Post by thread:[PICLIST] [pic] TRIPLE DC MOTOR CONTROL

Can anybody point me in the right direction concerning a small problem i have I want to control the speed of 3 permanent magnet dc motors(24v 2A)so that they are all running in tandem(same rpm). They are to be added to an existing circuit which uses a 16f84 I have thought about using a pwm system but dont know whether a dedicated pwm device (or one that controls 3 devices??), or whether a single or multiple pics would be easier driving a fet to each motor. I guess all 3 need to be monitored simultaneously to detect any speed differences. Any help would be appreciated Thanks Dave -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Weeeell, Dave, you have some choices. If you need exact speed matching, under line and load changes, then you have to have some sort of encoder feedback. Let us assume that you do not have encoder feedback. The best measure of motor speed is voltage. There is an outside chance of getting exotic and capturing the pulses of the back emf for a true RPM, but I would not count on it. So average DC volts is the ticket. Without an encoder, a load will influence this voltage to RPM relationship, especially at low speeds. If you try to drive a low turning voltage, and get a load, you will just bog down and stop. It will appear to have no 'torque'. So most folks take a touch of the current measurement, and add it into the feedback loop. Commonly known as IR feedback. This compensates a lot for this droop under load. The problem is that the relationship between volts and RPM's has gotten even less exact. 1 Is the load relatively constant for all three 2 Can you trim up the speeds to match by hand and expect to leave it alone (IE is the system running at one fixed speed). I presume not, as you wanted to use PWM. 3 Watch the PWM answer. I used straight 8 bit PWM, which when put into perspective only controlled to about 0.2VDC. The customer was used to having an analog feedback loop, and noticed the less acurate control right away. I started over. Chris~ Dave Selvester wrote: {Quote hidden}> > Can anybody point me in the right direction concerning a small problem i have > > I want to control the speed of 3 permanent magnet dc motors(24v 2A)so that they are all running in tandem(same rpm). > They are to be added to an existing circuit which uses a 16f84 > I have thought about using a pwm system but dont know whether a dedicated pwm device (or one that controls 3 devices??), or whether a single or multiple pics would be easier driving a fet to each motor. > I guess all 3 need to be monitored simultaneously to detect any speed differences. > > Any help would be appreciated > > Thanks Dave > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Chris Eddy wrote: > > Weeeell, Dave, you have some choices. If you need exact speed matching, > under line and load changes, then you have to have some sort of encoder > feedback. > > Let us assume that you do not have encoder feedback. The best measure > of motor speed is voltage. There is an outside chance of getting exotic > and capturing the pulses of the back emf for a true RPM, but I would not > count on it. There is another simple way that I have used with good results on a few occasions. Small DC commutated motors develop a sharp back-emf spike with every time the brush breaches a commutator segment. With a normal 3 segment com and two brushes this makes 6 evenly spaced spikes per rotation. These spikes are quite short but have a decent intensity once the motor is loaded. Just run the motor from a smooth DC supply, then couple one of the motor terminals through a small cap like 330pF, the spikes will be clearly visible on a CRO and will easily trigger a transistor base if the base is held just-off (like at 0.4v). This gives an encoder-like feedback system with no extra expense and works well even on the smallest cheapest DC motors. It works better with more motor current, so is more suitable to motors under load. :o) -Roman -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

Ah, yes.. one caveat in BOTH cases. With the commutation method, you have to apply clean DC to the motor to see the effect. The PWM would have to be cleaned up, probably with an LC filter, so that you can see the results. With V feedback, I found that the only way to measure voltage with any accuracy was to read the A/D just before the transistor on time begins. I rolled a PWM in software with a twist in it to allow the A/D to execute before on time began. This was no big deal, since the PIC PWM does not go down to the 50 to 100 Hz that you want for motor PWM, so I had to do software anyhow. Roman Black wrote: > There is another simple way that I have used with > good results on a few occasions. Small DC commutated > motors develop a sharp back-emf spike with every > time the brush breaches a commutator segment. > With a normal 3 segment com and two brushes this > makes 6 evenly spaced spikes per rotation. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservKILLspam@spam@mitvma.mit.edu with SET PICList DIGEST in the body

