encoder − software counting of quadrature encoder signals
loadrt encoder [num_chan=num | names=name1[,name2...]]
encoder is used to measure position by counting the pulses generated by a quadrature encoder. As a software-based implementation it is much less expensive than hardware, but has a limited maximum count rate. The limit is in the range of 10KHz to 50KHz, depending on the computer speed and other factors. If better performance is needed, a hardware encoder counter is a better choice. Some hardware-based systems can count at MHz rates.
encoder supports a maximum of eight channels. The number of channels actually loaded is set by the num_chan argument when the module is loaded. Alternatively, specify names= and unique names separated by commas.
The num_chan= and names= specifiers are mutually exclusive. If neither num_chan= nor names= are specified, the default value is three.
encoder has a one-phase, unidirectional mode called counter. In this mode, the phase-B input is ignored; the counts increase on each rising edge of phase-A. This mode may be useful for counting a unidirectional spindle with a single input line, though the noise-resistant characteristics of quadrature are lost.
encoder.update-counters (no floating-point)
Does the actual counting, by sampling the encoder signals and decoding the quadrature waveforms. Must be called as frequently as possible, preferably twice as fast as the maximum desired count rate. Operates on all channels at once.
encoder.capture-position (uses floating point)
Captures the raw counts from update-counters and performs scaling and other necessary conversion, handles counter rollover, etc. Can (and should) be called less frequently than update-counters. Operates on all channels at once.
The names for
pins and parameters are prefixed as:
encoder.N. for N=0,1,...,num-1 when using num_chan=num
nameN. for nameN=name1,name2,... when using names=name1,name2,...
The encoder.N. format is shown in the following descriptions.
encoder.N.counter-mode bit i/o
Enables counter mode. When true, the counter counts each rising edge of the phase-A input, ignoring the value on phase-B. This is useful for counting the output of a single channel (non-quadrature) sensor. When false (the default), it counts in quadrature mode.
encoder.N.counts s32 out
Position in encoder counts.
encoder.N.index-enable bit i/o
When true, counts and position are reset to zero on the next rising edge of Phase-Z. At the same time, index-enable is reset to zero to indicate that the rising edge has occurred.
encoder.N.phase-A bit in
Quadrature input for encoder channel N.
encoder.N.phase-B bit in
encoder.N.phase-Z bit in
Index pulse input.
encoder.N.position float out
Position in scaled units (see position-scale)
encoder.N.position-interpolated float out
Position in scaled units, interpolated between encoder counts. Only valid when velocity is approximately constant and above min-velocity-estimate. Do not use for position control.
encoder.N.position-scale float i/o
Scale factor, in counts per length unit. For example, if position-scale is 500, then 1000 counts of the encoder will be reported as a position of 2.0 units.
encoder.N.rawcounts s32 out
The raw count, as determined by update-counters. This value is updated more frequently than counts and position. It is also unaffected by reset or the index pulse.
encoder.N.reset bit in
When true, counts and position are reset to zero immediately.
encoder.N.velocity float out
Velocity in scaled units per second. encoder uses an algorithm that greatly reduces quantization noise as compared to simply differentiating the position output. When the magnitude of the true velocity is below min-velocity-estimate, the velocity output is 0.
encoder.N.x4-mode bit i/o
Enables times-4 mode. When true (the default), the counter counts each edge of the quadrature waveform (four counts per full cycle). When false, it only counts once per full cycle. In counter-mode, this parameter is ignored.
encoder.N.latch-falling bit in (default: TRUE)
encoder.N.latch-rising bit in (default: TRUE)
encoder.N.counts-latched s32 out
encoder.N.position-latched float out
Update counts-latched and position-latched on the rising and/or falling edges of latch-input as indicated by latch-rising and latch-falling.
encoder.N.counter-mode bit rw
Enables counter mode. When true, the counter counts each rising edge of the phase-A input, ignoring the value on phase-B. This is useful for counting the output of a single channel (non-quadrature) sensor. When false (the default), it counts in quadrature mode. encoder.N.capture-position.tmax s32 rw Maximum number of CPU cycles it took to execute this function.
for num_chan= specifier are:
Parameter names for names= specifier are:
encoder.N.min-velocity-estimate float rw (default: 1.0)
Determine the minimum true velocity magnitude at which velocity will be estimated as nonzero and postition-interpolated will be interpolated. The units of min-velocity-estimate are the same as the units of velocity. Setting this parameter too low will cause it to take a long time for velocity to go to 0 after encoder pulses have stopped arriving.