1.0 Motors

The chosen motors are:

• Finger grasping – Cytron IG32E-264K at a cost of USD $57.03 from RobotShop
• Finger sliding – Cytron MO-SPG-30E-20K at a cost of USD $22.58 from RobotShop

Both motors meet the torque speed requirements for this project.

1.1 Finger Grasping Actuation

From the initial budget, the budget set for actuators was set at $325.  The torque requirement needed for gripping was also calculated and each finger would require 4 Nm at its output to lift a 4 kg object using only the fingertips.  The evaluation criteria states that the device grasp as fast as possible and that the client would have the highest satisfaction if the device cost was less than $700.

Looking on several websites that sold motors, the following DC motor was chosen: Cytron IG32E-264K at a cost of USD $57.03 from RobotShop.  The specifications are as follows:

At 12 V with gearing included, the stall torque would be 20.7 Nm (without consideration of mechanical efficiency).  A motor with a high stall torque is also beneficial because it lowers the gear ratio required, which leads to the use of less space for gearing.  The motor also comes with its gearing, which contributes to its high torque.  Given the high stall torque of this motor addition gear reduction is not required.  During grasping, the motor will be exerting high torque at low speeds and therefore, the motor will be operating near the stall torque.  The torque here is more than sufficient even with mechanical efficiency losses accounted for.

For finger actuation before grasping occurs, there is a low load and therefore the motor will be operating closer to the no load speed.  The no load speed with gear reduction is 27.7 rpm (without consideration of mechanical efficiency).  Split among three output shafts, the speed would be 9.21 rpm.  Even with the load of the fingers accounted for, this is still acceptable. based on requirements and evaluation criteria.

While the torque is a lot higher than required, this motor was only more expensive than a motor of one size smaller by less than three dollars.  The electrical characteristics were also identical.  With a higher motor torque, the grasper would be comfortable in lifting objects even heavier than 4 kg, which is another benefit based on the evaluation criteria.

1.2 Finger Sliding Actuation

The motor required here does not require a large torque since motion in this degree of freedom is not expected to handle heavy loads.  This motor only requires enough torque to overcome the friction of the finger on the power screw that it is mounted on.  Therefore, a low-cost motor (relative to the grasping motor) will suffice.  The DC motor of choice is the Cytron MO-SPG-30E-20K at a cost of USD $22.58 from RobotShop.

The specifications are as follows:

The consistency of the stated specifications do not match the previous motor because the information that was provided by the manufacturer was not consistent.

An initial estimate of two 200 g fingers sliding on a 1/2” diameter power screw with a steel-steel sliding interface (friction coefficient of 0.8) shows that only 20 mNm of torque is required.  Therefore, this motor meets the requirement.  The power screw also has 13 threads per inch, and at the rated 185 rpm, the power screw would slide linearly at 6 mm/s.

1.3 Motor Type

DC and servo motors are motors that are suitable for the project.  An investigation between DC and servo motors showed that many manufacturers of motors did not provide the necessary specifications for choosing a servo motor in terms of the values of the no load, stall torque, and stall current.  While servos generate a lot of torque relative to their size when compared to DC motors, it is not feasible to use a servo motor because of the unknowns in the specifications.  It is possible to buy several servos and test them, but this is costly and time consuming.  At this stage in the project, the team is not able to commit the time to investigate further.

Comparison of the cost of the grasping motor relative to motors from Maxon Motor, a supplier of motors for UBC Thunderbots, shows that the price of a motor of the same power output costs approximately the same amount.  However, a fundamental difference is that the motor from Maxon does not include a gearbox, while the chosen motor for this project does.

2.0 Microcontroller

The microcontroller of choice is the Arduino Diecimila.  The primary motivator for using this board is because the UBC Thunderbots team already has this board and is currently unused.  The Thunderbots team has generously provided the board for use on this project.

The MCU will allow the turning the motors on and off, changing motor direction, changing motor speed, and reading encoder feedback.

3.0 Voltage Regulation for Encoders

Since the power supply is likely going to be run at 12 V, the encoders will require voltage regulation down to 5 V (the operating voltage of the encoder for the motors).  The chosen voltage regulator is the MC78L05A at a cost of USD from Digikey.  According to the typical application diagram of the datasheet, two capacitors will also be required (0.33 μF and 0.1 μF).  The cost of these are less than $1.

