dspic30f Quadrature Encoder Interface Module

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1 DS Digital Signal Controller dspic30f Quadrature Encoder Interface Module 2005 Microchip Technology Incorporated. All Rights Reserved. dspic30f Quadrature Encoder Interface Module 1 Welcome to the dspic30f Quadrature Encoder Interface Module web seminar Microchip Technology Inc. Page 1

2 Session Agenda What is a Quadrature Encoder? General Features Overview Programmable digital noise filters Quadrature Decoder The QEI as a Timer/Counter 2005 Microchip Technology Incorporated. All Rights Reserved. dspic30f Quadrature Encoder Interface Module 2 These are the main topics we will address during this seminar. First of all, we will see the purpose of the QEI module is. Then we will go through all the main functional blocks, the digital noise filters, the decoder, and the position counter. Finally we will see that this peripheral, if not used as an encoder, can behave as an up/down counter/timer Microchip Technology Inc. Page 2

3 What is a Quadrature Encoder? light source slotted wheel sensing device logic 2005 Microchip Technology Incorporated. All Rights Reserved. dspic30f Quadrature Encoder Interface Module 3 A Quadrature Encoder (or incremental encoder, or optical encoder) is used to detect the position and speed of rotors, enabling closed loop control in many motor control applications like switched reluctance and induction motors. Typically, an encoder includes a slotted wheel attached to the motor shaft, a light source, a light sensing device and some logic. During the rotation of the shaft, the light passes through the slots and hits the sensing element, generating electric signals. The three digital outputs are called Phase A, Phabe B and Index. With some decoding performed by the dspic quadrature encoder interface, these signals can tell us the speed and position of the rotor Microchip Technology Inc. Page 3

4 What is a Quadrature Encoder? Phase A leads Phase B Phase B leads Phase A 2005 Microchip Technology Incorporated. All Rights Reserved. dspic30f Quadrature Encoder Interface Module 4 An example of the encoder output waveform is illustrated here. In the upper plot we see the Phase A and Phase B signals coming in on the QEA and QEB pins respectively. In this case, Phase A leads Phase B: the shaft is spinning forward. If we reverse the rotor rotation, like in the lower part of the figure, Phase B will lead Phase A: so by detecting which rising edge comes first, we are able to detect the direction of rotation. The encoder outputs can only have four different states as indicated in the figure: 01, 00, 10, 11. Note that if we reverse the direction the sequence is reversed as well. The index occurs only once per revolution and is used to establish an absolute position. The quadrature decoder, which is part of the interface circuitry, will read these signals and converts them into a numeric count of the position pulses. The count will increment when the shaft is rotating in one direction and will decrement when the rotation is reversed Microchip Technology Inc. Page 4

5 dspic QEI Features QEI Features QEI decodes signals and accumulates count Logically swap A and B inputs Programmable noise filters on inputs x2 and x4 counting modes 16-bit Position count register Reset on index pulse (if enabled) Reset on rollover/underflow Count error status bit 2005 Microchip Technology Incorporated. All Rights Reserved. dspic30f Quadrature Encoder Interface Module 5 The QEI will perform all the operations needed to effectively use the information coming from the encoder. Since these signals are heavily affected by noise, a digital filter is available on each input. The filtered phase edges are counted by a dedicated 16 bit up/down counter, also referred to as the Position Counter. To establish a reference point for position and speed measurements, the counter can be reset either by the index signal or by a counter period match. The hardware can also perform some error checking on on the accumulated count Microchip Technology Inc. Page 5

