Finance provided by PayPal Credit. Terms and conditions apply. Credit subject to status, UK residents only, Modelsport Ltd acts as a broker and offers finance from a restricted range of finance providers, PayPal Credit is a trading name of PayPal (Europe) S.à.r.l et Cie, S.C.A. 22-24 Boulevard Royal L-2449, Luxembourg.Coming from Power HD is the new design waterproof HV servos LW-15MG and LW-30MG. Two servo models with different parameters for selection, 15kg/cm, 30kg/cm. With stable performance, high torque, stronger locking force double, less gear slop, double ball bearings, and 25T metal spline, they are ideal for RC crawler,boat, aircraft, etc.
Can servo motor run on 12V?
A 12V servo expects to operate on a minimum of 12V, and will have a long life. If you operate it at higher voltages it will operate proportionally faster and have a shorter life. Lead acid batteries maintain 12 volts when loaded, but they have 14 volts while being charged.
All payments are made in Euros. You can however browse the shop in other currency but the price then is not final. You will see the final Euro price in your shopping basket when paying for the product. The price is shown in “5. Send Order” part of the checkout form.
What are the strongest servos?
Overview. The HD-1218TH is one of the strongest standard-size servos we carry. Digital electronics combined with a coreless motor, titanium-shielded aluminum gears, and two ball bearings give this servo a distinct edge over typical standard-size servos.
Commonly servos run on 6 volts, but if you want to use a 2S LiPo receiver battery without a BEC, you need to choose a 7.4 V compatible servo, of which there are many alternatives available today. These servos can of course also be used with lower voltage, but their performance will be lower. Usually you find torque and speed figures for different voltages in the product information.While most servos are fairly well protected, it is only the special waterproof servos that are really made to survive even under water. If you do run a lot in wet conditions, a waterproof servo is the best choice for you.
If you feel that your product is defective please send us a email and we will let you know what to do next. Most items do not have a warranty as they will break in normal usage. For example if you hit something solid or take a larger tumble. Properly used items should easily last 3-12 months and during this period we are able to replace broken items if they have a manufacturing or material fault. We have years of experience with different makes and they all have different warranty policies. We have chosen to work with brands that have a good understanding how to serve their customers and you can be assured that you will get the best customer experience possible with us.
Please note that all parcels that are returned to us because customer has not picked it up / accepted it or paid customs fees of it will processed. Handling and shipping costs will be reduced from the refunds.The bigger and heavier your RC car is, the higher torque your servo should provide. It is important to choose a servo which is strong enough to handle the size and weight of your car. A normal 1/10 buggy or short course/stadium truck ideally needs a torque of around 10kg. A 1/10 on-road car will be alright with a little less, so around 8kg will be enough. Heavy 4WD trucks need 15kg and upwards, just like most 1/8 and larger vehicles.
We ship globally for example to Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Poland, Portugal, Romania,Slovakia, Slovenia, Spain, Sweden and United Kingdom, Norway, Turkey, Switzerland). And many other countries, just go to the checkout and see if you can checkout. If not just let us know and we will quote you a price as we are able to ship to any country in the world.Don’t worry you are safe with us! Most of our customers feel that they get excellent service in the rare occurrence that they encounter a problem with the product they have bought.
Are power hd servos good?
Power HD servos are far superior to Hitec servos. I have over 100 of these servos, been using them for over 10 years. Prior to using Power HD I used JR servos fairly exclusively. The Hitec servos I have tried ALL failed, wont use them ever again, even for throttle.
Some servos have full plastic servo cases, while others use a combination of plastic and aluminium, or full aluminium cases. Aluminium is used to improve heat dissipation, but this obviously adds a bit of weight over a plastic case. A full aluminium servo case eliminates flex for a more stable gear train and servo mounting points, which can be useful especially for heavier vehicles. Again, when you buy a quality servo, separate servo cases are available, should you need to replace it.After we receive the merchandise and verify its re-sellable condition, we will refund the money or give you a giftcard. Depending on the item this will take 1-3 days after we have received the item. If there is a delay we will contact you.
And remember if you run in to a problem or you feel that there is a problem with arrived products do not hesitate to contact us. We are here to help and we do our best to sort it out as soon as possible.
Always contact us before returning any product, we can not take responsibility for shipments that are sent without prior notice. Its very important that we can tell you how to mark the parcel so it will be processed in a timely fashion.
