Why Do RC Servos Chatter?

Radio-controlled cars (or RC cars for short) are miniature model cars or trucks that can be controlled from a distance using a specialized transmitter or remote. The term “RC” has been used to mean both “remote controlled” and “radio controlled”, where “remote controlled” includes vehicles that are controlled by radio, infrared or a physical wire connection (the latter is now obsolete). Common use of “RC” today usually refers only to vehicles controlled by radio, and this article focuses on radio-controlled vehicles only.

So why do RC servos chatter?

There are a few reasons why a RC servo chatter. The main reason is because your derivative is too high. If this happens, you need to lower your derivative, check your current loop, or try a different sampling.

If you are looking for a new servo for your RC car, you can find them by clicking here. 

If you would like to know more about why RC servos chatter, please keep reading and check out this video!

RC Cars

RC cars are powered by various sources. Electric models are powered by small but powerful electric motors and rechargeable nickel-cadmium, nickel metal hydride, or lithium polymer cells. There are also brushed or brushless electric motors.Brushless motors are more powerful and efficient, but also much more expensive than brushed motors. Most fuel-powered models use glow plug engines, small internal combustion engines fueled by a special mixture of nitromethane, methanol, and oil (in most cases a blend of castor oil and synthetic oil). These are referred to as “nitro” cars. 

Recently, exceptionally large models have been introduced that are powered by small gasoline engines, similar to string trimmer motors, which use a mix of oil and gasoline. Electric cars are generally considered easier to work with compared to fuel-driven models, but can be equally as complex at the higher budget and skill levels. Both electric and nitro models can be very fast, although electric is easier to upgrade and more versatile.

In both of these categories, both on-road and off-road vehicles are available. Off-road models, which are built with fully functional off-road suspensions and a wide tire selection, can be used on various types of terrain. On-road cars, with a much less robust suspension, are limited to smooth, paved surfaces. There are also rally cars, which fall somewhere between on-road and off-road and can be driven on gravel, dirt or other loose surfaces. In the past decade, advances in “on-road” vehicles have made their suspension as adjustable as many full scale race cars, today.

RC Servo           

A chattering servo is the most common issue that engineers come across when running their machine. This chattering is not only annoying but also, it can cause an increase in wear and tear on the motor and amplifier.

This problem comes from several areas. We will go over these reasons below. 

• Lower Your Derivative Term            

When using the ever-popular proportional, integral, derivative (PID) control, a high value for the derivative name tends to make the motor chatter. And in the utmost case, sound like a bag of ball bearings. Try lowering your derivative term in combination with your proportion gain to reduce noise.

If the noise disappears, but the performance is not at your level, factor in two sets of servo parameters. That is an active set and a holding set. Many applications are okay with a little noise during motion but need to be quiet while holding position.

Using a less aggressive quieter holding set can be a right solution since the servo usually doesn’t have to do much work to hold the axis in the same position.

• Check Out Your Current Loop      

If the position PID is set up just fine, some or all of the noise may be coming from an overly aggressive current circuit. If you happen to be using a digital amplifier, try to decrease the current loop gains. Or re-do the auto tuner with not much aggressive setting, if this quantity of control is available.

The current loop is where the electromagnetic rubber hits the way, and significant spikes in the voltage sent to the motor can cause the engine to act as a speaker.

Linear motors seem to be particularly sensitive to this, and reason could be because their mechanical layout is similar to the sounding board.

A faster test that can confirm a current loop diagnosis is to turn off the current loop by running the amplifier in voltage mode only.

Not every speaker permits this option, and if they do, you will probably have to retune the position loop to get comparable overall results. This is because the amplifier gain may be very different with and without current circuit enabled.

• Try A Different Sampling Time           

Changing the central servo loop time, or the derivative sampling time if it can be adjusted, may help with an audible noise. Even if the motion is not more correct, the pitch has a significant impact on perceived noise. Modern systems tend to run at very high servo loop rates, much of which is needed in a standard application.

When decreasing the servo loop rate, ensure you retune your PID parameters while the proportional term may or may not affect the integral and derivative values since they are time-dependent.

• Fancy Filters        

There are filtering strategies that you can try, but these tend to have their demerits. The most common approach applies some frequency-specific filtering. It’s typically done through a bi-quad filter.If your system supports these, you can try constructing a low pass or bandpass filter to see if they can.

Another filter modification that is effective at reducing noise is referred to as integral dead-band. You can use it in either the current loop or the position loop. This technique tells the integrator not to worry about small amounts of the windup, and only kick in for more significant corrections.

It has the effect of lowering the frequency of small corrective commands, thereby reducing noise.

• Sinusoidal Commutation

Brushless DC motors that are commutated with traditional 6 step control are susceptible to add instability. And therefore, noise at the hall sensor boundaries.

As the motor goes round, when entering each new hall state, the current flowing through the coils changes abruptly. This is what advances the commutation as the engine goes round.

But if your requested final position happens to fall on such a hall boundary, the motor may become unstable as the servo controller tries to hold the position. Servo loops like to have beautiful, proportional response curves.

Sinusoidal commutation or other techniques like field oriental control that advance the phase angle in tiny increments eliminates this problem.

Motor And Amplifier Getting Too Hot

When doing the work, motors and amplifiers generate heat. A lot of heat comes from the inefficient operation and can be avoided or minimized.

Note that many reasons that can overheat the motor can overheat the amplifier as well. So except where it’s indicated, the items below tend to improve performance for both the engine and amp.

• Increase the effective coil inductance

Modern motors are trending toward lower inductances. While healthy for motion performance, less inductance makes the work of the current control circuitry hard.

For a large class of switching amplifiers, low coil inductance means broad current ripple at each on and off switch cycle of the amp. These inductors will slow the rise and fall of the switch current, thereby reducing ripple and reducing heat generation in most cases.

• Switch at a higher pulse width modulation rate

A painless way of minimizing current ripple is to increase the switching frequency of your drive or select a new journey with a higher switching frequency. A higher switching frequency will decrease the magnitude of the current ripple due to switching, and thus lower waste heat generation.

• Try a different current control technique

It may only be possible if you are building your drive. But switching amplifier configurations like H-bridges, which are commonly used to manage step motor and DC servo motor coils can be controlled in numerous different ways.

Use A Holding Torque With Step Motors

Step motor has their peculiar heat generation issues. When moving a load, step motors usually have a heat disadvantage compared to servo motors because step motors must be driven with a current that can overcome the axis highest possible resistive force, whether or not power is present at any given time.

One approach to lowering step motor heat generation is to recall that in dynamic operation, many axes of the machine will have downtime.

In this case, switching the step motor to lower holding command will reduce average heat generation. You want sufficient holding torque to maintain the load from moving, but a low sufficient value to lower heat output while the axis is at rest.