Building robot drive trains tab robotics pdf

Some pointers that are worth mentioning, lowest cost can be found for components that have the greatest production volume for a competitive market with just enough saturation to make it worth while competing for. Being aware of the pointers and applying a bit of lateral thinking can yield some great results It also helps to be a skip rat.

The initial target audience is us the small home constructor so cost is a key consideration. A certain quantity of Industrial Class performance may be sacrificed to ensure that this is met whilst producing maximum flexibility. On cost effect and the home constructor I guess a final word is in order, the most cost effective solutions for most home constructors are what they have squirreled away or can source locally Often from Surplus.

Flexibility then is more cost friendly to the home constructor. The baseline for our performance requirements is in effect the current state of Darwin's Stepper System Driven art. That is to say we want to be able to use motors etc that will allow us to exceed this "baseline" specification whilst saving money by using motors that are more cost effective.

A Servo System without positional feedback quite clearly isn't a Servo System so we need to give this some thought. Positional feedback isn't simply an electronic problem. A universal controller will also have to be able to deal with the fact that it will need to be able to cope with each constructors preferred axis design and each constructors chosen resolutions of position.

Positional feedback that can directly read the absolute position of the tool head can do so without worrying a sensible amount about the mechanical tolerances in the drive train. This is because the drive train becomes part of the feedback loop.

It follows then that positioning of the tool head using the servo control method continues until the directly read position tool head position exactly matches that required. Printers that use a linear positioning strip along the carriage's length are a good example of this.

The print head is in effect the tool head of a printer. A printer though has very limited degrees of freedom and is mechanically speaking a strictly controlled environment. Production or Amateur fabbing and CNC work on the other hand requires many more degrees of freedom and flexibility not to mention the probability of a contaminated working environment, often making this method cost prohibitive if not impractical.

It can also be challenging to achieve the required degree of resolution. Precision implementation of mechanical drive trains is also often unavailable to the home constructor. Direct positional feedback of a tool head with sufficient resolution, precision and accuracy is actually quite difficult due to physical limitations. It is far too easy to limit the degrees of freedom or foul the tool head with the measuring method.

An alternative that has only recently become available and economicaly viable is Laser Navigation. The same techniques used in Laser Mice to detect how much desk surface the mouse has traveled over. Is it possible to use a cheap webcam looking at the toolhead tip to control position more precisely?

As a simple proof-of-concept, consider this camera watching several LEDs on a helicopter, using a single camera to estimate the XYZ position of the helicopter, and using that to drive the helicopter towards the desired position: RCgroups: "My tricopter design autonomous indoor hover etc. This is a kind of optical position control, talked above.

Mouse sensors are simple cameras with about 32x32 vision area. Their internal software processing optical pattern very quickly and output as TTL signals. This sensors are available for sale [5] or can be salvaged from old mice.

Annie Ogborn says [6] : I also just made a cool discovery. You know the rubbery ferrite fleximble magnet material? It's available in sheets to make 'fridge magnet' type things, with an adhesive backing. Any crafts store. Positional feedback that is a guestimate based upon how much the motor drive shaft has rotated can with high precision mechanics give as good if not better results than Tool Head Positional Feedback.

It can also yield fairly high resolutions of tool positioning on a very cost effective basis. However, unlike direct tool head positional feedback, the drive train does not form part of the feedback loop. Where the drive train and gearing have any degree of position error the motor mounted encoder will be oblivious to this.

We are in effect saying that if the motor shaft has traveled x distance then the tool head must have also traveled x distance. Where y is the error introduced by poor mechanical precision in the drive chain. Clearly the greater the error or y is the less precision our drive train has and therefore our tool head positioning. Please note that I have taken the liberty of embedding copies of data sheets and application notes here where ever possible. This only to ensure that most of these links stay live with the minimum of link maintenance allowing for the fact that manufacturers quite reasonably change specifications, retire product lines and reorganize their sites.

For the latest copy of this type of data please refer to the manufacturer. From RepRap. Motors are classified as:. AC alternating current motors are rarely used in mobile robots because most of the robots are powered with direct current DC coming from batteries. Also, since electronic components use DC, it is more convenient to have the same type of power supply for the actuators as well.

So, I will not explain about AC motors here. A motor controller is an electronic device that helps microcontroller to control the motor. Motor controller acts as an intermediate device between a microcontroller, a power supply or batteries, and the motors. The motor controller, on the other hand, can provide the current at the required voltage but cannot decide how the motor should run.

Thus, the microcontroller and the motor controller have to work together in order to make the motors move appropriately. Usually, the microcontroller can instruct the motor controller on how to power the motors via a standard and simple communication method such as UART or PWM. Also, some motor controllers can be manually controlled by an analogue voltage usually created with a potentiometer.