In the rare occurrence that the shipment has been damaged, please take pictures and send those to our customer service [email protected] immediately and we will make sure we get you sorted and back to enjoying our hobby. Also notify the courier bringing you the shipment that it has been damaged.All products listed as “in stock” are in our warehouse and ready to be shipped within the next day. Exceptions to this are pre-orders and possible, but unlikely, errors in stock numbers. All products include the VAT according to the destination country. Prices are subject to change without prior notice.
Is a higher kg servo better?
The bigger and heavier your RC car is, the higher torque your servo should provide. It is important to choose a servo which is strong enough to handle the size and weight of your car. A normal 1/10 buggy or short course/stadium truck ideally needs a torque of around 10kg.
We also accept returns after 14 days upto 50days, but we refund them with a giftcard to our shop so you can replace them with any products we sell. Please contact our customer service for details.Different servos come with different types of electric motors, the most common being: DC, coreless, and brushless. DC motors or the least expensive and best suited to less demanding applications. Coreless motors offer better power and reliability, and change direction more quickly and precisely. Brushless motors produce ultimate power, quicker torque, faster response, greater efficiency and are the longest lasting. Our shop automaticaly includes or excludes VAT percentage from the prices by the destination country. You can change the destination country from the checkout pages before you order. All our customers will pay the same price no matter the included VAT percentage. Companies who are buying with VAT numbers can buy products with exluded VAT according to destination country. A servo that is fast enough for the type of car is equally important, as a too slow servo will kill the steering feel. The smaller/lighter and faster your car is, the more important it is with a fast servo. On on-road cars you want a servo to be 0,1s or faster, to ensure you have the response needed for these cars. Going to off-road cars, a good 1/10 buggy servo should definitely have a speed of under 0,15s. The heavier the car is, the slower it will be in reaction, so you don’t need the most superfast servos, but the speed should be 0,2s or below for any type of car.Metal gears will improve the durability of a servo a lot. Metal gear servos recommended for most types of RC cars. For small and light weight cars or other low impact uses, you may choose a plastic gear servo. To save the gears, whether they are plastic or metal, it’s recommended that you use a servo saver. When you buy a quality servo, separate gear sets are available, should you need to replace the gears.
Different manufacturers use different servo output shafts, with either 23T, 24T, or 25T spline output shaft used on most small and standard sized servos. Some large scale servos use a larger output axle though, just like some small micro servos use a smaller output shaft. Because of this, it is important to check this what servo arm you have on your car, and if you need a different servo horn. Many cars include different servo horns for different servos.
We ship a lot of orders each day and rarely there are errors. We do however try to minimize the effect on the customer. Please contact us immediately if we have made an error and we will make it right.We accept returns of new, unused merchandise within 14 days from the delivery. Please contact us at [email protected] to receive return information. The customer is responsible for any return costs unless otherwise agreed to with our customer service. We try to return the money with the same payment method that was used to pay for the order, if its not possible we make a wiretransfer. You can also request a gift card if you are making a new order to speed up the process. We are answering emails everyday from the morning to the late evening, no matter if its Sunday or Christmas eve. We work in the EET time zone which is +2h from GMT. During the day our customer service is usually really fast and in the evening we are little slower due we have less people to answer questions. Easier things are usually handled within the hour and shipping related things we try to handle during the day so they can be shipped out the same day. Our goal is to answer all emails within 24h no matter how difficult the question is. The fastest way to get customer service is always to send us a email. Please include relevant pictures and a structured email with proper sentences so we can answer you faster. Take your time and read trough your email once before you send it to us. Below the standard size servos, there are “Mini”, “Micro”, and even super-small “Sub-Micro” sized servos, which are common on smaller 1/12 and 1/16 size vehicles. Above the standard size servos you find “Large Scale” or “XL” servos, which are much bigger in dimensions, and used for many large scale vehicles in 1/5 and 1/6 sizes.
Common MOSFETs require about 7 or 8 volts on their gates to properly turn them on. Because of this, most 4QD controllers have an internal supply of 9v – which gives nearly 8v on the MOSFET gate.
Motors are specified to run at a stated rpm at a particular applied voltage with a specified loading – that at which the motor takes its maximum continuous current.
What are the 3 types of servos?