The voltage regulator will be wired up in this way as the datasheet shows:

The input will be from the 12 V source, and the output will be to the encoders.

4.0 Motor Driver

To control the motors, an H-Bridge is to be used.  However, due to the different current flows for each motor, one motor driver will require a high current capacity while the other one will not.  The high current capacity motor driver consists of a driving circuit using two half-bridges.  The low current capacity motor driver is made up of 1 IC.

4.1 Finger Grasping Motor Driver

To design the H-Bridge for controlling the motor, an L6205 motor driver will be used.  Each of them costs USD $11.36 from Digikey and can withstand a continuous current of 2.8 A and a peak current of 5.8 A and can be interfaced with the microcontroller with PWM inputs.  The driver can also be run in a parallel mode to double the current ratings by using both of its full bridges together.  The datasheet provides an application circuit as follows:

Output pins from the Arduino can turn the motor on and off through the enable pins.  Direction is controlled by adjusting the voltage polarity between IN1 and IN2.

There are also two output pins for current sensing.  By using it to measure the current draw, the microcontroller program can determine what the torque is through the use of the motor curves provided in the motor specifications.

4.2 Finger Sliding Motor Driver

The motor driver of choice is the L293D and each one costs $2.50 from Digikey.  Bidirectional control of a DC motor can be achieved by adapting the application circuit from the datasheet:

The 1A and 2A pins determine the direction of the motor or the motor deceleration.  A truth table is provided in the datasheet.

5.0 Power Supply

To power the grasper, a power supply plugged into the wall will be used.  It is expected that the future robot will have a power source of its own which will power the hand as well.  Therefore, for the purposes of testing the grasper for this project only a wall power supply will be required.

Looking at the maximum electrical power consumption of the two motors, the total comes to ~65 W (12 V * 5 A + 12 V * 0.41 A).  The other components also consume power and so the final power consumption will be higher.  To compensate, a 12 V power supply with a maximum wattage and current of 84 W and 7 A was selected.  The part number is CENB1090 and each costs USD $70.48 from Digikey.

6.0 Power Supply Connector

The power supply chosen has one port that uses a Molex Mini-Fit Jr. Receptacle.  Therefore, a proper receptacle must be used so that proper wire routing can power all the circuitry.  This also means that the future robot will also require a matching plug for this receptacle.

7.0 Fuses

Fuses will also be used as a redundant mechanism to stop the motor in case the current draw is too high.  The primary method of stopping the motor is through the current sense pins on the motor driver.

The last two were pretty artsy, and I felt that as fun as it was, I never thought I was good enough to actually pursue it as a career (and I still think so).  It could also be because the way I was raised taught me that a career in the fine arts would be useless in the real world.  I’m not saying that it is, but that kind of thought very often prevailed when students made career decisions.  I forgot about pursuing web and graphic design as a career and left it alone as a hobby.  I still do this today and am still quite satisfied with this decision.

When it came to math and the sciences, I did really well in high school (which doesn’t really say much as university showed).  I thought about what I could do with these.  The few career choices that came to mind were careers like: computer programmer, mathematician, physicist, chemist etc.  Surprisingly, the thought of being an engineer never came across my mind.  At the time, I had only heard the word being used, but never knew what it was.

I talked to a few of my friends about what they were going to take in university.  A few of them told me they were going into engineering.  Intrigued, I looked to find out more.  The gist of what I got from the Internet was that “an engineer applies math and physics to design things that makes life better.”  I also found that UBC and SFU both had information sessions for engineering.  I visited the information session at SFU.

There, I got a whole slew of information about engineering.  Each department in the faculty and a lot of student teams set up booths explaining what they were all about.  To entice potential incoming high school students, they displayed a lot of cool things that they designed like helicopters, computer games, and just fancy gadgets.  At that point, I thought to myself, “wouldn’t it be so cool to build something like that?”  I had no idea what kind of work the projects entailed.  I was really focussed on the final product.  I had no concept of what happened in between.  To me, engineering seemed like the first half of that picture at the top.