The physical size and weight of a motor controller can vary significantly, from a device smaller than the tip of your finger used to control a mini sumo robot to a large controller weighing several Kg.

The size of a motor controller is usually related to the maximum current it can provide. Larger current means larger size. Since there are several types of motors, there are several types of motor controllers different type of motor requires different type of controller :.

A brushed DC motor is one which uses two brushes to conduct current from source to armature. Brushed DC motors are widely used in applications ranging from toys to push-button adjustable car seats.

Brushed DC BDC motors are inexpensive, easy to drive, and are readily available in all sizes and shapes. A Brush DC Motor consists of two magnets facing the same direction, that surrounding two coils of wire that reside in the middle of the Brush DC Motor, around a rotor.

The coils are positioned to face the magnets, causing electricity to flow to them. This generates a magnetic field, which ultimately pushes the coils away from the magnets they are facing, and causes the rotor to turn.

The Brush DC Motor has two terminals; when voltage is applied across the two terminals, a proportional speed is outputted to the shaft of the Brush DC Motor. A Brush DC Motor consists of two pieces: the stator which includes the housing, permanent magnets, and brushes, and the rotor, which consists of the output shaft, windings and commutator. The stator generates a stationary magnetic field that surrounds the rotor.

The rotor, also called the armature, is made up of one or more windings. When these windings are energized they produce a magnetic field. The magnetic poles of this rotor field will be attracted to the opposite poles generated by the stator, causing the rotor to turn.

As the motor turns, the windings are constantly being energized in a different sequence so that the magnetic poles generated by the rotor do not overrun the poles generated in the stator. This switching of the field in the rotor windings is called commutation. Unlike other electric motor types i.

Instead, the commutation of the windings of a BDC motor is done mechanically. A segmented copper sleeve, called a commutator, resides on the axle of a BDC motor. As the motor turns, carbon brushes slide over the commutator, coming in contact with different segments of the commutator.

The segments are attached to different rotor windings, therefore, a dynamic magnetic field is generated inside the motor when a voltage is applied across the brushes of the motor. It is important to note that the brushes and commutator are the parts of a BDC motor that are most prone to wear because they are sliding past each other.

Limitations: In addition to the audible whine from the commutator brushes, these motors create a lot of electrical noise which can find its way back into other circuitry and cause problems. By the term controlling I mean both direction control and the speed control. The direction of the DC motor can be reverse by simply reversing the polarity of the battery connection. The speed of the motor can be control by changing the voltage level and dc voltage level can be changed by PWM signal.

For higher voltage level speed will be higher and for lower voltage level speed will also be lower. Practically, Drive circuits are used in applications where a controller of some kind is being used and speed control is required.

The purpose of a drive circuit is to give the controller a way to vary the current in the windings of the BDC motor. The drive circuits discussed in this section allow the controller to pulse width modulate the voltage supplied to a BDC motor. It is more efficient way to vary the speed of a BDC motor compared to traditional analog control methods. In some cases the motor only needs to spin in one direction then a single switch topology with PWM modulation can be used to vary the voltage applied to the motor and thus to control its speed.

The higher the PWM duty cycle, the faster the motor will go. Note that in the circuit there is a diode across the motor. BEMF is generated when the motor is spinning. When the MOSFET is turned off, the winding in the motor is still charged at this point and will produce reverse current flow. D1 must be rated appropriately so that it will dissipate this current. Resistors R1 and R2 in the Figure are important to the operation of the circuit. R1 protects the microcontroller from current spikes while R2 ensures that transistor is turned off when the input pin is tristated.

When positioning is required or when both directions of rotation are needed most robots need a full H-bridge with PWM control is used. The H-Bridge is a 4-transistor circuit that allows you to reverse the current flow to the motor.

A H bridge is an electronic circuit that enables a voltage to be applied across a load in either direction. These circuits are often used in robotics and other applications to allow DC motors to run forwards and backwards. An H-bridge is a transistor-based circuit capable of driving motors both clockwise and counter-clockwise.

To understand this, the H-bridge must be broken into its two sides, or half-bridges. Referring to Q1 and Q2 make up one half-bridge while Q3 and Q4 make up the other half-bridge. Each of these half-bridges is able to switch one side of the BDC motor to the potential of the supply voltage or ground.

When Q1 is turned on and Q2 is off, for instance, the left side of the motor will be at the potential of the supply voltage. Turning on Q4 and leaving Q3 off will ground the opposite side of the motor.

The switching elements Q Note the diodes across each of the transistor D1-D4. These diodes protect the transistors from current spikes generated by BEMF when the transistors are switched off. The top-end of the bridge is connected to a power supply battery for example and the bottom-end is grounded. A capacitor can be used with parallel to the diode. But it is optional. The value of these capacitors is generally in the 10 pF range. The purpose of these capacitors is to reduce the RF radiation that is produced by the arching of the commutators.