Servo motors come in many sizes and in three basic types. The three types include positional rotation, continuous rotation, and linear.
Now if you view the terminal voltage of a 12v battery, with an oscilloscope, you will find that, when the controller draws chopped current from the battery, there is a squarewave of 2 volts amplitude shown. The battery may be 13v open circuit, but during the PWM periods when current is actually being drawn, the effective voltage is actually falling to 11 volts. If you want to know more about why there is a chopped current, see our circuits archive.There is another factor against 12v operation, MOSFETs need a good voltage to fully turn them on, so most of 4QD’s controllers use an internal 9v supply rail, which is adequate to ensure proper turn-on. However, there is not much difference between 9v and 12v. It does not take much current to be drawn from the battery before it drops 2v at its terminals. A small mount of extra drop in and wiring – and the 9v supply drops. After that, the available current from the controller drops quite quickly! Remember that the battery current is actually a chopped version of the motor current, so the inductance and resistance of the batteries and battery wiring all contribute to any voltage drop.If you run a 12v motor from 24v its current drain and speed will still depend on the mechanical loading. However under no load it will now run at twice the speed at which it originally ran with 12v. Heating in the motor is still related to the current – so you can still run it at its full rated mechanical load/current. However if the motor is badly balanced you may expect noise and vibration as the general construction may be inadequate for the faster speed. There may also be a problem with brush wear since the brushes are being asked to switch the current twice as fast. These effects are, however not very likely and usually the speed increase is quite OK.So it’s pretty difficult to fully use a 12v battery at high currents and get the full rated current out of the controller, as the 9v rail will drop and, with it, the available current.
We are often asked whether 12v or 24v is best for a battery powered motor system [the short answer is 24v], and also if we make a 12V motor controller.
Another consideration is that, if you put too much current through a permanent magnet motor, it is possible to slightly demagnetise the magnets. This is cumulative: the motor’s performance will drop slightly each time you do it. However, for battery motors, is is probably fairly safe to assume that, at the rated voltage, the current drawn when the motor is stalled will not reach this demagnetisation level. If you were to run a 12v motor off a 24v battery the stall current could then be excessive if it weren’t limited by the controller.Now electrical current causes heating. The motor, wiring and controller will all get hot which wastes power. The heat wasted is proportional to the square of the current multiplied by the resistance. Other things being equal, that would cause losses on 24v to be half those on 12v. so a 24v system is always better than a 12v system – provided you can physically fit two batteries. By the same token 36 or 48v would be even better [there is a page about using our controllers at 48v and above here].
If you increase the load, then the motor’s current consumption will increase and its speed will reduce. Obviously you are now exceeding the motor’s continuous rating so it will start to get hotter than it should. The greater the overload, the quicker the motor will heat so there is a time limit on such an overload. However it is usually safe to run a motor at a 300%-400% over current for, perhaps, a minute – although this will vary from motor to motor.
Consider also that a 12v battery may, when 80% discharged (a realistic level before recharging) has a terminal voltage (open circuit) of about 10.8v. So the PWM will be working from effectively 8.8 volts. So there is no way the 9v internal rail of the controller can stay at 9v! And that’s before we start to consider voltage drops in the battery wiring due to its resistance and inductance.Operation at high current from 12v can cause several problems, so 4QD are a little reticent about recommending this voltage. However our Porter, DNO, and Pro-160 models can operate on 12V. If you need a high current 12V controller then have a look at our new Pro-360.
What voltage is a 25kg servo?
7.4V FEETECH RC Servo 25KG Servo Motor 7.4V High Voltage High Torque Waterproof Full Metal Gear Digital Servo, Aluminium Case, Control Angle 180° for 1/8 1/10 RC Cars, TRAXXS x4, Baja, RC4WD, GTR and More.
