Before we shift to a discussion about LVDT, RVDT & Resolver simulation, let's review the key points and links from our series of blog articles on sensor simulation in ECU validation.
- Part 1: Understanding Sensor Simulation in ECU Validation presents an overview of the importance and role of sensors in real-time control systems and an introduction to Electronic Control Units (ECUs) and how they rely on sensor data.
- Part 2: Thermocouple Simulation in ECU Validation examines the Seebeck Effect, the importance of simulation accuracy, and cold junction compensation.
- Part 3: RTD Simulation in ECU Validation explores proportional resistance to temperature in simulation and suitability for low-temperature ranges where higher accuracy, stability, and repeatability are paramount.
- Part 4: Strain Gauge Simulation in ECU Validation discusses the importance of simulating strain gauges accurately across various applications and how they work.
Part 5: LVDT, RVDT & Resolver Simulation in ECU Validation
Linear Variable Differential Transformers (LVDTs), Rotary Variable Differential Transformers (RVDTs), and Resolvers are sensors that measure an object's movement. LVDTs measure linear displacement, as their name suggests, while RVDTs and resolvers measure angular movement or rotation.
Linear Variable Differential Transformers (LVDTs)
An LVDT consists of a stationary coil with primary and secondary windings that surround a moveable, cylindrical metal core. The LVDT primary winding is sourced by an AC reference signal, which magnetically couples to the secondary coils that are connected in series. When the core is in the middle of the primary winding—considered the null position—the output voltage will be zero. As the core is moved toward one of the secondary windings, an AC voltage will result at the output. The voltage is linear with respect to the core’s position within the winding, and the phase relationship between the input and output will determine the direction in which the core has moved.
Figure 1: Illustration of the main components in an LVDT
As illustrated in Figure 1, the primary signal is a fixed AC source. When the core is centered in the LVDT body, the amplitudes of the two secondaries will be the same and 180 degrees out of phase, resulting in a 0 V signal. The coupling increases on that secondary as the core moves to the right. The resulting amplitude at the output (Vsec2-Vsec1) indicates the core position relative to null, and the phase of the output relative to the input indicates direction.
Figure 2: Diagram of 4, 5 & 6 Wire LVDT Sensors
There are three variations of LVDT secondary winding connections illustrated in Figure 2. Some manufacturers internally connect the two secondaries to create a 4-wire LVDT. This configuration requires a simpler signal conditioner to process the displacement measurement while minimizing the interconnecting cabling. Alternatively, a 5-wire construction provides access to the secondary transformer center tap as a reference signal, allowing the signal conditioner to minimize the temperature sensitivity and phase differences between the primary and secondary coils. There is also a 6-wire configuration, which is less common but allows the user to monitor both secondary coils independently.
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Figure 3: Illustration of a 4-wire LVDT with two secondaries connected.
Figure 3 above shows a 4-wire LVDT with the two secondaries connected. This configuration requires a simpler signal conditioner to process the displacement measurement while minimizing the interconnecting cabling. Alternatively, a 5-wire construction provides access to the secondary transformer center tap as a reference signal, allowing the signal conditioner to minimize the temperature sensitivity and phase differences between the primary and secondary coils. There is also a 6-wire configuration, which is less common but allows the user to monitor both secondary coils independently.
It is important to note that LVDT measurement and simulation device manufacturers may refer to 2-, 3- and 4-wire types. The two input wires are not included in the count in these cases.
Rotary Variable Differential Transformers (RVDTs) and Resolvers
RVDTs and Resolvers are two types of transformer-based sensors that measure angular position with reference to a null point. They essentially perform the same function, though with some key differences. An RVDT has a 2-pole rotor that an external force can turn. The primary winding is coupled to the secondary windings, which have an output amplitude proportional to their angular position. As with an LVDT, the null position—or 0 degrees—occurs when the output amplitude of both secondaries is equal but 180 degrees out of phase. RVDTs typically have applications when measuring +/- 45-degree rotation, though some have an expanded range of +/- 90 degrees.
A resolver is constructed so the primary windings are part of the rotor. The primary is also proportionally coupled to the secondary windings depending on its position, but the design is such that a full 360 degrees of rotation can be measured. A resolver also has the capability to measure the speed of rotation.
Figure 4: Diagrams of RVDT & Resolver
As with all other sensor types discussed so far, LVDTs, RVDTs, and resolvers can be selected using a range of options depending on the application environment. There are a few key variables to consider when looking at simulation devices appropriate for validating control circuits that connect to these types of sensors.
- Sensors may connect to excitation sources ranging from 1 or 2 Volts to levels exceeding 30 V.
- Secondary output ranges may also vary from hundreds of millivolts to tens of volts.
- With LVDTs, there are also 4-, 5-, and 6-wire types to consider. Additionally, these sensors may be placed in settings where high common mode voltages are present.
- When validating control algorithms, it is important to be able to emulate short and open conditions.
Watch the video below to learn how linear & rotational sensors are utilized in aircraft technology.
PXI LVDT, RVDT, Resolver Simulator Modules
To the right is an example displacement simulator from Pickering. This range of high-density PXI/PXIe LVDT, RVDT, and Resolver simulators (model 41/43-670) can address most application settings.
- Onboard transformers and signal processing algorithms determine the coupling required between primary and secondary to represent a desired position setting.
- This product range can be configured with excitation range options spanning 250 mV to 38 Vrms and output range options from 450 mVrms to 31 Vrms, all with an operating frequency range of 300 to 20 kHz.
- Onboard relays allow for simulating open and shorted conditions, and phase offsets can also be simulated under program control.
Learn more about LVDT, RVDT & Resolver Simulation.
This article is part 5 of a 6-part series on sensor simulation.
Read Part 6: 4-20 mA Current Loop Simulation in ECU Validation
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