Adding a Shaft Encoder Directly Into A Servo
Here’s a nifty hack to add a shaft encoder directly into a servo box.
Parts:
1. Servo - Preferrably a Futaba or Tower Systems
2. Drill
3. Photointerrupter - Rat. Shack #276-142 (NOT the package with the diode and transistor separate),
or any standard ‘U’ shaped photointerrupter - Available in most printers, VCRs, etc.
4. Hot glue
5. Potentiometer
Background:
For most of the time that I’ve been doing amateur robotics I’ve always wanted to have some kind of feedback from my motors. Such set up would allow for dead reconing and accurate turning and other manuvers without having to worry to much about in-code timing or battery voltage. Unfortunately I’ve never been able to figure out an effective and cheap way of setting up such encoders.
A few months ago I started a science fair project about using a maze solving robot. After a few days of planning it became apparent that I would need wheel encoders for one of the robot’s maze solving algoritms. I briefly considered putting an interrupter across the spokes of the wheels. I soon ditched that idea because it was unreliable.
Soon after I realized my need, I saw a brief show-and-tell at the Triangle Amateur Robotics club. Ken Boone showed how he was going to add a built-in encoder to his micro servo. He planned on drilling holes through the metal gears, then positioning a photointerrupter across the gear to count the holes. I thought this was rather innovative, but didn’t consider the idea feasible for my application.
A month or so after that I picked up a copy of Karl Lunt’s Build Your Own Robot from the local library. We went camping that weekend, and most of my time was spent devouring the book. In it Karl describes how he added a built-in encoder to his servo, by setting up a photoreflector on the bottom most gear of the servo. Karl’s idea sounded more reasonable to me, because I thought that I would be able to change out the shaft gear after the modification was done. By being able to change the shaft gear I could convert the modified servo back to a regular servo at a moments notice. Ken’s idea sandwitched the shaft gear between the diode and transistor, thereby trapping the gear. It was only after I started the modification that I realized that Karl’s idea would do the same. Karl’s idea generated about 140 pulses per revolution, where Ken’s only generated 6 per revolution.
When I got home I quickly dismantled my servo and began to add an encoder ….
Modification #1 - The Frustration
Karl added an encoder to his servo by first drilling a hole into the case above the bottom most gear of the servo. Karl was using on of the standard Futaba servos. Fortunately my Futaba 3003s were exactly the same. The position of the hole was critical in the design. Karl using a Semens IR photodiode/phototransistor built-in pair. This allowed for a very small package. But, I didn’t have that luxery. My mission then was to find some good IR photodiodes and photo-transistors. Fortunatly the search was short lived.
Back in the early days of my robotics endevors I nievely bought 2 photointerrupters from Radio Shack. I had held the idea that they could be used in an IRPD setup. After learning of my mistake I had tossed the diodes and transistors into my ‘junk box.’ Amazingly enough the diodes and transistors were very small and perfect for my application. I drilled holes into the side of my servo case until I was able to position the diode/transistor pair above the bottom most gear. This is the gear that directly connect to the small gear on the motor. It is large, flat, and has a whole bunch of teeth.
I inserted the photodiode between the shaft gear and the bottom gear, and positioned the phototransistor directly above the bottom gear and below the top-middle gear. That way the infrared light would bounce off the bottom gear and hit the transistor.
I then took a ‘Sharpie’ marker and marked 1/4 to 1/3 of the bottom gear black.
I made sure that the mark was as dark as possible. The idea was to have the dark part absorbe most of the IR light, thereby preventing it from reaching the transistor, thereby causing a large change in the phototransistor’s resistance. Good idea in theory but hard to implement.
After much trial and error I was able to get the phototransistor to generate about a 50k resistance differenace between light and dark. At that point it was rather late at night and I decided to go to bed. I woke up that morning, and in my excitement I decided to hook it up again and test it. Being overly eager I bypassed the current limiting resistor on the diode and … *pop* it was dead.