Limits on motor speed are not simply the bearing quality. If you rev a motor hard enough – centrifugal force will take over and the rotor will fly to pieces. Also brush and commutator design is important. Depending on the design these will have a maximum switching rate and operating above this speed will cause tremendous brush arcing. In extreme circumstances this will generate severe noise transients which can destroy the controller. This is unlikely: we have only ever seen one customer do this: he was running 12v motors on 36v and blew two controllers! These motor limits are not things a controller manufacturer can really comment on: you need to consult the motor manufacturer. Lets put it in car terms for a minute, if you want to accelerate harder or have a higher top speed then you need more horsepower. In an electric vehicle the power usually comes from the battery, and is converted by the motor into energy. Electrical power is volts multiplied by amps so that 40 amps from a 12v battery is 480 watts. But 480 watts can also come from a 24v battery at a current of only 20 amps; so for any particular power, the higher the voltage, the lower the current will be. The amount of energy in the batteries is amps x hours x volts. Consider a 12v 60 Ampere Hour battery. Clearly this is exactly the same as two smaller 12v 30 AH batteries in parallel. But the total amount of energy in these two will not change whether we connect them in parallel or in series. So a 12v 60 AH battery can store exactly the same energy as a 24v 30 AH battery.The overheads on a 24v system are nowhere near as critical. The 2v drop, even 4v, will still take the battery supply nowhere near to the 9v rail. Motor resistances are also higher, so the extra effect of controller and wiring is less noticeable.
What voltage is a 25KG servo?
7.4V FEETECH RC Servo 25KG Servo Motor 7.4V High Voltage High Torque Waterproof Full Metal Gear Digital Servo, Aluminium Case, Control Angle 180° for 1/8 1/10 RC Cars, TRAXXS x4, Baja, RC4WD, GTR and More.
If you overload the motor, its current rises in the same way whether the motor is running from 12v or 24v. However on stall the current from 24v could be twice that from 12v, so the motor could get four times as hot (heating is proportional to the square of the current). This however won’t happen when you are using a good controller as the controller will limit the current to its designed value. Also the controller varies the voltage on the motor so you are probably not going to use the motor at full voltage in any case.Therefore, provided you chose a controller suitable for the motor you use, you can usually run a motor 12v motor from a 24v battery with no effect except that full speed is doubled.
There is one caveat on this. The motor is an inductive device and the commutator and brushes are a mechanical, switch. Such a mechanical switching system will have a limit on the maximum rate at which is can work and if this is approached, the commutation breaks down. Exactly what the limits are, I would not like to say but one effect is noise – and extreme noise can, on occasion, cause a controller to fail. The effect is quite rare – but beware of excessive over-revving.
Consider the stall current of a motor, for instance, the Sinclair C5 motor. On a freshly charged battery, its stall current can be 120 amps. This is limited by the motor resistance, the resistance of the leads supplying it and also on the internal resistance of the battery. Adding anything else into this loop will increase the loop’s resistance. So, if you have a system that works nicely without a motor speed controller, adding a motor speed controller will inevitably reduce its peak performance. Many 12v systems are simply not designed for operation with a speed controller and adding this will greatly reduce the performance.We have a huge range of RTR products on display in our showroom, why not come, pay us a visit, meet our passionate and experienced team and talk all things RC. We can’t wait to see you.The WH-65KG PowerHD servo is a digital standard servo with twoball bearings. The product is powered by direct current from6.0to8.4 Vand has a gear made of copper and steel. Operating temperature of the servo is from- 20\u00b0Cto60\u00b0C. The maximum torque is65 kg*cm. The angle of rotation of the servo is 180\u00b0.12-channel servo controller with support for digital and analog signals. The user can control servos via: PC application (USB), serial interface and simple script language. 18-channel servo controller with support for digital and analog signals. The user can control servos via: PC application (USB), serial interface and simple script language. The WH-65KG PowerHD servo is a digital standard servo with two ball bearings. The product is powered by direct current from 6.0 to 8.4 V and has a gear made of copper and steel. Operating temperature of the servo is from – 20°Cto 60°C. The maximum torque is 65 kg*cm. The angle of rotation of the servo is 180°.The WH-65KG PowerHD servo is a digital standard servo with two ball bearings. The product is powered by direct current from 6.0 to 8.4 V and has a gear made of copper and steel. Operating temperature of the servo is from – 20\u00b0Cto 60\u00b0C. The maximum torque is 65 kg*cm. The angle of rotation of the servo is 180\u00b0.6-channel servo controller with support for digital and analog signals. The user has at\u00a0his or her disposal, the control of the servos through: the PC app (USB), serial interface and a simple script language.The WH-65KG PowerHD servo is a digital standard servo with twoball bearings. The product is powered by direct current from6.0to8.4 Vand has a gear made of copper and steel. Operating temperature of the servo is from- 20°Cto60°C. The maximum torque is65 kg*cm. The angle of rotation of the servo is 180°.