I also went to the mechatronics information session there and that’s where I first learned about that discipline.  I really liked the idea of blending mechanical engineering, electrical engineering, and computer engineering together.  It seemed like a great combination.  The first thing that popped into my juvenile mind after was, “Cool! I can build robots like the ones people see in movies!” That kind of thought was quite naive, but regardless, it was the thought that dominated at the time.  Such was the thinking of a child.

That pretty much convinced me to go into engineering.  The next question was, “what school should I go to for that?”  I knew I didn’t want to go too far away from home, so I really only had two options: UBC or SFU.  I thought long and hard for a long time and eventually decided on UBC.  At the time, I didn’t know if I made the right choice, but I did have two reasons for doing so.

One was the international reputation that UBC commanded.  I could go to Hong Kong and talk about UBC and people would know what I’m talking about.  Even people in Germany have heard of UBC.  SFU?  Not so much.

UBC also let me live in a home away from home in a way.  I lived in Port Coquitlam and the commute from there to UBC took 1.5 hours each way.  In order to avoid having to deal with that intense time sink every day, I got a spot in residence.  That would teach me to live independently while having support from my parents.  To this day, I am still glad I made that choice.  I really did learn to live independently.  In fact, I wrote this while I was still in Germany, thousands of kilometers away from family.

After making my choice of doing engineering at UBC, I got acquainted with the UBC campus and the residences and so once school started I was prepared.  I was totally taken aback to how university life was different, but I managed to adjust.  In the end, I went through first year with pretty good grades (high enough to be guaranteed entry into whatever specialization I wanted).  Even then, I still had no idea what engineering was all about.  The stuff I learned was all just theory.  If anybody asked me to design anything, I would have been clueless.  I still had not bridged the gap between theory and practicality.

During the summer after first year, it became time to make yet another career-defining choice: What discipline in engineering should I do?  I researched pretty much every single one and I was left with three choices: mechanical engineering, engineering physics, and computer engineering.  Even though I researched them all, I had no idea what they were all about.

I remembered the mechatronics info session I went to a year ago.  The thought of building a robot still floated in my mind.  As naive of a thought as that was, it got me really considering the mechatronics option in mechanical engineering.  I looked on the Mech website and found out about the Mech 2 program.  I really liked the way the curriculum was organized.  I thought it had a great mix of learning theory and actually applying that theory to projects.  I agreed with its goal of breaking down the artificial barriers between subjects.  At that point, my mind was set on doing Mech.  I had already missed the deadline for going straight into mechatronics from first year, but I felt that I could get in after second year.

Second year in mechanical engineering really taught me what it meant to be a mechanical engineer.  I found out that Mech wasn’t just about cars, planes, trains, and boats, and that those things were only a small facet of a discipline with an abundance of possibilities.  I found out that not every single engineer does design – some are in research, product safety, and even maintenance.  Despite those other kinds of engineers, my primary interest still lay in design.  I learned the process of actually going from just an idea to a final product – something that no info session ever taught me.  In fact, some engineering curricula programs don’t teach it either.  They seem to half-expect its students to figure out on their own and know how to apply the theory themselves.  I had really interesting and useful classes that I could really apply in engineering designs.  I could go on forever here, but the important thing is that after knowing all this, I still wanted to stay.  It’s what I wanted to do.  I wanted to design and build things.  Every now and then someone will talk about some other profession having a better salary, but that doesn’t matter.  Interest trumps that.

Looking back, I found it quite absurd as to how I ended up in mechanical engineering, but it’s what happened.  I started off knowing next to nothing about the profession, and now I can’t think a better option.  I am now in my fourth year of mechanical engineering, and if someone gave me the chance to go back and pick something else, my choice would not change.

Eight months is a long time to be away from family.  It will be my first time going away for so long.  Before this, I had only been living at UBC and with my parents.  This is a big step for me to learn how to be independent.  My parents were always kind enough to do all the cooking and chores.  I guess I am kind of spoiled.

It helps that I am going with a friend of mine from UBC already.  Having a companion helps so much more than being alone.

