AP_Scripting: added example that slew a servo with controls

useful for measuring servo capabilities by looking at achieved
deflection vs frequency

libraries/AP_Scripting/examples/servo_slew.lua

@@ -0,0 +1,45 @@
+-- move a servo in a sinisoidal fashion, with settable limits and frequency
+
+local PARAM_TABLE_KEY = 135
+local PARAM_TABLE_PREFIX = "STEST_"
+
+-- bind a parameter to a variable
+function bind_param(name)
+   local p = Parameter()
+   assert(p:init(name), string.format('could not find %s parameter', name))
+   return p
+end
+
+-- add a parameter and bind it to a variable
+function bind_add_param(name, idx, default_value)
+   assert(param:add_param(PARAM_TABLE_KEY, idx, name, default_value), string.format('could not add param %s', name))
+   return bind_param(PARAM_TABLE_PREFIX .. name)
+end
+
+-- setup script specific parameters
+assert(param:add_table(PARAM_TABLE_KEY, PARAM_TABLE_PREFIX, 10), 'could not add param table')
+
+local STEST_CHAN      = bind_add_param('CHAN',          1, 0)
+local STEST_PCT       = bind_add_param('PCT',           2, 100)
+local STEST_PWM_MIN   = bind_add_param('PWM_MIN',       3, 1000)
+local STEST_PWM_MAX   = bind_add_param('PWM_MAX',       4, 2000)
+local STEST_FREQ      = bind_add_param('FREQ',          5, 1)
+
+function update()
+   local chan = STEST_CHAN:get()
+   local freq = STEST_FREQ:get()
+   if chan > 0 and freq > 0 then
+      local t = 0.001 * millis():tofloat()
+      local pi = 3.1415
+      local output = math.sin(pi * t * freq * 2.0) * STEST_PCT:get() * 0.01
+      local pwm_min = STEST_PWM_MIN:get()
+      local pwm_max = STEST_PWM_MAX:get()
+      local pwm_mid = 0.5*(pwm_min+pwm_max)
+      local pwm = math.floor(pwm_mid + (pwm_max-pwm_mid) * output)
+      SRV_Channels:set_output_pwm_chan_timeout(chan-1, pwm, 100)
+      --gcs:send_text(0, string.format("pwm=%u", pwm))
+   end
+   return update, 5 -- 200Hz
+end
+
+return update()