There is another simple way that I have used with good results on a few occasions. Small DC commutated motors develop a sharp back-emf spike with every time the brush breaches a commutator segment. With a normal 3 segment com and two brushes this makes 6 evenly spaced spikes per rotation. These spikes are quite short but have a decent intensity once the motor is loaded. Just run the motor from a smooth DC supply, then couple one of the motor terminals through a small cap like 330pF, the spikes will be clearly visible on a CRO and will easily trigger a * What is a CRO? Thanks, Duane transistor base if the base is held just-off (like at 0.4v). This gives an encoder-like feedback system with no extra expense and works well even on the smallest cheapest DC motors. It works better with more motor current, so is more suitable to motors under load. :o) -Roman -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservKILLspam.....mitvma.mit.edu with SET PICList DIGEST in the body

Hello, while we're on the subject, a friend of mine needs a motor to make a precise number of turns. Lets say it puts on his program that he wants the motor to take 129 turns. The motor have to make this turns fast and precisely stop at the end of the 129th turn. How this can be made? Step motors are the only alternative? There's fast and robust step motors? Best regards, Brusque ----------------------------------- Edson Brusque Research and Development C.I.Tronics Lighting Designers Ltda (47) 323-2685 / (47) 9993-6453 Blumenau - SC - Brazil http://www.citronics.com.br ----------------------------------- -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email EraseMElistservspam_OUTTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

> while we're on the subject, a friend of mine needs a motor to make a > precise number of turns. Lets say it puts on his program that he wants the > motor to take 129 turns. The motor have to make this turns fast and > precisely stop at the end of the 129th turn. > > How this can be made? Step motors are the only alternative? There's fast > and robust step motors? You can fit an encoder to the motor which gives multiple pulses per revolution. Then its a "simple matter of programming". With a stepper it's simpler still, BUT you would need to specify what "fast and robust" meant in this case. If you are dealing with whole turns then you may be able to use a single indicator per revolution sensor (hall cell, optical, Giant magnetic or whatever) or a limited number of pulses per rev (eg 2). Russell McMahon -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservspam_OUTmitvma.mit.edu with SET PICList DIGEST in the body

> From: Edson Brusque [@spam@ebrusqueKILLspamTERRA.COM.BR] > Subject: Re: [pic] TRIPLE DC MOTOR CONTROL > > Hello, > > while we're on the subject, a friend of mine needs a motor to make a > precise number of turns. Lets say it puts on his program that he wants the > motor to take 129 turns. The motor have to make this turns fast and > precisely stop at the end of the 129th turn. > > How this can be made? Step motors are the only alternative? There's fast > and robust step motors? If he doesn't need much torque, a stepper motor is probably the easiest solution. Otherwise, I'd propose a DC motor with an encoder (e.g., magnet wheel with two Hall sensors to give a quadrature signal). Then apply those standard closed-loop control algorithms from the textbook... Regards, Tobias ---------------------------------------------------------- Tobias Ulmer, Engineer, Electronics Development Dept. (DE) AFL Stribel GmbH, Benzstrasse, D-72636 Frickenhausen, Germany KILLspamTobias.UlmerKILLspamalcoa.com -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email RemoveMElistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

I thought so, just hadn't hear of one referred to as such. Thanks, Duane -----Original Message----- From: Roman Black [TakeThisOuTfastvidEraseMEspam_OUTEZY.NET.AU] Sent: Thursday, April 05, 2001 10:20 AM To: RemoveMEPICLISTTakeThisOuTMITVMA.MIT.EDU Subject: Re: [pic] TRIPLE DC MOTOR CONTROL Duane Brantley wrote: > visible on a CRO and will easily trigger a > * What is a CRO? A Cathode Ray Oscilloscope. :o) -Roman -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservEraseME.....mitvma.mit.edu with SET PICList DIGEST in the body -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email EraseMElistservmitvma.mit.edu with SET PICList DIGEST in the body

>If he doesn't need much torque, a stepper motor is >probably the easiest solution. Believe me there are stepper motors with a lot of torque. I used to service a microfiche collator that had a couple of motors about 4 inches diameter and 6 to 8 inches deep that drive a wire rope system to move the collator carriage. This was definitely keep hands clear time while it was operating. As with anything there will be a version to do what you want if you need it. Providing power to run it at the speed you want may be another matter - these motors ran on twenty something volts from memory. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email RemoveMElistservEraseMEEraseMEmitvma.mit.edu with SET PICList DIGEST in the body