Servo amplifier gain

A servo drive is an electronic amplifier used to power electric servomechanisms. A servo drive monitors the feedback signal from the servomechanism and continually adjusts for deviation from expected behavior. A servo drive receives a command signal from a control system, amplifies the signal, and transmits electric current to a servo motor in order to produce motion proportional to the command signal. Typically, the command signal represents a desired velocity, but can also represent a desired torque or position. A sensor attached to the servo motor reports the motor's actual status back to the servo drive.

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Content:

• Servo Motor Compensation
• Series 59G / 59H 2-Axis Low Gain Linear DC Servo Amplifier
• SA-500 Closed Loop Servo Amplifier
• DC Servos - Tips, Traps & Applications
• Mitsubishi MDS-B-V24-3520 High Gain Servo Amplifier
• What is servo tuning and why is it important?
• PEES Components AN430 Servo amplifier / PID controller
• Supported EtherCAT Slave Device
• What are auto tuning methods for servo drives?

WATCH RELATED VIDEO: 06 Servo Tuning Basics (Sigma-7 Servo Tuning)

Servo Motor Compensation

A number of audio circuits use a DC servo circuit, with the idea being to remove all traces of DC from the output of a preamp or power amp. Apart from the IMO complete futility of making audio equipment DC coupled throughout, it's also potentially dangerous to loudspeakers in particular.

Operating any audio gear with response to DC is asking for trouble, and it should be obvious that a DC servo will not by definition allow operation to DC. Unless the DC servo is set for an unrealistically low frequency 0. At question here is whether this is more or less 'intrusive' than a couple of capacitors.

A good part of this has come from the stupid idea that "The best cap is no cap. It will usually be polyester, sometimes people insist on polypropylene, and in many cases an electrolytic cap is used. Despite all the objections, provided the voltage across any capacitor is low enough, the distortion contributed is negligible. Phase shift is often stated as a 'good' reason to avoid using an input cap, but a DC servo can actually make it worse. It's easy to ensure that there is close to zero phase shift at any frequency of interest, simply by using a larger cap than normal.

When you include a DC servo system, it creates issues of its own, and these are rarely discussed by anyone. There is also additional complexity in the overall circuit, which is sometimes considerable. That means additional regulation is needed, which may only include a couple of resistors and zener diodes, but may use regulator ICs instead. In a combined preamp and power amp, the DC servo s can be run from the preamp supplies, and now two supplies are needed for the power amp board s - operating voltages and servo supply voltages.

This all means that there are more parts, more connectors and obviously more things that can go wrong. If any part of the DC servo circuit fails, there's every chance that the circuit will develop a DC output as a result, and that may be sufficient to cause speaker failure in a fully DC coupled system.

The chance of a capacitor failing in such a way as to cause the same problem is very small - so small as to be considered negligible in most cases.

For anyone who thinks that caps are 'evil' hint; they aren't the only way to ensure a low DC offset is to use a DC servo, but as you'll see these impose their own special constraints. In many cases, the servo may be more intrusive than using capacitors, and I can't see how this can be considered a sensible approach.

However, DC servos definitely have their uses, and dismissing them out of hand would be just as silly as rejecting capacitors because they 'ruin' the sound another hint; they don't.

If a faulty preamp is connected with say 5V DC at its output, the DC servo system will not have enough range to remove that much, so the power amplifier will provide DC straight to the speakers which will announce their displeasure by liberating 'magic smoke'.

Consider that just about every piece of music you listen to has already passed through countless capacitors within the recording process. Not just coupling caps, but those used for equalisation whether vinyl or CD - EQ is almost invariably used during recording , and even in microphones such as capacitor aka 'condenser' mics or any other that has electronic circuitry. It's unrealistic to imagine that every piece of equipment used for recording only contains capacitors with the most advanced dielectrics available, because the vast majority will include no such thing.

It's equally unrealistic to assume that if no capacitors are used in the playback audio chain that it will make anything sound 'better'. By definition, an amp or preamp using a DC servo cannot reproduce DC. The servo will operate and remove or try to remove the DC component, but if it's large enough to saturate the servo opamp then DC will get through anyway. Everything has its limits, and no ideal devices exist, so the end result will always be a compromise.

This is not to say that the DC servo is 'pointless'. There are countless pieces of equipment that rely on a DC or other servo for their operation, and the purpose of this article is to provide useful information, and not to dissuade anyone from adopting a DC servo if it suits their purpose.

When used for some perceived benefit such as eliminating capacitors from the signal path , then the actual benefit may be far less than expected. All circuit building blocks have their place in electronics, and it's up to the designer to determine what is necessary to achieve the desired goals. If this includes a DC servo, then that's what should be used. Before continuing, not everyone will know what a DC servo is or how it's used, so some explanations are in order.

If a circuit has a DC error i. The servo is almost always a fairly simple integrator, most commonly using a FET input opamp to allow low values of capacitance and high resistances. Some practical examples are shown further below. The integrator is set up so that it provides negative feedback, but with very high DC gain to maintain a low final error. Even a 'pedestrian' opamp such as a TL has a DC open loop gain of at least , dB and often more. The primary error term in the final system is the opamp's input offset voltage typically mV, but usually less in practice.