The Power HD LF-13 MG is excellent for a 30cc or 50cc plane. This Digital servo has metal housing around the middle which allows for heat to rapidly dissipate. With double ball bearings you will have smooth movement and powerful torque when you need it. You cant beat the performance of this servo and at such a great price it’s a no-brainer.
The HD Storm 5s are high efficiency and perform better. They run cooler and quieter and with less vibration – because there are no brush arcings, the radio frequency interference and electronic noise is significantly reduced.Similar to the Storm series of our new Power HD servos – the high voltage D-series are a coreless servo with the same hardened aluminum case and splash water protection.
Can I use 24v on 12V motor?
Therefore, provided you chose a controller suitable for the motor you use, you can usually run a motor 12v motor from a 24v battery with no effect except that full speed is doubled.
The Power HD LF-20 servos are a PERFECT Bargain servo with respectable performance. If you are looking for a top notch budget servo that has better centering and resonlution, you may consider the 8309/8312 Power HD servos, but for this price you won’t find a better servo!”What the heck does that have to do with my servo?” Well now they are incorporating that smart technology into these HD Storm 4 high voltage servos – so they are not going to be sucking the life of your Redwing LiFe battery when it doesn’t have to – which means not only would you be getting a great and powerful servo – it’s smart! Your “LiFe” will thank you.
“What the heck does that have to do with my servo?” Well now they are incorporating that smart technology into these HD Storm 3 high voltage servos – so they are not going to be sucking the life of your Redwing LiFe battery when it doesn’t have to – which means not only would you be getting a great and powerful servo – it’s smart! Your “LiFe” will thank you.
“What the heck does that have to do with my servo?” Well now they are incorporating that smart technology into these HD Storm 5 high voltage servos – so they are not going to be sucking the life of your Redwing LiFe battery when it doesn’t have to – which means not only would you be getting a great and powerful servo – it’s smart! Your “LiFe” will thank you.
The Power HD LF-20 MG is excellent for a 30cc or 50cc plane. This Digital servo has metal housing around the middle which allows for the heat to rapidly dissipate. With double ball bearings you will have smooth movement and powerful torque when you need it. You cant beat the performance of this servo and at such a great price it’s a no-brainer.
The Power HD Storm 3 high voltage servos are made with brushless technology. Now – we’ve heard about this “brushless” stuff before but it is time to ask the question on all of our minds – why is that so great? Take a brushless motor for example – no brushes in a brushless motor (no kidding…) nor does it have the commutator – instead a small circuit board coordinates the energy delivery to the windings. Think about this technology being used in a power tool – since the electronics communicate directly with the stationary windings the tool can adapt to its surroundings – only using enough juice necessary to accomplish the job. So if you are using a brushless drill to drive a screw into Styrofoam – it will sense the lack of resistance from the foam and pulls only enough charge from the battery that it needs. Whereas a brushed drill will run as fast and hard as it can no matter the job simultaneously wasting your battery life.The HD Storm 3s are high efficiency and perform better. They run cooler and quieter and with less vibration – because there are no brush arcings, the radio frequency interference and electronic noise is significantly reduced.
The Power HD Storm 4 high voltage servos are made with brushless technology. Now – we’ve heard about this “brushless” stuff before but it is time to ask the question on all of our minds – why is that so great? Take a brushless motor for example – no brushes in a brushless motor (no kidding…) nor does it have the commutator – instead a small circuit board coordinates the energy delivery to the windings. Think about this technology being used in a power tool – since the electronics communicate directly with the stationary windings the tool can adapt to its surroundings – only using enough juice necessary to accomplish the job. So if you are using a brushless drill to drive a screw into Styrofoam – it will sense the lack of resistance from the foam and pulls only enough charge from the battery that it needs. Whereas a brushed drill will run as fast and hard as it can no matter the job simultaneously wasting your battery life.
These HD D18 servos are capable of 7.4 volts in 10 seconds and are made with the finest titatanium and aluminum gears. The long lasting coreless motor guarantees quality performance for a lifetime at lower temperatures.All aluminum housing, high voltage and the new brushless technology gives you the best strength and speed while being incredibly efficient and long lasting.There’s a storm brewing in Redwing RC! These new HD Storms can handle anything thrown at them. Now, we can’t imagine your servos heading into battle facing a hurricane or tornado – but they sure behave as if they have fought and conquered the fiercest of storms – taking all of that power and knowledge with them when they are installed into your bird. Rain or shine, these servos are ready for anything!
