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Mechatronics: The amplifier

I am primarily targeting analog novices in this upcoming tutorial series because I see a great many designers in our industry who have expertise in the more digital, processor and software side of design in the new millenium. These upcoming blogs are by no means an exhaustive treatment of Mechatronics design, but my main focus will be on the basic analog content of Mechatronics architectures. I will not be able to completely discuss all the ‘Black art’ analog intricacies, however, without also briefly discussing the digital, processor and software aspects of Mechatronics architectures.

The main architecture of Mechatronics (Image courtesy of Reference 6)

The main architecture of Mechatronics (Image courtesy of Reference 6)

Now let’s discuss some aspects of amplifiers as they relate to Mechatronics.

Amplifiers5

First, let’s look at some power driver circuitry.

Class D Power driver amplifiers

Since we are primarily dealing with the Analog architectures in Mechatronics, which is actually an integration of mechanical engineering, control theory, computer science, and electronics to manage complexity, uncertainty, and communication in engineered systems as described by NYU Tandon School of Engineering, let’s begin our analog discussion with the Class-D amplifier. A Class D amplifier is a high-power, switch-mode architecture that is typically used in nanometer-accurate mechatronic systems1 .

See Figure 1 for the driver architectural design portion of a Mechatronics architecture which can be used to translate commands that have been computed by the processor into physical action to some load such as a motor in a robotic arm on an automotive assembly line in Detroit.

Figure 1

A switch-mode amplifier with high-precision output that can be used for single- or three-phase loads is shown here. In this closed loop control system, disturbances are attenuated from power supplies or from low-frequency harmonics which can emanate from the switching stage. (Image courtesy of Reference 1)

A switch-mode amplifier with high-precision output that can be used for single- or three-phase loads is shown here. In this closed loop control system, disturbances are attenuated from power supplies or from low-frequency harmonics which can emanate from the switching stage. (Image courtesy of Reference 1)

In Figure 1 we have the following:

u I (t ) is the amplifier input voltage

u NH (t ) is a source of different disturbing spectral components

Z I is an inductive-resistive series impedance

Learn more about Class D audio power amplifiers here on EDN:[Class D audio power amplifiers: Adding punch to your sound design]

Smart gate power drivers

Another key application for power drivers in Mechatronics is the Motor Gate Driver; these are not typically only Class D amplifier architectures, but also can be power FETs driven by a gate driver circuit that may or may not have intelligence integrated into the IC. Texas Instruments has an excellent variety of Brushless DC (BLDC) Motor Drivers that can be accessed here. I will be discussing these more in detail in my next blog.

Next, we can take a look at some typical amplifiers used on the input circuitry of a Mechatronics system design.

Input amplifiers and associated circuitry

Analog voltage sensing tips for best performance on the front-end/input of a Mechatronics architecture.

The amplifier portion of the front end must be able to properly and efficiently take a real-world analog input signal and amplify, filter, isolate and convert it to send on to an Analog-to-Digital converter in most cases so that a processor can make decisions on that optimized signal and translate that to the output driver circuitry.

Let’s first take a look at a voltage input signal in which an amplifier, filter, and ADC will be involved. We want a good noise performance and low distortion as well in most cases.

The resistive voltage divider

When the input signal to an operational amplifier or op amp, is a large voltage, we can adjust that large signal via resistive dividers. If the input signal is too small we can amplify it via an op amp with gain.

Now let’s take the case of a high input signal and look at the resistive voltage divider technique as a method of lowering its level to a usable, safe signal for the circuitry to follow. We will be able to achieve a very good linearity with resistive dividers if the power dissipation of the resistors in the divider are kept low, meaning a few mW. Care needs to be taken with possible resistor heating at higher dissipation which will affect the resistance variation.

We need to be aware that resistors will also generate electrical noise which will mostly be thermal noise and excess/flicker noise.

Read more about noise here: [Strategies for minimizing resistor-generated noise]

Now let’s look at a divider example that is frequency compensated and can measure voltages of up to 400V with only 3.3V circuitry like those typical in Mechatronic amplifiers. See Figure 2.

Figure 2

A high-voltage divider compensated with a Signal-to-Noise Ratio (SNR) of 130 dB and a thermal power dissipation of only 10 mW per resistor which allows good linearity performance. (maximum) (Image courtesy of Reference 1)

A high-voltage divider compensated with a Signal-to-Noise Ratio (SNR) of 130 dB and a thermal power dissipation of only 10 mW per resistor which allows good linearity performance. (maximum) (Image courtesy of Reference 1)

The parallel capacitors, in the Figure 2 divider, help to compensate parasitic capacitances and enable a relatively flat frequency response with a fairly wide bandwidth.

The operational amplifier for buffering or gain

In the case of low level signals or when a current sense resistor is part of the input circuit, amplification is needed and hence the use of an operational amplifier is necessary. In the case of the current sense resistor, where low voltage drop levels in the sense resistor are in the mV range, an op amp or a differential amplifier can be a good choice. A good example in Mechatronics would be in motor control, because knowing the magnitude and direction of the motor current can tell you the speed and direction of the motor. See current sense amplifiers here.

See this Texas Instruments article on EETimes for some good detail regarding Current Sensing [A Current Sensing Tutorial—Part 1: Fundamentals]

The difference amplifier

Difference amplifiers include integrated matched resistors that can easily be configured for a wide range of fixed gains.

The Instrumentation amplifier

Instrumentation amplifiers are precision gain blocks that have a differential input and an output that may be differential or single-ended with respect to a reference terminal. These ICs amplify the difference between two input signal voltages and will reject any signals that are common to both inputs. These amplifiers have dc precision and gain accuracy which must be maintained within a noisy environment, as well as where large common-mode signals (usually at the ac power line frequency) are present. See this article on Instrumentation amplifier tips on Planet Analog: 6 Burr-Brown secrets to success you need to know about instrumentation amplifiers.

A very helpful tool in understanding and proper usage of an instrumentation amplifier is the Common-Mode Input Range Calculator for Instrumentation Amplifiers in Reference 6. This is an area that can cause a great deal of problems because the instrumentation amplifier is not just a simple amplifier but typically has two or three amplifiers internally configured for best common mode rejection (CMR) of input noise6 .

In out next part in this series, I will discuss other types of power driver amplifiers for such things as motor control. In another later article in this series I will discuss amplifiers as filters in a front-end design.

References

  • Voltage/Current Measurement Performance and Power Supply Rejection in All-Digital Class-D Power Amplifiers M. Mauerer, A. Tüysüz and J.W. Kolar, IEEE 2016

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