The overall open-loop gain i. The DC servo provides a very large open-loop gain improvement over the amplifier circuit by itself. This is by design limited to sub-audible frequencies, and the additional DC gain provided by the servo's opamp is able to remove DC offset almost completely. By design, few power amplifiers have a high enough open loop or DC gain to be able to effectively eliminate any DC offset.

The opamp and associated integrator ensure that there is more than sufficient DC gain to reduce overall DC offset to negligible levels. Note: The ultimate limitation of any DC servo is the DC input offset voltage of the opamp used for the servo itself. For an opamp such as the TL, the 'typical' input offset voltage is 3mV, and unless you include a DC offset control for the servo opamp, the main amplifier's output DC offset can be no better than this. I mention the TL because it's ideal for this purpose, having very low input current which minimises errors due to this factor.

The integrator's input DC offset has been assumed to be zero for the following discussion, but it will rarely be so in practice. Figure 1 - Basic DC servo Principle. The basics of a DC servo are shown above. If it happens to be some value of DC, then the output of the integrator will be just that, provided of course that the AC component is at a frequency high enough to be 'ignored' by the integrator itself.

Note that the integrator is inverting. The circuit has been shown connected to a loudspeaker this site is mainly about audio after all , but in reality it can be any transducer, as may be used for scientific, medical, industrial or other application s. DC servos are used in some unlikely places, but the same principles apply regardless.

Because they are DC servos, much of the complex feedback loop stability criteria may not be necessary, but as you'll see below, just the addition of an input capacitor can mess that up badly. If the amplifier shows any sign of DC at the output, this is integrated by U1, and that signal is applied to the amp's input to correct the offset. Let's say that the amplifier for whatever reason has an output DC offset of mV corresponding to an input DC offset of around 27mV.

While that won't hurt a loudspeaker power into an 8 ohm driver is only 49mW it may cause a small but unacceptable shift in the speaker cone's static position. In some other applications, it may be catastrophic for example, driving a transformer. When the DC servo is connected, the initial DC is still mV, but the servo circuit reduces that to less than 1mV within a few seconds.

Any DC at the output of the amp is integrated by U1 via R6 and the integration capacitor C2 , and once settled the output of U1 applies exactly the right amount of DC offset to the input to force U1's output to close to zero. The passive summing point is the junction of R1, R2 and R3. However, the circuit shown is now sensitive to the source resistance, which has to be in excess of 20k or the DC servo is unable to make the correction needed.

U1 can supply a maximum output voltage of around 13V, and this can't force enough current through the bias network R3, R2 and R1 to cope with low impedance inputs. This is obviously unacceptable, since most sources have an output impedance of close to ohms, so the DC servo can't function. There's another problem as well, in that if the source is connected or disconnected while the amp is on, it takes time for the servo to reset itself to suit the changed conditions.

With an audio system, the speaker will make a fairly loud 'thump' as the input is changed. You also can't use an input pot, because the DC will make it noisy and it will cause more issues with source impedance. One answer is to include C1 shown greyed out so the DC servo feedback path is isolated from the source. This has some unexpected consequences though, because there are two time constants involved in the feedback path, which cause some potentially serious issues.

This means that we do need to concern ourselves with feedback loop stability. The frequency response shows a peak of more than 6dB at 0. If C1 is reduced to nF, settling time is as close to perfect as you'd ever need, but response is about 2dB down at 20Hz.

This is almost certainly unacceptable. Unfortunately, the servo makes the input capacitor value critical for proper circuit behaviour, something that isn't usually a problem. We've come to expect that altering the low frequency response is simply a matter of changing the input capacitor, but once a DC servo of the form shown is in place, the capacitor value becomes a critical part of the circuit.

In particular, the response of the red trace is not simply undesirable, it's potentially dangerous! There's more on that further below. While they are used sometimes, inverting DC servos are the least desirable way to achieve the goals expected.

The capacitor value has to be selected with care, and extensive tests are needed to ensure that the circuit is absolutely stable. A damped oscillation or premature rolloff will result if the cap is too large or too small respectively. Consider that many sources e.

This overcomes the problem of the input capacitor, because it's no longer part of the DC feedback loop. The value can be changed at will or even left out if you are particularly brave without affecting the response of the DC servo. Note that if there is any DC potential at the amp's input, that can cause issues, and the servo may not have sufficient range to change that.

The resistance of the DC feedback resistor now becomes part of the the main amp's feedback circuit, so it has to be high enough as to not adversely affect the desired gain. With the values shown below, the gain is affected very marginally, but it won't normally be a problem. You need to be aware that when used like this, the opamp's output noise and any distortion that may be created will be injected into the amplifier's feedback loop, so that needs to be considered in circuits designed for very low noise.