Thought our other servos were great? Well of course they are! But wait until you check out these new Power HD servos – they will rock your wheel pants off* (no wheel pants will be harmed by using this product – but you get the idea).
The HD Storm 4s are high efficiency and perform better. They run cooler and quieter and with less vibration – because there are no brush arcings, the radio frequency interference and electronic noise is significantly reduced. The LF-13 Power HD Servos are a PERFECT Bargain servo with respectable performance. If you are looking for a top notch budget servo that has better centering and resonlution, you may consider the 8309/8312 Power HD servos, but for this price you won’t find a better servo! The Power HD Storm 5 high voltage servos are made with brushless technology. Now – we’ve heard about this “brushless” stuff before but it is time to ask the question on all of our minds – why is that so great? Take a brushless motor for example – no brushes in a brushless motor (no kidding…) nor does it have the commutator – instead a small circuit board coordinates the energy delivery to the windings. Think about this technology being used in a power tool – since the electronics communicate directly with the stationary windings the tool can adapt to its surroundings – only using enough juice necessary to accomplish the job. So if you are using a brushless drill to drive a screw into Styrofoam – it will sense the lack of resistance from the foam and pulls only enough charge from the battery that it needs. Whereas a brushed drill will run as fast and hard as it can no matter the job simultaneously wasting your battery life.Warning: Your browser failed to connect to our secure payment processor, Braintree Payments. You will be unable to enter your credit card information at the payment step of checkout unless this is fixed.
The HD-1218TH is one of the strongest standard-size servos we carry. Digital electronics combined with a coreless motor, titanium-shielded aluminum gears, and two ball bearings give this servo a distinct edge over typical standard-size servos. Unlike typical hobby servos, the 1218TH has an operating voltage range of 6 V to 7.4 V. This servo works with standard RC servo pulses at frequencies up to 333 Hz. The 12″ (300 mm) lead is terminated with a standard “JR”-style connector, which is Futaba-compatible. The picture below shows an example of the hardware that might be included with this servo (hardware might vary).
This force that is applied at a position r from the axis of rotation (which is directly out of the page) is known as torque. Torque is often represented by the Greek letter τ. The equation for torque is:It looks like that pink dot is above the curve, so we might need a stronger motor. Let’s double check to see if that point is, in fact, above our torque vs. speed curve.
To calculate the torque requirement, we start at the end effector (i.e. gripper) of the robotic arm and then work our way to the base of the robot. We need to account for all the pieces of the robot that are impacted by gravity:
You can see how the torque required to overcome the force of gravity is more than 3x the required torque to accelerate a link from rest (which we calculated as 200 oz-in).
What is the voltage range of a servo motor?
The servo operates off of a 4.0 to 7.2 V power supply. It can draw as much as 0.5 A if it must deliver a large amount of force but may run directly off the RED-V power supply if the load is light.
The reason why this curve below gives the minimum acceleration is because acceleration is the slope of the velocity curve (i.e. change in angular velocity/change in time = angular acceleration).
The force of gravity on the links and payload (i.e. the box) is only part of the calculation of the torque requirement for a motor. We want our robotic arm to move and do useful work in the world…not just sit there with its arm stretched out, holding a box in place.
There is a torque required for a joint to move (i.e. generate angular acceleration) a link (or payload) from a rest position. Therefore, the total torque requirement for a servo is:
However, in our analysis we have not taken into account the rotational inertia of the motor, and we have assumed that there is no payload attached to the link. So, in a real-world use case, we need to account for those by incorporating them into our original calculation of I (i.e. the rotational inertia). You can see that the magnitude of the slope is minimized on the black curve. The red and green curves get steep in some parts, which means the angular acceleration is high (and thus more torque is needed for the servo motor). It is worth repeating, but with torque we are only concerned about the component of the force that is perpendicular to the position vector. Any force component that is parallel to the position vector doesn’t generate torque.If you go to this list at Wikipedia, you will see that this object can be considered a “point mass”. The equation for rotational inertia for a point mass is:
Where r/2 is the distance from the axis of rotation to the center of mass of the link. The center of mass of the link is noted with a pink circle below.