That normally wouldn’t have been so bad, except for the fact that the only other IR photodiode I had, that was near the other’s size, was just slightly too big to fit between the bottom gear and the shaft gear. Oh well, I just decided I’d move it up beside the phototransistor.
I tired and tried with this setup to get an acceptable resistance difference, but alas and alack it wasn’t going to happen. At this moment of desperation I decided to take my Dremel tool and shave down the surface of the diode, so it would fit where the othe one had. Even after all of that I was still unable to get the desired change in resistance.
It was time for another idea …. Modification #2 - Finally!
To prevent the risk of premature baldness I decided to try out Ken’s modification idea. Actually, as I later found out, his idea is a bit different. See the note at the end of this article. Thankfully this worked much better. Below are the steps describing the modification:
1. Remove the main/shaft gear from the gear train. Around the perimeter of the gear, inside the line of teeth, you should be able to make out 6 small circualr formations, that were created when the gear was molded. This 6 circles make perfect outlines for drilling holes. Find a good bit for the job and put 6 holes though the gear.
2. Replace the gear back in the housing and drill a hole for the diode and transistor above and below the gear.
3. If you need more room you can shave the gear down a bit. Most gears have a ridge around the outside that can be shaved down to make more room. Place the photodiode and transistor above and below the gear so that they face each other. Hot glue them in place and make sure that they don’t hamper the movement of the gear.
4. Put a resistor in series with the the photodiode and power it up. Measure the resistance of the phototransistor as you turn the shaft. Make sure that your have the negative lead of your multimeter hooked up to the negative lead of the transistor and the same for positive. Reversing the leads causes the resistance to be very high. You should be able to get a sizeable resistance difference. Usually above 50k ohm. If the resistance doesn’t change that much you can try adding foil backing to the gear then puch holes through it.
5. Hook up the transistor to the schematic below.
6. Now, turn the shaft till you reach highest reistance. This means that it is not above the hole. Measure the resistance across the transistor and find a potentiometer that is about twice that value. Connect up the pots middle pin to Vin and one of the wipers the to the transistor ‘+’. Measure the voltage across ‘Vin’ and the pot/transistor connection. Turn the pot till the voltage is greater than 2.7 or so volts. Now turn the servo shaft till the diode/transistor pair is over a hole. The voltage at this point should read below 1.5v. Measure the resistance of the pot and insert a similar value in its plce. If you prefer to do this process by manual calculations use this formula to figure out the resistor value:
Vout = Vin (R2 (R1 + R2))
In this case Vin is 5v. R2 is equal to the resitance of the transistor and R1 is what you are trying to figure out. You will want the Vout to swing from at least 1.5v to 2.7v. You should probably check the specification of your chip to make sure that this voltage swing will generate a pulse.
This is a note I received from Ken. He’s clarifying his idea a bit. It seems that I got it a bit wrong.
My conversion uses the same bottom gear you started with. It has 8 holes through it and aluminum foil on the bottom to completely block the IR. There is around 320 pulses per revolution of the output gear. The detector in below the gear inside the bottom case and there is a ground down IR LED suspended above the gear. I get a sine wave output and I use [a custom] circuit for line detection… My goal is to do speed control on each drive servo so I can accurately go straight and accurately do several different sweep turns with the fire fighting robot. I also want to count the pulses to accurately rotate the robot and determine how far it has gone.
Well, there you have it! A nice compact encoder inside your servo. It’s just wating to be hooked up to your Stamp’s PULSIN command or your OOPic’s oCounter object.
Update - Modification #3 - Hopefully the last!
Well, after hours and hours of agonizing work I finally just gave up and decide to switch gears (pun intended). I finnally realized the obvious truth - 6 counts per wheel just won’t work! As per Ken’s hack I decide to move the photointerrupter to the bottom gear. I drilled 4 holes into it. Be careful not to crack the gear - I speak from experience. And drilled out the top of the bottom part of the case. And placed a red LED (my IR went dead) at the top and put a phototransistor in the bottom. This gives me about 180 counts per revolution. I put black electrical tape and black perminant marker to block the light going throug the gear.