During these eight months, I will be working for a company named Robert Bosch, which is based out of Stuttgart, Germany.  My work will be on microelectromechanical systems (MEMS).  At the same time, being in Germany enables me to travel around Europe as much as I can (provided that I can afford it first).  I am hoping this will be an extremely positive experience.

Some pictures:

Flying Above the Clouds

My First Stop: Heathrow International Airport

Concept

I was given the circuit shown below (from Microelectronic Circuits 6th Edition by Sedra and Smith):

I was asked for the voltage of the output terminal and the current flowing through the diode.  Finding the current was easy, but I didn’t quite get the voltage.  I thought to myself, “from what two points do I measure the potential difference?”, because after all, voltage measurements are always taken relative to two points.  It took me the longest time before I could just use ANY two points and the measure the voltage.

What I figured is that, for part a, since the diode is in forward bias, the diode will act as a short circuit.  I then can say that V – (-5) = 0, since the voltage difference along a short is 0.  From there, I can easily determine that V = -5.  I can apply that same logic to the other three.

Nodal Analysis

I was given the circuit shown below (from Microelectronic Circuits 6th Edition by Sedra and Smith):

From class, I remember the professor saying that since we cannot tell right away which diodes are on and off, we can only make an assumption about the states of the diodes and then apply the usual analytical techniques.  In the event that the result is unreasonable, we know that our assumption wrong.  I made my assumptions, but afterward, I didn’t know where to go next.  I had forgotten how to analyze these circuits.

Later, I realized that I could use nodal analysis.  Since I assumed the two diodes in part a were both on, I replaced them with short circuits.  I used the intersection right before the current splits as my node.  For part a, the current into the node was 3 V/5 kΩ = 0.6 mA.  The current leaving was some unknown current for diode 1, and a current for diode 2.  I could determine the current of diode 2 by taking the potential difference between the node and the -3 V output and dividing that by the resistance in the path.  The result was (0 – (-3))/ 10 kΩ = 0.3 mA.  From the nodal analysis equation 0.6 mA = current from diode 1 + 0.3 mA, the current from diode 1 is 0.3 mA.  And as for the voltage V, the potential difference between V and the -3 V output is equal to the voltage across the resistor, (0.3 mA)(10 kΩ) = V – (-3).  Therefore, V = 0.  Alternatively, I can use the potential difference between the node and the output, which equals zero, thus yielding the same result.

Boolean Logic

The next issue I had was determining the Boolean outputs of a diode logic circuit.  Again, the picture below is from Electronic Circuits 6th Edition by Sedra and Smith.

The issue I had here is that I did not know current sources have no voltage if there is no current running through them.  Looking back that sounded pretty intuitive.  Finding all possible combinations of inputs A and B for this logic circuit, we get the truth table below:

A B X Y
0 0 0 0
1 0 0 1
0 1 0 1
1 1 1 1

The outputs of X reveal an AND gate.  Setting A or B to a logic 0 in (a) will cause current to flow through that path.  The potential difference X – A or X – B (depending which one was set to 0), will yield 0, thus the output X will be zero as well.  When A and B are both set to logic 1, it causes the diodes to go into reverse bias, and node X will be powered by the current source.

The outputs of Y reveal an OR gate.  Setting A or B in (b) to have different inputs, either 1,0 or 0,1, will cause the diode to be in forward bias for the input that has the logic 1.  For the input with logic 0, the diode will in reverse bias.  Since one of the diodes is on, current will flow and output Y will yield a logic 1.  In the case of both diodes being off, no current will flow, thus causing output Y to yield logic 0.

Other Notes

In the case of multiple parallel diodes in the same direction, if one of them is on, chances are they are all on (provided that the voltage across them is positive).  However, there may be a case where one parallel diode will want to flow into the other because of a greater voltage difference.  For example, if there were two parallel diodes, one with a 3V input and the other with a 2 V input both going to the same node, current from the 3 V input will want to flow into the diode connected to the 2 V input.  This will cause a reverse bias in the diode connected to the 2 V input, thus turning it off.

There is also another case where there are parallel diodes with different output voltages.  If current is flowing toward it, they will all be on, so they will all be a short circuit.  Which path will the current go to?  The path with the larger current.