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Allow tendon springlength attribute to take two values between which the spring produces no force (a deadband).

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If given two non-decreasing values, `springlength` specifies a deadband for spring stiffness, inside of which the force is 0 and outside of which force behaves like a regular spring, with the setpoint corresponding to the nearest value. This can be used to create tendons whose limits are enforced by springs rather than constraints, which are cheaper and easier to analyse. See test/engine/testdata/tendon_springlength.xml example model.

Spring-limited spatial tendons whose lower range value is 0 are rendered as catenaries.

Fixes #520.

PiperOrigin-RevId: 484509706
Change-Id: I6698e94ee36168f52d501e83057559805e05172d
This commit is contained in:
Yuval Tassa
2022-10-28 05:57:05 -07:00
committed by Copybara-Service
parent 3a445d0fdd
commit 893942a729
20 changed files with 259 additions and 31 deletions
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src/engine/engine_vis_visualize.c
+19 -4
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@@ -1350,14 +1350,24 @@ void mjv_addGeoms(const mjModel* m, mjData* d, const mjvOption* vopt,
if (vopt->flags[mjVIS_TENDON] && (category & catmask)) {
for (int i=0; i<m->ntendon; i++) {
if (vopt->tendongroup[mjMAX(0, mjMIN(mjNGROUP-1, m->tendon_group[i]))]) {
// stiff tendon has a deadband spring
int limitedspring =
m->tendon_stiffness[i] > 0 && // positive stiffness
m->tendon_lengthspring[2*i] == 0 && // range lower-bound is 0
m->tendon_lengthspring[2*i+1] > 0; // range upper-bound is positive
// non-stiff tendon has a length constraint
int limitedconstraint =
m->tendon_stiffness[i] == 0 && // zero stiffness
m->tendon_limited[i] == 1 && // limited length range
m->tendon_range[2*i] == 0; // range lower-bound is 0
// conditions for drawing a catenary
int draw_catenary =
!mjDISABLED(mjDSBL_GRAVITY) && // gravity enabled
mju_norm3(m->opt.gravity) > mjMINVAL && // gravity strictly nonzero
m->tendon_num[i] == 2 && // only two sites on the tendon
m->tendon_limited[i] == 1 && // limited length range
m->tendon_range[2*i] == 0 && // range lower-bound is 0
m->tendon_stiffness[i] == 0 && // no stiffness
(limitedspring || limitedconstraint) && // either spring or constraint length limits
m->tendon_damping[i] == 0 && // no damping
m->tendon_frictionloss[i] == 0; // no frictionloss
@@ -1400,7 +1410,12 @@ void mjv_addGeoms(const mjModel* m, mjData* d, const mjvOption* vopt,
mju_copy3(x1, d->wrap_xpos + 3*d->ten_wrapadr[i] + 3);
// length of the tendon
mjtNum length = m->tendon_range[2*i+1];
mjtNum length;
if (limitedconstraint) {
length = m->tendon_range[2*i+1];
} else {
length = m->tendon_lengthspring[2*i+1];
}
// points along catenary path
int npoints = mjv_catenary(x0, x1, m->opt.gravity, length, catenary);