feed based on the feet/min rate of the decreasing circumference.
    In my case I used electronically geared servo's however.
    Max.
     
  9. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Yes, the speed of the groove under the stylus will decrease as the stylus moves from the outer edge of the LP to the inner edge. But, the grooves themselves are roughly evenly spaced from the outer to the inner edge.
    So, my thought is that the tone arm/stylus would need to advance the same distance for each revolution of the platter.
    To be workable, the tone arm would need a certain amount lateral movement at its connection point to the linear slide mechanism.
     
    Last edited: Sep 8, 2013
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,509
    2,369
    But it would be a linear movement that will increase in rate as the dia gets smaller.
    Not a step/rev, however..
    Max.
     
  11. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Yes, that's what I'm thinking.

    As the stylus tracks the groove it is pulled inward. The small angle change is detected by the hall sensor, and the stepper control circuit moves the rear of the tone arm to compensate.
     
    Last edited: Sep 8, 2013
  12. Trackmann

    Thread Starter New Member

    Sep 7, 2013
    6
    1
    Hello tubeguy,

    I'll answer your second question first:
    I'm not building a turn table, just the tone arm.

    To your first question: Yes, the Hall Effect sensor detects a change in the angle of the tone arm and tells the stepping motor to correct the position of the tone arm carriage.

    I'm attaching a block diagram to show how that works.

    Hello killivolt,

    in response to your second sentence:
    As mentioned above, I didn't build the turn table, just the tone arm, but I built that from scratch.

    Hello MaxHeadRoom,

    The tone arm servo is basically a so-called follow-up system. For instance, when you're driving down the road in a straight line let's say between Regina and Fort Q'apelle you are actually not driving in a straight line but a long drawn out zig zag. Between your eyes (the sensors) and your hands on the wheel (the stepping motor) you're constantly correcting the direction of your car. That's how the Hall Effect sensor and the stepping motor work together in my tone arm design.

    Hello again tubeguy,

    Grooves on an LP record are spaced from .002" to .011". That's called "variable pitch cutting". The run-out groove pitch at the label is .250". That requires the stepping motor to constantly change it's step rate to keep up.

    Hello again MaxHeadRoom,

    Actually, the step rate constantly decreases as the tone arm approaches the label.

    Sincerely,

    Ralf
     
    tubeguy likes this.
  13. John P

    AAC Fanatic!

    Oct 14, 2008
    1,632
    224
    I'm afraid you're using obsolete technology two different ways. The first (of course) is spending time on anything involving vinyl records. You must be sentimental about that past invention! Second, that arrangement of 555 timers may be workable, but you'd do a lot better nowadays to switch to a microcontroller. You could have the basic servo function, a detector for the center of the record, which would call for lifting the stylus and running the carriage back to the start at high speed, then responding to a sensor to tell it when to stop (remember the stylus has to be clear of the record before you can put a new one on, so you can't leave the tone arm in the center, even if it lifts up). And of course, changing features of the operation would just involve typing something on the computer, not replacing components.

    Sometimes it is boring to keep saying "You need a microcontroller" but those things are so much better than a pile of chips.
     
    killivolt likes this.
  14. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Some of my most favorite recordings are on vinyl. I still like to listen to them.
    But, no matter how hard I try, I just can't get them to fit in the DVD player. :rolleyes::D
    (Burning a disc??? Sounds dangerous...:cool:)
     
    Last edited: Sep 9, 2013
    killivolt likes this.
  15. killivolt

    Active Member

    Jan 10, 2010
    341
    310
    John P, I have a project for some home automation, I'll be working on in the future. All of this applies, I think I'll use sensors, hex inverters and Micro to control some servos.

    All of this is helping me to understand the method means and way to get there.

    kv
     
  16. Trackmann

    Thread Starter New Member

    Sep 7, 2013
    6
    1
    Well, I had several responses to my original question. Most related to understanding what I am doing. One response was from a mean individual and no help at all. But not one would comment on the circuit I've posted. Like maybe there was a misplaced resistor or a wrong value etc.
    So I thought I could approach the problem by doing it in a piecemeal fashion. Part of the circuit involves a 555 timer based VCO. The output frequency should range from 0.27Hz to 27Hz with a variable DC input. The components connected to the 555 timer should not change for the output range. Once I have that, I can work on the LM 324 op amp to amplify the output of the Hall Effect device.

    John P, please don't respond. I won't read it!

    Sincerely,

    Ralf
     
  17. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,128
    266
    This project positively *aches* for a micro controller.
    You could read the hall signal with the ADC, then generate the stepper phase signals to directly dive a really simple uni-polar stepper drive with 4 transistors.
     
  18. Trackmann

    Thread Starter New Member

    Sep 7, 2013
    6
    1
    You are absolutely right. If you look at the block diagram I have posted, you'll see that I have specified a micro controller. But if I am not able to debug the circuit I have posted, I'm certainly not qualified to program a micro controller. I have actually purchased a Cypress micro controller development board from Schmartboards. I also obtained the programming software from Cypress. They have tons of tutorials on their website all for free. The problem is that, the tutorials start at a level of knowledge well above my own level of knowledge. The tone arm servo control does not have to be state of the art (although I would like it to be) Once the music comes out of the speakers, I don't care if the circuit has vacuum tubes! I presently push the tone arm carriage along its track with my finger which is a relatively clumsy operation. But it shows that the servo does not need to be of the precision type.

    Sincerely,

    Ralf
     
Loading...