This makes a lot of sense. Imagine holding your arm stretched out horizontally and trying to hold a bucket of water in place while trying not to bend your elbow. It would be pretty difficult!The official metric (SI) units of torque is the Newton-meter (Nm). However in datasheets for servo motors, you’ll often see ounce-force-inch (oz-in) or kilogram-force centimeter (kg-cm).
In the real world, a servo motor’s axis is the blue nail. This is the robotic arm’s joint. The orange bar is the link of a robotic arm. The force F is the force acting on an object (that the robotic arm is trying to lift) due to gravity.
Let’s assume that we want the motor to move 90° in 1.0 second and then stop (e.g. common in pick and place tasks). We can draw a graph of angular velocity ω (radians/second) vs. time t (seconds).
We see that the motor generates 83 oz-in of torque when the speed is 30rpm. However, we need to have 200 oz-in of torque, so this motor is not strong enough for our project.
Now we need to add the rotational inertia of the object attached to the link. Let’s assume the object has a mass of 1.2kg (just like the mass of the link). The equation becomes:Why is torque so important? When you want to build a robotic arm to perform some task in the world, you have to make sure that each joint of the arm (i.e. servo motor) is strong enough to lift whatever you want it to lift. In robotics jargon, the maximum weight that a robotic arm can lift is referred to as the maximum payload.
We will reposition the joint-link combination we worked with in the previous section so that the body of the motor is now parallel to the surface. Here is how that looks:
Let’s suppose we searched around on the Internet at various electronics stores and found a motor with a no load speed of 45 RPM and a stall torque of 250 oz-in (18 kg-cm). Let’s draw the torque vs. speed curve. Note that the “no load speed” is the revolutions per minute of a motor when it is running at top speed with nothing attached to it.Consider this joint below that is connected to a link. In this case, the motor will rotate counterclockwise in a direction that is at a 90 degree angle to the force of gravity (which is straight down). The only torque we have here is the torque required to generate angular acceleration.Note that we are only concerned about the component of the force that is perpendicular to the position vector. The position vector is drawn from the axis of rotation (i.e. the blue nail) to the force vector (in blue below). The position vector must be perpendicular to the force vector.Now let’s look at an example where we need to take both angular acceleration and the force of gravity into account in order to calculate the torque requirement.
The mass of the link is 1.2 kg, and the link length is 0.75 meters. In the worst case, cos(θ) = 1. Therefore, the amount of torque that the motor needs to have the link overcome the downward force of gravity is:
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What does 35kg mean for a servo?
Example 1. If you see a servo with a torque of 35kg-cm, what does that mean? Typically, when you see a value like 35 kg printed on a servo motor, what they are referring to is the stall torque, which, in this case, is 35 kg-cm. Stall torque is the torque load that causes a servo motor to “stall” or stop rotating.
Consider this diagram of a robotic arm below. It has four servo motors and a gripper on the end (at right). Each servo motor (i.e. joint) is connected by a link (typically a metal piece like you see in this six degree of freedom robotic arm here).
Now, that we’ve identified the torque requirements for the motor in motion (we covered the stationary motor case in the previous section of this post), we need to make sure we select a motor that will be able to exert 14.410 kg-cm of torque at all of the speeds in the curve below.
We want to minimize acceleration as much as possible so that we minimize the torque requirements for our servo motor. We could have any velocity curve we want, but the curve that minimizes acceleration is the one that increases linearly from rest, reaches a peak at the halfway point, and then decreases linearly from there.In the example above, we have accounted for the force of gravity when calculating our torque requirements. However, we assumed that the links have no weight. In reality they do.
In this tutorial, I will show you how to calculate how much torque you need for the servo motors (i.e. joints) on a robotic arm you are trying to build.Typically, when you see a value like 35 kg printed on a servo motor, what they are referring to is the stall torque, which, in this case, is 35 kg-cm. Stall torque is the torque load that causes a servo motor to “stall” or stop rotating.
This six degree of freedom robotic arm we built here can lift a pair of sunglasses, but the servos don’t have enough torque to lift a car, for example.The value for I will vary depending on what is generating the angular acceleration (e.g. solid cylinder, thin rod, slab, etc.). Inertia is the “resistance an object has to any change in its velocity.” Therefore, rotational inertia in the case of a servo motor is the resistance the motor has to any change in its velocity.