The basic operating mode of an H-bridge is fairly simple: if Q1 and Q4 are turned on, the left lead of the motor will be connected to the power supply, while the right lead is connected to ground. If Q2 and Q3 are turned on, the reverse will happen, the motor gets energized in the reverse direction, and the shaft will start spinning backwards.

In a bridge, you should never ever close both Q1 and Q2 or Q3 and Q4 at the same time. If you did that, you just have created a really low-resistance path between power and GND, effectively short-circuiting your power supply. There are many different models and brands of H-Bridge IC is available.

It has two bridges, one on the left side of the chip and one on the right, and can control 2 motors. It can drive up to 1 amp of current, and operate between 4. The small DC motor generally used in robot bots can run safely off a low voltage so this H-bridge will work just fine.

Small motors are engineered for applications where compactness is valued over torque. While there are small high-torque motors, these tend to be expensive because they use rare earth magnets, high efficiency bearings, and other features that add to their cost. Large motors may produce more torque, but also require higher currents. Therefore, match the size of the motor with the rest of the robot. When decided on the size of the motor, compare available torque after any gear reduction.

Gear reduction always increases torque. The increase in torque is proportional to the amount of gear reduction: if the reduction is , the torque is increased by about three times but not quite, because of frictional losses. As you already know DC motor must not connect directly to arduino pin because it can burn your arduino. So you must connect a transistor between arduino and motor. Let's first control a small DC motor using single transistor.

Using single transistor you know only speed can be control. PWM is used to control speed of a DC motor. Connect your circuit as Figure Arduino PWM pin must be connected to the base pin of transistor. We can control both speed and direction now. Pin 9 is used as PWM pin and a switch is added to control the speed. L is a dual H bridge IC.

So, you can control two motor by single IC. Connect two motor to the IC as like figure 5 and use the following code. Make modification according to your requirement. Geared DC motors can be defined as an extension of DC motor which already had its Insight details demystified before.

A geared DC Motor has a gear assembly attached to the motor. The speed of motor is counted in terms of rotations of the shaft per minute and is termed as RPM. The gear assembly helps in increasing the torque and reducing the speed. Using the correct combination of gears in a gear motor, its speed can be reduced to any desirable figure. This concept where gears reduce the speed of the vehicle but increase its torque is known as gear reduction.

This Insight will explore all the minor and major details that make the gear head and hence the working of geared DC motor. The DC motor works over a fair range of voltage. The higher the input voltage more is the RPM rotations per minute of the motor.

The working of the gears is very interesting to know. It can be explained by the principle of conservation of angular momentum. The gear having smaller radius will cover more RPM than the one with larger radius. However, the larger gear will give more torque to the smaller gear than vice versa. The comparison of angular velocity between input gear the one that transfers energy to output gear gives the gear ratio.

When multiple gears are connected together, conservation of energy is also followed. The direction in which the other gear rotates is always the opposite of the gear adjacent to it.

Hence the gear having more torque will provide a lesser RPM and converse. In a geared DC motor, the concept of pulse width modulation is applied. For example, an unloaded DC motor might spin at rpm and provide 0. The motor will now be able to move significantly more weight at a more reasonable speed.

In a geared DC motor, the gear connecting the motor and the gear head is quite small, hence it transfers more speed to the larger teeth part of the gear head and makes it rotate. The larger part of the gear further turns the smaller duplex part.

The small duplex part receives the torque but not the speed from its predecessor which it transfers to larger part of other gear and so on.

DC gear motor can be controlled exactly the same way DC motor control. Limitations: This is especially a problem with low-cost plastic gear trains used with low-voltage motors. The extra resistance can make these gear-trains balky at low speeds. Brushless DC BLDC motors are called by many names: brushless permanent magnet, permanent magnet ac motors, permanent magnet synchronous motors etc.

The confusion arises because a brushless dc motor does not directly operate from a dc voltage source. However, as we shall see, the basic principle of operation is similar to a dc motor. A BLDC has a rotor with permanent magnets and a stator with windings. It is essentially a dc motor turned inside out. The brushes and commutator have been eliminated and the windings are connected to the control electronics.

The control electronics replace the function of the commutator and energize the proper winding. As shown in the animation, the winding are energized in a pattern which rotates around the stator.

The energized stator winding leads the rotor magnet, and switches just as the rotor aligns with the stator. There are no sparks, which is one advantage of the BLDC motor. The brushes of a dc motor have several limitations; brush life, brush residue, maximum speed, and electrical noise.

