Post by thread:DC motor and tachometer

Chenhee I am currently doing a project where i use the feedback from the DC motor and using a PIC output the speed to and LCD. The best and probably easiest way is to use a Voltage to Frequency converter, this is then put into PIC where an A/D conversion is undertaken, it is then just a case to output this to the LCD. If you want some circuits and source code, email me personally. spam_OUTamb1engTakeThisOuTmail.bolton.ac.uk

I alreay saw several suggestions to use a DC motor as a generator, to measure rpm, speed and so on. This solution involves losses at the voltage generation at the motor (also temperature dependent), losses at the conversion ADC, and requires an ADC or as you say below, or a conversion voltage - frequency... I always wanted to suggest to do a surgery at the motor, and transform it into a pulse generator. See, a DC motor has coils connected to the collectors that are in constant contact with the brushes. Rotating the motor, will generate a voltage that will be transfered to the brushes, and the outside world. The following procedure can works in motors with 4 or more collector contacts. a) Carefully open the motor. b) Locate the coils and collector contacts. c) cut all the wires from the coils to the collector contacts. d) short circuit two collector contacts only, one should be in opposite side of the shaft to another. Be careful to use a very small wire and install it in such way that it will not interfere with the motor mechanic rotation. It should have at least one floating collector contact between the shorted ones, this is why using 4 contacts only 2 got shorted. e) close the motor. When rotating, the motor will not generate any voltage anymore, but a solid and strong short circuit per turn. At your PIC, using a pullup resistor connected to the port pin, also goes to one motor wire, other motor wire goes to ground, just count those pulses in time, it will be one per revolution... Wagner Andy Bassam wrote: > I am currently doing a project where i use the feedback from the > DC motor and using a PIC output the speed to and LCD. > > The best and probably easiest way is to use a Voltage to > Frequency converter, this is then put into PIC where an A/D > conversion is undertaken, it is then just a case to output this to the > LCD.

Why go to such trouble with a slotted disk, if you can buy a ready made encoder, ball bearings, 3 years warranty? Wagner Donald L Burdette wrote: {Quote hidden}> > Wagner Lipnharski wrote: > > >I always wanted to suggest to do a surgery at the > >motor, and transform it into a pulse generator. > > Wagner - > > Why go to such trouble, when a simple slotted disk will provide the same > effect? > > Don

andy howard wrote: [snip] > > When rotating, the motor will not generate any voltage anymore, but a > > solid and strong short circuit per turn. > > At your PIC, using a pullup resistor connected to the port pin, also > > goes to one motor wire, other motor wire goes to ground, just count > > those pulses in time, it will be one per revolution... > > Two, surely? ...hehe, yes, two..., except if the short is made by a diode :)

Because those things are EXPENSIVE. Don >Why go to such trouble with a slotted disk, if you can buy a ready made >encoder, ball bearings, 3 years warranty? >Wagner > >Donald L Burdette wrote: >> >> Wagner Lipnharski wrote: >> >> >I always wanted to suggest to do a surgery at the >> >motor, and transform it into a pulse generator. >> >> Wagner - >> >> Why go to such trouble, when a simple slotted disk will provide the same >> effect? >> >> Don