6 Block Diagram Clock Divider Tcy INDEX Digital Filter Logic QEB QEA Digital Filter Logic Digital Filter Logic Prescaler and Sync. Logic Tcy 0 1 Timer Mode Timer Mode Quadrature Decoder Logic DIR Clock Reset 16-Bit Up/Down Counter Comparator UPDN TQCS Max. Count Register 2005 Microchip Technology Incorporated. All Rights Reserved. dspic30f Quadrature Encoder Interface Module 6 This block diagram depicts the internal architecture of the QEI modules. We can see the input pins and the associated digital filters. There is also an up/down input pin that is mainly used when the unit operates as a counter. The quadratrute decoder logic is responsible for analysing which edge comes first and the counter accumulates the edge count and is compared with the internal Max Count Register Microchip Technology Inc. Page 6

7 Digital Filters Multiple clock options to digital filter Tcy, 2Tcy, 4Tcy, 8Tcy, 16Tcy,, 256Tcy Signal must be stable for 3 clock cycles Adjust clock divide bits to change noise filtering characteristics Use of digital filter generates latency 2005 Microchip Technology Incorporated. All Rights Reserved. dspic30f Quadrature Encoder Interface Module 7 The digital filters are responsible for rejecting noise from the three inputs. The instruction cycle clock can be divided down by 2, 4, 16, 32, 64, 128, 256 before being used in the filter. The lower the clock frequency the lower frequencies are rejected by the filter. The prescaled clock is used to sample the input signal: if and only if three consecutive samples have the same value the input is considered stable and the value is output from the filter. One of the effects of this sampling is an added latency, because there is a propagation delay of the input signal through the filter Microchip Technology Inc. Page 7

8 Digital Filters T CY QEA/B Filter 2005 Microchip Technology Incorporated. All Rights Reserved. dspic30f Quadrature Encoder Interface Module 8 Here we can see that the input signal at the QEA or QEB pin is sampled using the selected clock, in this case the instruction cycle period Tcy. If at least three samples having the same value are detected the output is updated, otherwise the input signal changes are disregarded. Glitches and spikes can be efficiently filtered out by the digital filters Microchip Technology Inc. Page 8

9 Quadrature Decoder Four basic modes x 2 mode with Index Pulse reset X 2 mode with reset by match X 4 mode Index Pulse reset X 4 mode with reset by match 2005 Microchip Technology Incorporated. All Rights Reserved. dspic30f Quadrature Encoder Interface Module 9 As we have already seen, the quadrature decoder must determine the direction of rotation looking at the two incoming phase signals, and generate the clock that will be used by the position counter. We can select between two modes: in the first one (x2) the decoder only generates a clock impulse at the rising and falling edges of Phase A signal; in the other mode (x4), the clock pulses are generate at each edge of phase A and Phase B. The position counter can be reset either by the index pulse coming from the encoder or by the matching of the current position counter value with the number in the Maximum Count Register Microchip Technology Inc. Page 9

10 Timing Diagram PHASE A PHASE B COUNT UPDN 2005 Microchip Technology Incorporated. All Rights Reserved. dspic30f Quadrature Encoder Interface Module 10 This is an example of how the decoder works. We are using the x4 mode, where the clock pulse is at each edge of both phases. In the first part of the timing diagram Phase A leads Phase B, so that the counter is counting up. Then, in the second half, the rotation of the rotor is reversed, Phase B now leads Phase A and the counter is counting down. This is why an up/down counter is required in this application. The count direction can be determined by reading the UPDN bit in the QEI control register, but the UPDN pin can also be used to indicate the count direction status. With the x4 mode we can get a very high angular resolution, but we also get a relatively high output clock frequency. With the x2 mode, the resolution is twice as fine, but the frequncy is lower. The maximum allowed quadrature frequency is one-third of the instruction cycle frequency Fcy Microchip Technology Inc. Page 10

