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No. 4041, Bhandup Industrial Estate, L. B. S. Road, Pannalal Compound, Bhandup West
Mumbai - 400078, Maharashtra, India
  Tel. : (+91-22) 25961671 / 25963478
 
   
 

 

Bipolar Stepper Motor Drives

 


Hello sir, I have come from Orion Electronics.
Q. Please Come in.
Sir, we know that you have earned enviable reputation as a manufacturer of innovative SPM’s. We feel that our products can be a very good addition to your ideas. They will make some things simpler and some other more economic.
Q. That sure, sounds interesting. But I don’t have much time. Tell me in short, about your products.
Our core product is stepper motor drives, but we believe in providing a solution, rather than offering only drives. We can proudly say that we have a reasonably big list of successful applications. We have worked with quite a few machinery manufacturers. We have provided and complemented systems ranging from computer-controlled, PLC’s, microprocessors and some having manual controls.
Q. Which are the most common ones?
Many of the ‘plastic bag making machines’ use our stepper motor drives with our own controller developed specially for the purpose. In short it is a “cut-to-length” system.
The ‘tube filling and sealing machines’ use our drive with built in control for rotating and stopping the tube at a correct place before filling and sealing. Essentially this is a “sensor start and sensor stop” system. We have incorporated some additional features, extremely useful for these machines.
Q. But these are very standard ones and people like me, working in SPM’s need intricate logics.
Sir, it will take a while for me to explain. If you are in a hurry, I can come again, some other time.
Q. No, it’s okay. I think the things can wait. Tell me. Mean while I will ask for some tea. Yes?
We have implemented many customized sensor logics too. These include homing, slowing down, changing directions etc. Relay outputs for sequential operations are possible. Opto-coupler outputs also can be provided for communication with a master control system. You can have a motor status (start / stop or direction) and clock to keep track of position.
Q. But what about, speed? I have heard that steppers have a limitation of speed. What are the speeds up to which the stepper can run?
This is a million dollar question. The speed will depend principally on two things.
1) Choice of motor and drive
2) Requirement of torque
The motor manufacturers provide torque-speed characteristics. The torque decreases at higher speeds. So it’s a matter of proper mechanical design and calculation of torque requirement. As a thumb rule, motor with less winding inductance and a drive with higher voltage will give you good speed and better torque at that speed. One of our regular customers, who is manufacturing ‘wire cutting and stripping machines’ runs the motors at 3,000 RPM, with our drives. Speeds from 400 to 900 RPM are common.
Q. How do we calculate the torque?
You are testing me sir, you know it better than me! However, we do it by first principle. We attach a pulley of radius R cm and go on adding weights in the attached pan until the system starts moving. This is of course done with maximum load on the system.
T (Torque) = R (Radius) X W (Weight in pan)
But we have to remember that the motor manufacturers specify “holding torque” and not the “pulling torque” that we need. Generally, pulling torque can be taken to be 70% of holding torque. A safety factor of 1.5 is recommended. So if your experiment gives the torque requirement of say 10 Kg-Cm, you should select a motor of {(10 ÷ 0.7) X 1.5} = 21 Kg-Cm, or more.
Q. What kind of accuracy can we expect?
The stepper basically runs in steps. Give one pulse to drive and the motor moves by one step. Counting of pulses is an easy task in electronics. So if ‘x’ number of pulses are given to the drive, the motion will be ‘x’ steps only. Therefore you can get a fantastic repeat accuracy. But yes one thing has to be kept in mind. A proper speed profile – a trapezoidal one – must be maintained. Start at 60 RPM, accelerate to the required and achievable speed, retard back to 60 RPM before stopping and then stop. All our controllers are equipped for this.
Q. Actually what is the value of one step?
The drive can run in “full step” or “half step”. In full step you will get a step of 1.8 degrees whereas in half step it will be 0.9 degrees. Thus in full step 200 steps = one revolution and in half step, 400 steps = one revolution. The half step is more popular.
Q. If I use this for a linear indexing table, let’s calculate the accuracy level.
Ok. Let’s say you use a ball screw of 4 mm pitch. If we attach motor directly to the screw, and use a half step drive, 400 steps will mean 4mm motion. So one step will be (4 ÷ 400) = 0.01 mm or 10 microns.
Q. And if I need better?
There are two alternatives. One is using a timing pulley ratio as per requirement or use a micro-stepping drive. We make micro-stepping drives which can give 800 / 1600 / 3200 / 6400 / 12800 steps / rev.
So in the example of 4mm pitch ball screw, you can get 0.313 microns / step!
Q. What about backlash?
We have some solutions for backlash correction, for initial referencing of home position, considering all subsequent operations are unidirectional. A wise combination of mechanical and electronic corrections can be worked out.
Q. But in another application that comes to my mind right now, the motor will have to move at a speed, 10% of the main motor.
We have designed an add-on for such requirements. The percentage can be set on site with a potentiometer. You may say, it’s a variable gearing. Only thing is it will need either an encoder feed back or an analog input (4 to 20 mA or 0 to 10 VDC) from say VFD. Such add-on is used in winding machines.
Q. All this is okay for single axis applications. But for two or three axis machines, we may need interpolation.
Sir, we also supply our systems to some other customers, who make engraving machines in jewelry industry for bangles, wood working machines, special purpose welding equipments etc. We have also developed packages for up-gradation and modernization of milling machines, lathes. These packages come with our proprietary computer software, a dedicated controller, MMI and stepper motors with proper drives. Your autocad or corel files can be translated physically on the job.
Q. Where can I see such a machine working?
We have no track of the machines, once they leave our customer’s place. But you can visit our marketing office at Bhandup, Mumbai; where you can see the photographs and animations or video clips. Please come with concrete requirement in your mind, so that we can discuss at length. We believe in co-creation. Let’s do some business together and grow together.
Q. Sure. But one more question, before you leave. What service back up will we need?
Sir, our drives are really good. The failure is rare. Only two things can fail the drive. One is rampant voltage fluctuations and the other is direct shorting of motor outputs. 195 to 255 VAC is the acceptable mains voltage span. If, power conditions are so bad, that the span is exceeded, we advice the customer to use a servo stabilizer of proper rating. For some models, we have developed special SMPS which gives improved span of 170 to 270 VAC. Another important thing is that, the traders of imported drives do not provide service support. But our drives can be repaired. We can repair them, as we have developed and manufactured them.
Q. Yes, that’s a valid point.
Thanks for sparing time. Have a good day sir. Hope to see you soon.