Parts:
1. Servo - Preferrably a Futaba or Tower Systems
2. Drill
3. Photointerrupter - Rat. Shack #276-142 (NOT the package with the diode and transistor separate),
or any standard ‘U’ shaped photointerrupter - Available in most printers, VCRs, etc.
4. Hot glue
5. Potentiometer
Background:
For most of the time that I’ve been doing amateur robotics I’ve always wanted to have some kind of feedback from my motors. Such set up would allow for dead reconing and accurate turning and other manuvers without having to worry to much about in-code timing or battery voltage. Unfortunately I’ve never been able to figure out an effective and cheap way of setting up such encoders.
A few months ago I started a science fair project about using a maze solving robot. After a few days of planning it became apparent that I would need wheel encoders for one of the robot’s maze solving algoritms. I briefly considered putting an interrupter across the spokes of the wheels. I soon ditched that idea because it was unreliable.
Soon after I realized my need, I saw a brief show-and-tell at the Triangle Amateur Robotics club. Ken Boone showed how he was going to add a built-in encoder to his micro servo. He planned on drilling holes through the metal gears, then positioning a photointerrupter across the gear to count the holes. I thought this was rather innovative, but didn’t consider the idea feasible for my application.
A month or so after that I picked up a copy of Karl Lunt’s Build Your Own Robot from the local library. We went camping that weekend, and most of my time was spent devouring the book. In it Karl describes how he added a built-in encoder to his servo, by setting up a photoreflector on the bottom most gear of the servo. Karl’s idea sounded more reasonable to me, because I thought that I would be able to change out the shaft gear after the modification was done. By being able to change the shaft gear I could convert the modified servo back to a regular servo at a moments notice. Ken’s idea sandwitched the shaft gear between the diode and transistor, thereby trapping the gear. It was only after I started the modification that I realized that Karl’s idea would do the same. Karl’s idea generated about 140 pulses per revolution, where Ken’s only generated 6 per revolution.
When I got home I quickly dismantled my servo and began to add an encoder ….
Modification #1 - The Frustration
Karl added an encoder to his servo by first drilling a hole into the case above the bottom most gear of the servo. Karl was using on of the standard Futaba servos. Fortunately my Futaba 3003s were exactly the same. The position of the hole was critical in the design. Karl using a Semens IR photodiode/phototransistor built-in pair. This allowed for a very small package. But, I didn’t have that luxery. My mission then was to find some good IR photodiodes and photo-transistors. Fortunatly the search was short lived.
Back in the early days of my robotics endevors I nievely bought 2 photointerrupters from Radio Shack. I had held the idea that they could be used in an IRPD setup. After learning of my mistake I had tossed the diodes and transistors into my ‘junk box.’ Amazingly enough the diodes and transistors were very small and perfect for my application. I drilled holes into the side of my servo case until I was able to position the diode/transistor pair above the bottom most gear. This is the gear that directly connect to the small gear on the motor. It is large, flat, and has a whole bunch of teeth.
I inserted the photodiode between the shaft gear and the bottom gear, and positioned the phototransistor directly above the bottom gear and below the top-middle gear. That way the infrared light would bounce off the bottom gear and hit the transistor.
I then took a ‘Sharpie’ marker and marked 1/4 to 1/3 of the bottom gear black.
I made sure that the mark was as dark as possible. The idea was to have the dark part absorbe most of the IR light, thereby preventing it from reaching the transistor, thereby causing a large change in the phototransistor’s resistance. Good idea in theory but hard to implement.
After much trial and error I was able to get the phototransistor to generate about a 50k resistance differenace between light and dark. At that point it was rather late at night and I decided to go to bed. I woke up that morning, and in my excitement I decided to hook it up again and test it. Being overly eager I bypassed the current limiting resistor on the diode and … *pop* it was dead.