BLDC motors are potentially cleaner, faster, more efficient, less noisy and more reliable. However, the BLDC motor requires electronic control. There are two types of Brushless RC motors, inrunners and outrunners. The permanent magnets of inrunner brushless motors are positioned on the inside of the electromagnets.

An outrunner brushless motor has the permanent magnets on the outside of the electromagnets. The faster a motor spins, the more efficient it is. Inrunner motors turn very fast and are much more efficient than outrunner motors. The downside of an inrunner is the added parts that can and do fail.

The gears get stripped, and the gearbox shafts are easily bent. It can also be an obstacle when mounting the gearbox motor combination for your RC airplane neatly, especially under a cowling. The mechanics of a brushless motor are incredibly simple. The only moving part is the the rotor, which contains the magnets. Where things become complicated is orchestrating the sequence of energizing windings. The polarity of each winding is controlled by the direction of current flow.

The animation demonstrates a simple pattern that controllers would follow. The trick is keeping this pattern in sync with the speed of the rotor. There are two widely used ways this can be accomplished.

Most hobby controllers measure the voltage produced back EMI on the un-energized winding. This method is very reliable in high velocity operation. As the motor rotates slower, the voltage produced becomes more difficult to measure and more errors are induced. Newer hobby controllers and many industrial controllers utilize Hall effect sensors to measure the magnets position directly. This is the primary method for controlling computer fans. The control of the brushless DC motors is very different from the normal brushed DC motor, in that it this type of motor incorporates some means to detect the rotors angular position or magnetic poles required to produce the feedback signals required to control the semiconductor switching devices.

Using Hall effect sensors, the polarity of the electromagnets is switched by the motor control drive circuitry. Then the motor can be easily synchronized to a digital clock signal, providing precise speed control. Brushless DC motors can be constructed to have, an external permanent magnet rotor and an internal electromagnet stator or an internal permanent magnet rotor and an external electromagnet stator. Brushless dc motor are actually three phase ac motors.

To controll the speed an electronic speed control or ESC is used. Brushless ESC systems basically create a tri-phase AC power output of limited voltage from an onboard DC power input, to run brushless motors by sending a sequence of AC signals generated from the ESC's circuitry, employing a very low impedance for rotation. Brushless motors, otherwise called outrunners or inrunners depending on their physical configuration, have become very popular with "electroflight" radio-control aeromodeling hobbyists because of their efficiency, power, longevity and light weight in comparison to traditional brushed motors.

However, brushless AC motor controllers are much more complicated than brushed motor controllers. The correct phase varies with the motor rotation, which is to be taken into account by the ESC: Usually, back EMF from the motor is used to detect this rotation, but variations exist that use magnetic Hall Effect or optical detectors.

Computer-programmable speed controls generally have user-specified options which allow setting low voltage cut-off limits, timing, acceleration, braking and direction of rotation. Reversing the motor's direction may also be accomplished by switching any two of the three leads from the ESC to the motor.

To handle more power, the ESC needs to be larger, heavier, and is more expensive. This determines the current rating you should look for in an ESC. Always choose an ESC with a current rating that is higher than what you need. The 10A ESC will probably overheat and cook, even if you only fly at half throttle. Choosing the correct type and identifying the minimum current rating are the two big steps.

The next choices depend on your preferences. All ESCs have voltage limits. Some even have more than one! What is your battery voltage? Choose an ESC that is designed to work with an equal or higher voltage. Some ESCs are designed for low voltages below 13V , some for medium voltages below 25V , and some for high voltages above 25V.

However their disadvantage is that they are more expensive and more complicated to control. Brushless motors designed for autonomous and remote control aircraft and vehicles typically require a separate controller. These are typically of the sensorless type and use standard servo type pulsed signals for speed control. Controlling BLDC motor is very easy. Most of the ESCs need a 50Hz frequency i. The values between them give you a variation in speed. To achieve that all the circuit must be powered from an external power supply connected directly to the ESC and not via the Arduino, which will be powered by the BEC circuit of the ESC.

The rest of the circuit is pretty easy: from pin 9 of the Arduino we have the signal for the ESC, and into pin 0, the voltage reading from the potentiometer comes in. Sometimes ESC needs calibration and in terms of ESCs, calibration means to set the max and min speeds of the motor in relation to the max and min width of the PWM signal sent by the Arduino. The ESC sets the speed of the motor depending on the ratio of high to low signals.

Calibration involves programming the ESC to understand the PWM waves corresponding to the stop and maximum speeds of the motor. The default signal range for most servo motors and ESCs is a high signal width between and microseconds over a repetition period of 20 milliseconds assuming a 50hz PWM signal.

For the quad copter, however, we want the range to be as wide as possible to allow for greater incremental control of the motor.