11 Position Counter Up/down counter Counts pulses generated by the decoder Count is accumulated in POSCNT register POSCNT can be accessed, both for read and write Its value can be compared to MAXCNT register 2005 Microchip Technology Incorporated. All Rights Reserved. dspic30f Quadrature Encoder Interface Module 11 The position counter can be used either for position or speed measurement. To measure motor position, we must know the relationship between the displacement and the number of phase pulses we get from the encoder. This relation can be known in advance, or can be measured during initialization by accumulating the total count for the maximum allowed displacement. We can set a constant value in the Maximum Count register, which is typically the number of pulse edges generated by one encoder revolution. As soon as we have a match between the Position Count and the Maximum Count, an interrupt is generated. In the Interrupt Service Routine, the user software can increment or decrerment a software counter containing the most significant bits of the position count. For speed measurement application, the time interval between two index pulses or count match events gives a measure of the angular velocity Microchip Technology Inc. Page 11

12 QEI as a 16 bit Timer/Counter 2005 Microchip Technology Incorporated. All Rights Reserved. dspic30f Quadrature Encoder Interface Module 12 If the quadrature decoder functionality is not needed, the QEI peripheral module can be configured as an additional 16 bit counter/timer. In this mode, the Position Counter register has the same functionality as the Timer registers in general-purpose timers, while the Maximum Count register serves as a period register. An additional feature compared to the generalpurpose timers is that the QEI counter is able to both increment and decrement its count, thus providing up-down counter functionality. The timer clock can be either internal or external; in the latter case the input pin is the QEA pin and the input clock, after digital filtering, will be synchronized with the instruction cycle. As in the general-purpose timers, gated time accumulation is also possible. The counting direction can be selected either with the UPDN bit in register QEICON, or with the QEB pin. An interrupt is generated when the value in the Position Counter register maches the value in the Maximum Count register Microchip Technology Inc. Page 12

13 Key Support Documents Device Selection Reference Document # General Purpose and Sensor Family Data Sheet DS70083 Motor Control and Power Conv. Data Sheet DS70082 dspic30f Family Overview DS70043 Base Design Reference Document # dspic30f Family Reference Manual DS70046 dspic30f Programmer s Reference Manual DS70030 MPLAB MPLAB C30 C Compiler User User s Guide DS51284 MPLAB ASM30, LINK30 & Utilities User DS51317 dspic Language Tools Libraries DS Microchip Technology Incorporated. All Rights Reserved. dspic30f Quadrature Encoder Interface Module 13 For more information, here are references to some important documents that contain a lot of information about the dspic30f family of devices. The Family Reference Manual contains detailed information about the architecture and peripherals, whereas the Programmer s Reference Manual contains a thorough description of the instruction set Microchip Technology Inc. Page 13

14 Key Support Documents Device Specific Reference Document # dspic30f2010 Data Sheet DS70118 dspic30f3010/3011 Data Sheet DS70141 dspic30f4011/4012 Data Sheet DS70135 dspic30f6010 Data Sheet DS70119 Microchip Web Site: Microchip Technology Incorporated. All Rights Reserved. dspic30f Quadrature Encoder Interface Module 14 For device-specific information such as pinout diagrams, packaging and electrical characteristics, the device datasheets listed here are the best source of information Microchip Technology Inc. Page 14

15 Related Material Apps Notes on Motor Control AN901 Using the dspic30f for Sensorless BLDC Control AN908 Using the dspic30f for Vector Control of an ACIM AN957 Sensored BLDC Motor Control Using dspic30f Microchip Technology Incorporated. All Rights Reserved. dspic30f Quadrature Encoder Interface Module 15 We also have some application notes on motor control, in which the peripheral is used. All these documents can be obtained from the Microchip web site, by clicking on the dspic Digital Signal Controllers or Technical Documentation link. This wraps up the seminar on dspic30f QEI. Thank you for your interest in the dspic30f Family of Digital Signal Controllers Microchip Technology Inc. Page 15

Section 16. Quadrature Encoder Interface (QEI)

M Section 16. Quadrature Encoder Interface (QEI) HIGHLIGHTS 16 Quadrature Encoder Interface (QEI) This section of the manual contains the following major topics: 16.1 Module Introduction... 16-2 16.2 Control