The opamp's output is also part of the feedback loop, and by extension is also part of the signal chain. The input to the DC servo opamp must be constrained so that it's within the opamp's input voltage range. Now, we can either add an attenuator which will badly affect performance or get clever the preferred choice whenever possible. If a passive integrator is used we can ensure that nothing below 1Hz can cause a problem, and the opamp's input can be protected easily because of the high impedance.

This is fortunate, because it means that only one time constant is involved 2. The benefit of the circuit shown is that it has far greater gain at DC and below 1Hz. The two diodes protect the opamp's input from fault voltages.

Despite the capacitor from the servo opamp's output to input, this is not an integrator. The cap allows the opamp to run at maximum gain for DC voltages, but doesn't add any usable AC filtering. In theory it can use lower or higher values, but it's more sensible to maintain C2 and C3 at the same value.

Series 59G / 59H 2-Axis Low Gain Linear DC Servo Amplifier

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SA-500 Closed Loop Servo Amplifier

A number of audio circuits use a DC servo circuit, with the idea being to remove all traces of DC from the output of a preamp or power amp. Apart from the IMO complete futility of making audio equipment DC coupled throughout, it's also potentially dangerous to loudspeakers in particular. Operating any audio gear with response to DC is asking for trouble, and it should be obvious that a DC servo will not by definition allow operation to DC. Unless the DC servo is set for an unrealistically low frequency 0. At question here is whether this is more or less 'intrusive' than a couple of capacitors. A good part of this has come from the stupid idea that "The best cap is no cap. It will usually be polyester, sometimes people insist on polypropylene, and in many cases an electrolytic cap is used. Despite all the objections, provided the voltage across any capacitor is low enough, the distortion contributed is negligible.

DC Servos - Tips, Traps & Applications

Servo amplifiers, or servo amps for short, are drives that are used to power electronic servomechanisms, such as servo motors. A servo amp transfers signals from the command module of the robot, and translates them for the servo motor, so the motor knows how much it should move at any given time. By using servo amplifiers, servo motors are able to perform more consistently, meaning that the path trajectory and overall motion of the robot is more smooth and consistent during its application cycles. There are several advantages to using servo amplifiers and their subsequent motors for a robotics system, as opposed to using traditional AC or DC motors. The main advantage is the feedback that these amplifiers are able to pass on to the motor and the command control.

Mitsubishi MDS-B-V24-3520 High Gain Servo Amplifier

A lot of motion control terminology is used interchangeably, situationally, and sometimes incorrectly. We'll be the first to admit that it can be a bit confusing at times, and understandably so. Some of the motion control lingo has changed over time , and it hasn't changed the same way in every region. Linguistic preferences can also change depending on the industry of the application. This blog should hopefully clear up some of the ambiguities surrounding the motion control terminology of drives, controllers, motors, and more.

What is servo tuning and why is it important?

Because mechanical systems have inertia and compliance, the target value is rarely achieved on the first position command—hence, the need for feedback and correction commands. Servo tuning is a method of adjusting the feedback to determine how hard the system tries to correct the error. The process of servo tuning means tweaking the various gains and motion parameters in the servo controller so that performance is optimized—i. In other words, the goal of tuning is to achieve the fastest response from the system, while avoiding or minimizing overshoot of the target value typically either position or velocity. Auto-tuning uses the servo drive and controller to test the system at multiple frequencies and set the tuning parameters often more numerous than those used for manual tuning to achieve the best response.

PEES Components AN430 Servo amplifier / PID controller

A dual mode, digital servo positioning system controls servo motor current by a processor-generated current control set point in conjunction with velocity feedback. When the motor has advanced to the vicinity of a target position, the processor opens the velocity feedback loop and reduces servo amplifier gain, such that the set point controls motor current without velocity feedback. This open loop condition provides a springy servo response that eliminates oscillation due to small deviations from target position caused by electronic drift or gravity.

Supported EtherCAT Slave Device

Tuning a servo system is a complex and iterative process. While manual tuning has been the predominant method for many years, most servo drives now incorporate functions that will automatically tune the system. Although in the beginning, auto tuning functions were useful only when the load was rigidly coupled and the system dynamics were relatively simple, more complex algorithms and faster computing power have enabled the development of auto tuning functions that are sophisticated enough to address even the most complex systems, with minimal input or effort from the user. Auto tuning is based on the same principles as manual tuning. That is, the performance of the motor is evaluated relative to a given command, and the servo drive automatically adjusts the gains until values are found that give the best performance. In most cases, the auto tuning process can also add filters to the control loop to suppress oscillations caused by resonance frequencies in the system.

What are auto tuning methods for servo drives?

Servo gain adjustment is complete just by turning on the one-touch tuning function. The advanced vibration suppression control II suppresses two types of low frequency vibrations owing to vibration suppression algorithm which supports three-inertia system. This function is effective in suppressing residual vibration generated at the end of an arm and in a machine, enabling a shorter settling time. Adjustment is easily performed on MR Configurator2. With advanced filter structure, applicable frequency range is expanded to between 10 Hz and Hz. Additionally, the number of simultaneously applicable filters is increased to five, improving vibration suppression performance of a machine.

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