That normally wouldn’t have been so bad, except for the fact that the only other IR photodiode I had, that was near the other’s size, was just slightly too big to fit between the bottom gear and the shaft gear. Oh well, I just decided I’d move it up beside the phototransistor.
I tired and tried with this setup to get an acceptable resistance difference, but alas and alack it wasn’t going to happen. At this moment of desperation I decided to take my Dremel tool and shave down the surface of the diode, so it would fit where the othe one had. Even after all of that I was still unable to get the desired change in resistance.
It was time for another idea …. Modification #2 - Finally!
To prevent the risk of premature baldness I decided to try out Ken’s modification idea. Actually, as I later found out, his idea is a bit different. See the note at the end of this article. Thankfully this worked much better. Below are the steps describing the modification:
1. Remove the main/shaft gear from the gear train. Around the perimeter of the gear, inside the line of teeth, you should be able to make out 6 small circualr formations, that were created when the gear was molded. This 6 circles make perfect outlines for drilling holes. Find a good bit for the job and put 6 holes though the gear.
2. Replace the gear back in the housing and drill a hole for the diode and transistor above and below the gear.
3. If you need more room you can shave the gear down a bit. Most gears have a ridge around the outside that can be shaved down to make more room. Place the photodiode and transistor above and below the gear so that they face each other. Hot glue them in place and make sure that they don’t hamper the movement of the gear.
4. Put a resistor in series with the the photodiode and power it up. Measure the resistance of the phototransistor as you turn the shaft. Make sure that your have the negative lead of your multimeter hooked up to the negative lead of the transistor and the same for positive. Reversing the leads causes the resistance to be very high. You should be able to get a sizeable resistance difference. Usually above 50k ohm. If the resistance doesn’t change that much you can try adding foil backing to the gear then puch holes through it.
5. Hook up the transistor to the schematic below.
6. Now, turn the shaft till you reach highest reistance. This means that it is not above the hole. Measure the resistance across the transistor and find a potentiometer that is about twice that value. Connect up the pots middle pin to Vin and one of the wipers the to the transistor ‘+’. Measure the voltage across ‘Vin’ and the pot/transistor connection. Turn the pot till the voltage is greater than 2.7 or so volts. Now turn the servo shaft till the diode/transistor pair is over a hole. The voltage at this point should read below 1.5v. Measure the resistance of the pot and insert a similar value in its plce. If you prefer to do this process by manual calculations use this formula to figure out the resistor value:
Vout = Vin (R2 (R1 + R2))
In this case Vin is 5v. R2 is equal to the resitance of the transistor and R1 is what you are trying to figure out. You will want the Vout to swing from at least 1.5v to 2.7v. You should probably check the specification of your chip to make sure that this voltage swing will generate a pulse.
This is a note I received from Ken. He’s clarifying his idea a bit. It seems that I got it a bit wrong.
My conversion uses the same bottom gear you started with. It has 8 holes through it and aluminum foil on the bottom to completely block the IR. There is around 320 pulses per revolution of the output gear. The detector in below the gear inside the bottom case and there is a ground down IR LED suspended above the gear. I get a sine wave output and I use [a custom] circuit for line detection… My goal is to do speed control on each drive servo so I can accurately go straight and accurately do several different sweep turns with the fire fighting robot. I also want to count the pulses to accurately rotate the robot and determine how far it has gone.
Well, there you have it! A nice compact encoder inside your servo. It’s just wating to be hooked up to your Stamp’s PULSIN command or your OOPic’s oCounter object.
Update - Modification #3 - Hopefully the last!
Well, after hours and hours of agonizing work I finally just gave up and decide to switch gears (pun intended). I finnally realized the obvious truth - 6 counts per wheel just won’t work! As per Ken’s hack I decide to move the photointerrupter to the bottom gear. I drilled 4 holes into it. Be careful not to crack the gear - I speak from experience. And drilled out the top of the bottom part of the case. And placed a red LED (my IR went dead) at the top and put a phototransistor in the bottom. This gives me about 180 counts per revolution. I put black electrical tape and black perminant marker to block the light going throug the gear.
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